Dispersion, Spatial Behaviour and Burrows of the
ghost crabs Ocypode cordimana (Fabricius) and
Ocypode ceratophthalma (Pallas), (Decapoda,
Brachyura, Ocypodidae)
By: Louise Raggett
Abstract
Acknowledgments
I am particularly grateful to Martyn Drabik-Hamshare, to whom this studies’ data collection
was a combined effort (see Drabik-Hamshare, 2010). I would also like to thank my supervisor
John Allen for the opportunity to go to Bird Island and conduct this study and for his support
during the preparation of this document. Further thanks go to Dr. Colin Little whose
enlightenment in the habits and ecology of Ocypode, helped me decide upon this studies
focus.
Abbreviations
H.W.M. High water mark
C.S.R. Complete spatial randomness
Contents
1. Introduction.......................................................................................................................................... 8
1.1 Dispersion and ecological significance ........................................................................................... 8
1.2 Environmental factors .................................................................................................................. 10
1.3 Population density ....................................................................................................................... 10
1.3.1 Seasonal variations in population ......................................................................................... 10
1.3.2 Temperature effects.............................................................................................................. 11
1.3.3 Human disturbance ............................................................................................................... 11
1.4 Ocypode zonal distribution .......................................................................................................... 11
1.4.1 Beach exposure ..................................................................................................................... 12
1.4.2 Sympatric species .................................................................................................................. 13
1.4.3 Distribution and crab size ...................................................................................................... 13
1.5 Ocypode burrows: function and distribution of morphotypes .................................................... 14
1.5.1 Zonal distribution and burrow morphotype ......................................................................... 14
1.6 Social and behavioural implications of Ocypode Burrows ........................................................... 16
1.6.1 Interactions- agonistic and non-agonistic ............................................................................. 16
1.6.2 Aggregative behaviour .......................................................................................................... 17
1.6.3 Female centred competition ................................................................................................. 18
1.6.4 Use of burrows by reproductively active males .................................................................... 18
1.6.5 Pyramid building as sematectonic signals ............................................................................ 19
1.6.6 Pyramid building in enforcing male territories ..................................................................... 21
1.6.7 Visual, acoustic and vibrational communication .................................................................. 21
1.7 Summary ...................................................................................................................................... 23
2. Aims of Study...................................................................................................................................... 24
3. Method ............................................................................................................................................... 24
3.1 Study site ...................................................................................................................................... 24
3.2 Decapod species found at the site ............................................................................................... 25
3.3 Choice of beach for sampling ....................................................................................................... 29
3.4 Mapping burrows in a ‘point pattern’ .......................................................................................... 32
3.5 Data collection for transects of Beach A ...................................................................................... 32
3.6 Mapping of Nearest Neighbours for transects 2,3 and 4 ............................................................. 35
3.7 Activity data collection ................................................................................................................. 38
3.7 Observations of adult O. ceratophthalma ................................................................................... 38
4. Analysis of Data .................................................................................................................................. 40
4.1 Population distribution and density ............................................................................................. 40
4.1.1 Population density ................................................................................................................ 41
4.1.2 Crab activity........................................................................................................................... 41
4.2 Dispersion pattern of Ocypode burrows ...................................................................................... 41
4.2.1 Frequency distributions among quadrats ............................................................................. 41
4.2.2 Nearest Neighbour Analysis .................................................................................................. 43
4.3 The relationship between crab size and distance to neighbours ................................................ 44
4.3.1 Size of burrow effect on nearest neighbour ......................................................................... 44
4.4 Dispersion of different juveniles and adults ............................................................................ 45
5. Results ................................................................................................................................................ 45
5.1 Beach morphology ....................................................................................................................... 45
5.2 Population Distribution ................................................................................................................ 47
5.3 Density of burrows ....................................................................................................................... 49
5.4 Population estimate ..................................................................................................................... 50
5.5 Zonal distribution ......................................................................................................................... 51
5.6 Burrows types............................................................................................................................... 52
5.7 Crab activity.................................................................................................................................. 54
5.8 Dispersion pattern: using frequencies in quadrats ...................................................................... 56
5.9 Dispersion of Ocypode burrows: using nearest neighbour distances .......................................... 58
5.10 Dispersion pattern of different size categories and mature males ........................................... 59
6. Discussion .............................................................................................. Error! Bookmark not defined.
6.1 Differences between O. cordimana and O. ceratopthalma ............ Error! Bookmark not defined.
6.3. Distribution of burrows .................................................................. Error! Bookmark not defined.
6.3.1 Variation of burrow density along the beach .......................... Error! Bookmark not defined.
6.3.1 Distribution of different burrow sizes ...................................... Error! Bookmark not defined.
6.3.2 Distribution of juvenile versus adult burrows .......................... Error! Bookmark not defined.
6.3.3 Variation up the beach ............................................................. Error! Bookmark not defined.
6.3.4 Burrow morphology ................................................................. Error! Bookmark not defined.
6.4 Dispersion of burrows ..................................................................... Error! Bookmark not defined.
6.4.1 Burrow defence ........................................................................ Error! Bookmark not defined.
1. Introduction
Ghost crabs of the genus Ocypode are the most widespread members of Ocypodidae,
occurring on all major oceans, (Daumer et al, 1963; George and Knott, 1965, Mclachlan and
Brown, 2006). They are a dominant feature of sub-tropical and tropical sandy beaches,
(Barrass, 1963; Taylor, 1968; Chan et al, 2006), and mostly appear to have crepuscular and
nocturnal activity, (Cott, 1930; Hagasaka, 1935; Barrass, 1963; Hughes, 1966; Hill and Hunter,
1973; Shuchman and Warburg, 1978; Strachan et al, 1999). Ocypode are primarily
predacious, (Gibson and Hill, 1947; Hughes, 1966; Wolcott, 1978; Strachan et al, 1999), but
are also scavengers, and deposit feeders, (Hagasaka, 1935; Hughes, 1966; Jones, 1972; Trott,
1988; Strachan et al, 1999). Due to their abundance, relative size, extensive bioturbation
activities and omnivory, Ocypode perform a key function in sandy shore ecosystems,
(Wolcott, 1978; Chan et al, 2006; Tureli et al, 2009). As all Ocypode individuals burrow
regardless of size or sex, (De, 2005), they are ideal organisms to study dispersion and space
related behaviours, (Lighter, 1977).
1.1 Dispersion and ecological significance
Studying the dispersion of organisms has considerable, wide-ranging ecological significance,
(Moore and Chapman 1986; Dale 1999; Southwood and Henderson, 2000). The dispersion
pattern of a population is defined here as the spatial distribution of individuals within their
population’s geographical range at any one point in time, (Browns and Orians, 1970). In a
continuum of dispersion patterns, three categories are generally recognised. Dispersion may
be Regular, where individuals are more evenly spaced than expected by chance, aggregated,
where individuals are closer than expected by chance, and random, when there is an equal
probability of any individual occupying any point in space irrespective of the position of
others, (Southwood and Henderson, 2000; Fowler et al, 1998). However, simply identifying a
deviation from random is of little ecological interest, (Dale, 1999). Instead, the discernment
of the proximate and ultimate factors behind the observed spatial behaviour is the major
focus of most studies.
Dispersion is scale-dependant, (Dale, 1999) and is a function of both environmental factors,
such as resource availability, which dictate where animals are distributed at a large scale
perspective, and behavioural factors, which dictate the spacing of conspecifics usually at a
finer scale, (Pearson, 1949, Dale, 1999, Fero and Moore, 2008). The latter include factors
such as social units, (Brown and Orians, 1970), species specific individual distances, (Hediger,
1955), agonistic interactions such as territorial behaviour, (Maher and Lott, 1995), and social
dominance, (Hemerijic, 2000), which are in turn impacted by environmental factors, (Fero
and Moore, 2008). In crustaceans, many studies focus upon either environmental or
behavioural impacts, (e.g. Shuchman and Warburg, 1978 versus Lighter, 1977), however both
should be considered, (Fero and Moore, 2008) . Studies on Ocypode in general have been
relatively few compared to their relatives the fiddler crab in the genus Uca, (Popper et al,
2001), and many gaps in our knowledge remain.
1.2 Environmental factors
Studying the spatial distribution of Ocypode indirectly through their burrows requires an
understanding of their ecology and the environmental factors which affect them, (Lighter,
1977; Barrass, 1967, Alberto and Fontoura, 1999; Turra et al, 2005). As most species inhabit
heterogeneous habitats, from a large-scale perspective an aggregated dispersion pattern is
most common, (Moore and Chapman, 1986), especially on coastal shores where there are
strong environmental gradients, (Benson, 2002). Previous studies provide only a limited
picture of what principal factors determine density, zonal distribution and burrowing
behaviour of Ocypode.
1.3 Population density
The population densities of Ocypode species are variable, with large differences observed
even between neighbouring populations, (Frey and Mayou, 1971; Chakrabati, 1981).
However, identifying the primary factors determining densities is challenging due to the
dynamic nature of beaches, (Jarmillo et al, 2000), and their multiple confounding factors
(Quijon et al, 2000). Little is known about the impact of resource availability, (Lucrezi et al,
2008), but high cannibalism rates appear to reduce juvenile density, (Cowles, 1908; Williams,
1965).
1.3.1 Seasonal variations in population
Recruitment, (Strachan et al, 1999), winter dispersal (Tureil et al, 2009) and growth (Wolcott,
1978) affect seasonal variations dramatically. Beach type and exposure appear to impact
density, but not consistently intraspecifically, (Jones,1972; Jarmillo and Mclachlan, 1993;
Dugan and Hubbard, 1996; Chan et al, 2006).
1.3.2 Temperature effects
Temperature tends to dictate ghost crab burrowing activity, (Shuchman and Warberg, 1878),
which generally occurs between temperatures of 16 and 30˚C, (Christoff, 1986; Weinstein,
1998).
1.3.3 Human disturbance
Many studies suggest human disturbance from pedestrians and vehicles is a predominant
factor affecting density of different Ocypode species, (Hughes, 1966; Frey and Mayou, 1971;
Chakrabati, 1981; Barros, 2001; Blankesteyn, 2006; Maccarone and Matthews, 2007; Moss
and Mcphee, 2006; Neves and Benvenuti, 2006; Schlacher et al, 2007; Lucrezi et al, 2008;
Hobbs et al, 2008; Brook et al, 2009; Magalhaes et al, 2009; Yong and Lim, 2009). These
results suggest Ocypode species would therefore make good biological indicators, although
densities can increase around food refuse, (Hill and Hunter, 1973; Steiner and Leatherman,
1981), while hot weather and short term human trampling can lead to temporarily plugged
and imperceptible burrows, reducing burrow counts erroneously (Lucrezi et al, 2009). Tidedominated beaches also have too many confounding factors impacting density as they are
particularly heterogeneous habitats, (Turra et al, 2005).
1.4 Ocypode zonal distribution
Ghost crabs usually burrow in distinct zones on beaches. Most studies suggest Ocypode
classically burrow in just above the H.W.M., (Grubb, 1971; Jones, 1972; Hartnoll, 1975;
Burggren and McMahon, 1988), contrary to the belief of De, (2005). For example, O.
ceratophthalma, (Barrass, 1963; Hughes, 1966; Jones, 1972), O. urvillei, (Burggren and
McMahon, 1988), O. saratan, (Clayton, 2001), O. macrocera, ,(Burggren et al, 1988), O.
cursor, (Tureli et al, 2009), O. ryderi (Vannini, 1980b), and O. platytarsis, (Naidu, 1951). Some
species have adapted to supralittoral areas, (O. aegyptiaca,, (Magnus, 1960), O.
gaudichaudii, (Quijon et al, 2001), O. jousseaumei, (Clayton, 2005) whilst others even extend
into sandy soils inland, e.g. O. cordimana, (Horch, 1975; Stoddart, 1984), O. kuhlii, (Macnae
and Kalk, 1962; Jones, 1972), O. Africana, (Strachan et al, 1999), O. albicans, (Pearse et al,
1942) and O. quadrata, (Borradaile, 1903; Pearse et al, 1942; Hill and Hunter, 1973; Strachan
et al, 1999; Alberto and Fontoura, 1999; Clayton, 2005). However there are intraspecifc
differences in Ocypode zonal distributions as well as density, occurring spatially, temporally
and between different sexes and ages. At this coarse scale, (Brown and Orians, 1970),
dispersion appears to be a reflection of burrowing conditions and crab physiology rather
than social interactions.
1.4.1 Beach exposure
Beach exposure appears to affect zonal distribution, (Jarmillo et al, 2000). Indeed during
storms, zonal distributions can shift landwards, (Alberto and Fontoura, 1999; Neves and
Benvenuti, 2006; Hobbs et al, 2008). For intertidal species, distribution may be determined
by exposure times between tides, as distributions often shift in sync with annual tidal
changes (Barrass, 1963; Jones, 1972). Hughes, (1966) suggests for O. ceratophthalma, these
changes are indirect, reflecting shifts in sandpacking, suggesting crab distribution is primarily
determined by suitable burrowing substrate. Contrary to Takahasi (1935), Ocypode burrows
do not reach the water table to renew respiratory water like most fiddler crabs, (Chakrabati,
1981; Strachan et al, 1999; Chan et al, 2006). Instead crabs regular moisten gills in the sea,
(Williams, 1965), and can absorb capillary moisture from sand via setae found on their
abdominal segment, (Hatnoll, 1973) and third and fourth ambulatory legs, (Wolcott, 1976).
O. cursor at least, appear to be sensitive to very specific moisture levels, (Warburg and
Shuchman, 1978) and studies have found distribution is often largely determined by the
water gradient, (Lucrezi et al, 2009), with shifts closer to the sea observed during summer,
(Shuchman and Warburg, 1978; Lucrezi et al, 2009; Tureli et al, 2009). This is a contrast to
temporal stability observed in species adapted to dry sand areas such as O. gaudichaudii,
(Quijon et al, 2001). Many studies indicate that density decreases away from the sea, (Hill
and Hunter, 1973; Tureli et al, 2009) and it appears this may be linked to increasing height
from the water table and decreased capillary water, (Turra et al, 2005).
1.4.2 Sympatric species
The impact of sympatric species has never been examined, perhaps due to the great
variation in zonation compared to rocky shores. However, both O. cursor and O. laevis
appear to be competitively displaced by the larger O. africana, (Strachan et al, 1999) and O.
cordimana, (Fellows, 1975) respectively.
1.4.3 Distribution and crab size
There is a clear trend that as crabs increase in size, their distribution migrates landwards, as
found in O. quadrata, (Duncan, 1986; Frey and Mayou, 1970; Hill and Hunter, 1973, Fisher
and Tevesz, 1979; Turra et al, 2005), O. ceratophthalma, (Hayasaka, 1935; Chakrabati, 1981,
Chan et al, 2006), and O. cursor, (Shuchman and Warburg, 1978; Strachan et al, 1999; Tureli
et al, 2009). As juveniles have proportionately smaller gills, (Chakrabati, 1981) and loose
water faster than adults, (Eshky, 1985), this may be a function of physiological competence,
(Fisher and Tevesz, 1979; Chakrabati, 1981; Chan et al, 2006). However, some studies have
observed no such trend, even within the same species, (Barrass, 1963; Fellows, 1975;
Maccorone and Matthews, 2007; Seike and Nara, 2008; Yong and Lim, 2009). This suggests
the relationship between physiology and distribution is not clearcut. (see Drabik-Hamshare,
2010 for review).
1.5 Ocypode burrows: function and distribution of morphotypes
Several factors that impact burrow morphology and thus burrowing behaviour should be
considered in the context of Ocypode distribution. Burrowing is typical of macroinvertebrates inhabiting soft sediments, (Katrak et al, 2008) and is essential to Ocypode
survival, (Wolcott, 1984). Burrows provide several functions, (Berti et al 2008; Tureil et al,
2009) including a refuge from predators, (mainly seabirds), (Cowles, 1908), and a
“homebase”, (Browns and Orians, 1970) which feeding, breeding and territorial behaviour
orientate around, (Tureil et al, 2009). As Ocypode crabs are ‘quasi-terrestial’, their gills must
remain moist (De, 2005) and thus burrows also provide a refuge from surface temperatures,
(Chan et al, 2006), a supply of moisture, (Barrass, 1963), and a plugged air column to breathe
during tidal submersion, (De, 2005).
1.5.1 Zonal distribution and burrow morphotype
Ocypode burrow shape varies intraspecifically and studies have found distinct zonal
distributions of different burrow ‘morphotypes’. Although morphotypes vary among
populations and beach types, (e.g. Chakrabati, 1981), generally it is found that there is a
landward transition from shallow, vertical unbranched tubes to increasingly deeper, sloping,
Y shaped burrows, representing the letters IJUY, (Frey and Mayou, 1970; Allen and
Curran, 1972; Hill and Hunter, 1973; Shuchman and Warburg, 1978; Vanini, 1980a,
Chakrabati, 1981; Chan et al, 2006). Similar transitions have been seen in other burrowing
crustaceans, (Perez-Chi et al, 2005; Li et al, 2008; Berti et al, 2008), although other
Ocypodidae such as Uca species tend to have consistently, simple ‘J’ shaped burrows,
(Koretsky et al, 2002; Lim, 2006). Increased depth may reflect a changing watertable,
(Strachan et al, 1999; Tureli et al, 2009) or sediment layer level, (Borradaile, 1903; Barrass,
1963), whilst burrow slope is likely to be a function of crab weight, substrate consistency,
(Fellows, 1966), and beach slope, (Duncan, 1986). This transition in increasing complexity
often coincides with an increase in crab size, (Frey and Mayou, 1970; Hill and Hunter, 1973;
Lighter, 1977; Chakrabati, 1981; Chan et al, 2006). It is thought that juveniles usually dig
simple, shallow tubes as their burrows are less permanent as they must often wet their gills
at sea, and as they grow larger and become more terrestrial, burrows are more permanent,
allowing deeper, complex burrows with added functions in the form of chambers and
secondary branches, (Chakrabati, 1981; Chan et al, 2005) which may be associated with
copulation, (Milne and Milne, 1946), and/or provide a refuge against up-wash, (Chakrabati,
1981) or predators, (Cowles, 1908). Indeed the temporary burrows of other decapods are
often shallow, single tubes, (Braithewait and Talbot, 1972), compared to the complex
galleries of species with permanent burrows, (Green, 2004).
However, it appears that both the size of the crab and their sub-environment, affect burrow
shape, as size increases are not always correlated with this landward burrow shape
transition, (Seike and Nara, 2008), and often sharp transitions occur between zones rather
than sizes, (Hill and Hunter, 1973; Chakrabati, 1981). Irrespective of age, burrowing
behaviour may be affected by temperature, grain size and packing, slope and sand moisture,
(Chakrabati, 1981). Interestingly, the suggestion that ghost crabs subject to tidal action
maintain pressure equilibrium between external hydrodynamics and internal capillarity by
making their burrow larger, with added branches and curves during a single diel tidal cycle,
contradicts the findings of previous studies drastically, (De, 2005).
1.6 Social and behavioural implications of Ocypode Burrows
The social behaviour of Ocypode has not been studied in great detail, excepting perhaps the
behaviour of O. saratan males, O. ceratophthalma and O. laevis, (Linsenmair, 1963; Lighter,
1977). Many studies are observational and this is associated with the difficulty in observing
the behaviour of these crabs without disturbing them, (Clayton, 2005). The general focus has
been on Ocypode communication and mate choice which have implications for spacing.
Although all Ocypode individuals require a burrow for their survival, (Lighter, 1977), burrows
may be either transitory or permanent and maintained, and many ‘wanderers’ can
temporarily lack a shelter, (Wolcott, 1978; Strachan et al, 1999).
1.6.1 Interactions- agonistic and non-agonistic
Generally non-agonistic interactions appear minimal, (Hughes, 1966; Lighter, 1977), whereas
defensive or agonistic interactions are typical of communal decapods. Many wandering crabs
actively compete with residents for burrows and digging space, e.g. in O. ceratophthalma,
(Jones, 1972; Hughes, 1973; Lighter, 1977; Brooke, 1981), O. gaudichaudii, (Schober and
Christy, 1993), O. laevis, (Lighter, 1977), O. saratan, (Linsenmair, 1967), O. jousseamei, O.
rotundata and O. platyarus, (Clayton, 2005). Interactions are generally ritualised, (Schone,
1968), like many in the genus Uca, (Altevogt, 1957; Crane, 1958). Lighter (1977) found that as
crabs developed, not only did their sand disposal behaviour get more complex, but so did
their defensive behaviours, with added behavioural repertoires, ‘pushing’ and ‘grappling’
with chelae in O. ceratophthalma and ‘pushaside’, in O. laevis, being added to ‘chase’ and
‘threat’ behaviour observed in juveniles.
1.6.2 Aggregative behaviour
Wandering crabs often aggregate at the water’s edge before seeking a burrow, (Vannini,
1976; Wolcott, 1978; Eshky, 1985; Trott, 1988; Strachan et al, 1999; Tureli et al, 2009). This is
possibly to forage an unidentified food source, (Strachan et al, 1999) but parallels the
seemingly aimless wandering stage exhibited by soldier crabs, (Cameron, 1966). This
behaviour has clear social implications, and despite aggregating, individuals maintain
individual distances between conspecifics, (Clayton, 2005), similar to that seen during
foraging behaviour, (Wolcot, 1978). Such behavioural spacing appears, however, to be
overridden when food sources are concentrated, (Hughes, 1966; Lighter, 1977).
Burrows appear to be maintained if they are not naturally damaged often, (Hughes, 1966;
Vannini, 1980a). For example O. ceratophthalma individuals subject to tidal action and
burrow collapse tend to have temporary burrows (Hughes, 1966), whereas supratidal
individuals maintain burrows for at least 10 days, (Hughes, 1966). It is not known whether
temporary residency has an effect on social interactions, but from studies on other crabs,
(Schembri, 1981), reduced agonistic interactions seem likely and is supported by the
observation that few competitive interactions are apparent in O. quadrata, (Clayton, 2005).
These have temporary burrows and prefer to takeover abandoned burrows, (Fimpel, 1975).
In fiddler crabs, long lived burrows can lead to complex social interactions, for example
neighbour recognition, (Detto et al, 2006) and territorial coalition, (Backwell and Jennions,
2004).
1.6.3 Female centred competition
Some behavioural studies focus their attention on sexually mature males, which show strong
burrow defence behaviour, (Linsenmair, 1963). Male Ocypode individuals encounter
receptive females unpredictably in time and space due to their highly mobile nature and
infrequent matings, (Christy, 1987). Females also do not appear to require a suitable
incubation burrow that they cannot provide easily for themselves, unlike most Uca females,
(Hughes, 1966). Therefore Ocypode males have evolved ‘female centred competition’, a
mating strategy where males attract and defend females, (Christy, 1987). This is rare in
decapods and found only in the family Ocypodidae, (Wada, 1981, 1983, 1984; Christy et al,
2001).
1.6.4 Use of burrows by reproductively active males
The basis of reproductive activities is the burrow, from which the male ‘advertises’ himself to
females, (Christy, 1987) and in which copulation usually occurs, (Netto et al, 2007).
These burrows are not part of the spectrum of burrow shapes described in 1.1.5, but spiral
tightly in the direction of the handedness of the crab, as a function of digging posture
(Barrass, 1963), and differ from the spiral burrows of Uca dug as predator defences, (Basan
and Frey, 1977). Some studies appeared to have overlooked these inter-sexual differences,
(Barrass, 1963; Hughes, 1966) or concluded no sex differences were apparent due to low
female sample sizes, (Chakrabati, 1981; Chan et al, 2005). However, it seems these burrows
are dug and defended by males of O. ceratophthalama,(Farrow, 1961; Fellows 1966; Lighter,
1977; Hughes, 1973), O. saratan, (Linsenmair, 1967; Eshky, 1985), O. kuhlii, (Jones, 1972), O.
jousseaumei, (Clayton, 2005) and O. gaudichaudii, (Schoeber and Christy, 1993). Some
species such as O. occidentalis and O. quadrata have been observed in surface matings,
(Hughes, 1973) and do not have spiral burrows.
Although copulations have remarkably never been witnessed, (Clayton, 2005), males and
females with post-copulation evidence have been found, (Linsenmair, 1967). The androgenic
gland is responsible for eliciting spiral burrow construction behaviour in O. ceratophthalama,
(Lighter, 1977) and due to its cyclic activity, (Thampy and John, 1970; Payen, 1972), such
behaviour appears to be synchronised so that it peaks when tides are lowest (Lighter, 1977).
Although juveniles often dispose excavated sand in fan shapes, (thought to decrease burrow
conspicuousness), (Hughes, 1966; Jones, 1972; Clayton, 2005) and females and nonreproductively active males in loose, inconspicuous piles, (Lighter, 1977, Hughes, 1973),
these active males construct “pyramid” structures.
1.6.5 Pyramid building as sematectonic signals
Ocypode construct ‘pyramid’ structures which appear to act as sematectonic signals, (Wilson,
1975). These pyramidal structures are similar to those built by many males of other species,
as they play no role in the care for eggs and young, (Linsenmair, 1973). These include the
pillars, (Christy, 1988a), chimneys, (Wada and Murata, 2000), mudballs, (Burford et al, 2001),
hoods, (Zucker 1981; Clayton, 1998), shelters, (Zucker, 1974), and barricades, (Wada, 1994),
built by 17 Uca species, (Burford et al, 2001). Similar to many of Uca sand structures,
(Christy, 1988a, 1988b, 2001; Muramatsu, 2010), the pyramids function as highly developed
forms of signals for mate attraction, obviating the need for an active display, (Wilson, 2000).
These signals are only for long distance attraction, (Linsenmair, 1967), possible only in such
flat, featureless habitats as beaches and tidal flats, (Kitaura et al, 2002; Takedo, 2005).
Pyramids appear to be analogous to the long distance signal ‘hoods’ constructed by male
Uca musica, (Christy et al, 2001) and may similarly attract females though both landmark
orientation behaviour, (Herrnkind, 1983) and optical conspicuousness (Zeil and Ak-Mutairi,
1996). Pyramids may be ‘honest’ signals signifying male quality as males do appear to aim for
the largest pyramids which are clearly energetically costly, (Linsenmair, 1967) however the
signal strength of such ‘displays’, (krebs and Davis, 1993), would have to have little withinmale variation compared to inter-male variation and evidence that females select males with
large pyramids, to increase reproductive fitness would have to be shown, (Burford et al,
2001).
Pyramids are also important ‘sign-stimuli’ in colony formation by stimulating other
reproductively active males to dig pyramids close by. They also enforce other males to
construct their own spiral/pyramid complexes at a minimum 134cm distance apart, acting as
‘petrified display signals’, and allowing males to maintain active territories, (Linsenmair,
1967). This may be why pyramids are often built earlier than the rest of the population
during daylight hours, allowing visual signals to be strong, (Hughes, 1966). Indeed a
nocturnal population of O. saratan appeared to lose the signal function of pyramids,
(Linsenmair, 1967).
1.6.6 Pyramid building in enforcing male territories
The role of pyramids in enforcing male territories and maintaining neighbour distances is
similar to the sand structures of many other Uca species, (Oliveira et al, 1998; Muramatsu,
2010) as well as the mounds built by Ilyopax pingi, (Wada et al, 1994). Whether the pyramids
reduce inter-neighbour aggression like many large structures, (Zucker, 1974; Clayton, 1987;
Wada, 1994) is unclear, but displacement by dominant intruders does occur. Interestingly
these conflicts and displacements are nearly always between crabs of the same handedness,
(Linsenmair, 1967), of which there is usually a 1:1 ratio in Ocypode, (Barrass, 1963;
Linsenmair, 1967; Eshky, 1985; Strachan et al, 1999). Unless there are large size
discrepancies, the owner also usually wins, (Linsenmair, 1967) probably due to a prior
residency effect, found in many conflicts between burrowing crustaceans, (Peeke et al, 1995;
Jennions and Backwell, 1996; Takahashi et al, 2001).
1.6.7 Visual, acoustic and vibrational communication
Other than the structural signals built by males, Ocypode crabs exhibit various acoustic, vibrational
and visual behaviours which appear to be intraspecific signals, (Clayton, 2008, Horsch and Salmon,
1969). The function of these have not been conclusively identified, (Popper et al, 2001), partly due to
the inability to elicit the natural behaviours during manipulations, (Clayton, 2005).
Several behaviours are clearly agonistic, one being the universal ‘threat’ posture taken up by
ghost crabs during defensive interactions, (Hughes, 1966; Lighter, 1977; Brooke, 1981;
Kuriharaetal, 1989). The carapace is tilted vertically, with the merus held sideways, and the
chelae vertically, (Lighter, 1977), characteristic of semi-terrestrial crabs, (Schone, 1968).
Brooke, (1981) suggests this is classic threat behaviour, where size is accentuated, much like
hair raising seen in mammals.
Acoustic and or vibrational signals have had comparatively less research in decapods in
general, (Popper et al, 2001). Sounds produced by Ocypode species are species specific and
include ‘rapping’, where both chelae drum the ground, (Horch and Salmon, 1969),
‘stridulation’, where stridulatory teeth on the major chelae are rubbed against the crab’s
plectrum, (Clayton, 2005) and ‘ tamping’, where sand in patted using chelae and inwardly
flexed dactylii, (Warburg and Shuchman, 1979). Many studies tentatively suggest these
signals are used by crabs to interact with neighbours, (Alcock, 1892; Cott, 1929; Crane, 1941;
Hughes, 1966; Horch and Salmon, 1972;Vannini, 1980a). Clayton (2005) suggests rapping in
O. jousseamei is a signal to advertise the crabs territorial presence and increases
indiscriminately at the sighting of any specie or sex. Such ‘advertising signals’ are in fact
common, in insects, (Dumontier, 1963) and anurans, (Wells, 1977). Stridulation is a close
range signal, quickly attenuating due to its higher frequency and is thought to be associated
with inter-male burrow defence, (Clayton, 2005).
Some studies suggest claw waving and acoustic/ vibrational signals are produced mainly by
males, with peaked activity coinciding with lunar cycles suggesting they play a role in female
attraction, (Barrass, 1969; Imafuku et al, 2001; Popper et al, 2001). Waving is a widespread
signal in Uca to attract females, Imafuku et al, 2001), and has also been observed in O.
ryderii, (Vannini, 1976, 1980a), O. stimpsoni, (Wada, 1978; Imafuku et al, 2001), O.
gaudichaudii, (Wright, 1968) and O. platytarsus, (Clayton, 2001, 2008). Also similar to certain
Uca species, (Altevogt, 1959; Salmon and Stout, 1962; Müller, 1989), these are combined
with auditory ‘rapping’ in O. stimpsoni, (Clayton, 2008). O. platytarsus have perhaps the most
developed displays involving complex rapping with stridulation and even dances, (Clayton,
2008). Ocypode populations which lack pyramids appear to utilise acoustic signals more
frequently , e.g. O. stimpsoni, O. macrocera and O. platytarsus, (Clayton, 2001; Imafuku et al,
2001), whilst species observed in surface matings display little burrow advertisement
signalling at all, (Horch and Salmon, 1969).
1.7 Summary
Although recent studies are increasing our understanding, (Clayton, 2008), more research is
clearly required to understand these communicative behaviours before their impact on
conspecifics spacing in Ocypode can be understood, as has been done extensively in many
Uca species, (Crane, 1966; Salmon and Atsaides, 1968; Latruffe et al, 1999). The same is true
for the impact of environmental factors as discussed above. A good starting point is to
identify a population’s dispersion pattern at different spatial scales, (Dale, 1999) as this
indicates whether potential negative interactions are occurring between conspecifics
(Connell, 1962; MacArthur and Connell, 1966) and whether the environment is homogenous
or heterogeneous, (Brown and Orians, 1970; Piou and Feller, 2009).
2. Aims of Study
Despite recent interest in their potential as biological indicator species, there is a general lack of
knowledge on the ecological and social factors implicated in the distribution and abundance of these
organisms. The main aim of this study was therefore to examine the dispersion pattern of active
ghost crab burrows found on a sandy beach on Bird Island, in the Seychelles, to gain an insight into
both their social nature and the impact of the environment upon their burrows and burrowing
behaviour. In association, this study also aimed to investigate whether densities and morphotypes of
burrows varied on the beach, and as the size of a crab is likely to have both social and physiological
implications, the impact of crab size, determined indirectly from burrow diameter, upon spacing,
dispersion and burrow morphology were also examined. Burrows were indistinguishable between
two overlapping species, O. ceratophthalma and O. cordimana, and therefore similar to many
Ocypode studies, the study aims were not specific to a particular species. Consequently the daytime
activities of each species were recorded to identify any species-specific ecological differences.
3. Method
3.1 Study site
This study was carried out on a low lying, reef derived sand cay in the Seychelles, figure 1,
called Bird Island, (previously known as Île aux Vaches), currently 1.4km2 in size. The climate
is seasonal tropical with a mean temperature of 28°C, (Walsh, 1984). The study took place
over a period of nine days at the beginning of the NW monsoon period where most rainfall
occurs. Humans only began to have a significant impact on the island in the early 20 th
century when bird guano extraction from Bird’s sooty turn colony began, shortly followed by
a coconut plantation, (Feare, 1979). Today the island is a managed conservation resort with
a small permanent human population, (Brook et al, 2009).
Figure 1 Central Seychelles showing position of Bird Island in the north. (from Hill et al, 2002).
3.2 Decapod species found at the site
Four species of Decapods were observed on the island during the study, colour plate 1. Two
individuals of Geograpsus grayi (purple nipper) were observed in the islands centre and
Coenobita rugosa hermit crabs were also noticed at the tops of beaches.
On the beaches, only two ghost crabs were observed; Ocypode ceratophthalma, found up to
20m inland; and surprisingly Ocypode cordimana, which rarely extends from its inland
habitat in to beach areas, (Taylor 1968, 1971). O. cordimana colouration varied considerably,
from a dark purple to a pale buff morph, as found by Grubb, (1971) on Aldabra. Ocypode
juveniles were undistinguishable, being without pigment or horns until maturity, and were
highly cryptic on the white sands. The low species diversity on the beaches is made up for by
the large ghost crab population sizes, observed across the Seychelles islands, (Taylor, 1971).
Colour plate 1
a) Ocypode cordimana, Purple morph, b) O. cordimana buff morph, c) Ocypode ceratophthalma,
d)juvenile ghost crab of unknown species, e) Geographus grayi, f) Coenobita rugosa. (a, d-f taken
by author/co-worker whilst b and c were taken by Chapman also on Bird Island:
http://planetchapman.net/main.php/v/2008/Seychelles/Bird/DSC_9697.JPG.html)
a)
b)
c)
d)
e)
f)
3.3 Choice of beach for sampling
Although only one beach could be effectively sampled, all beaches on the island were
observed. Large differences in morphology and crab distribution were noticed, table 1.
Beach A was chosen for the study, having many burrows to sample, few tourists, even width
and a sheltered location, figure 2. According to Feare, (1979), there is a cycle of erosion and at the
time study this would have meant beach A in particular had net deposition. However annually, net
erosion is occurring at a rapid rate at that side of the island, (Pers. Comm. with island staff).
Figure 2: Arial view of bird island and its beaches (appear blue in photo so are outlined in black for
clarity). Beach A was selected for the site of study (shown in yellow).
Table 1: Initial observations of crab activity on different beaches illustrated in Figure 1 .
A. 16m wide, large numbers of crab burrows of different sizes with even coverage across beach
although higher numbers on left side.
B. Too windy (burrows filled with sand). Very wide foreshore. Similar crab burrows patterns to A.
C. 10m wide (narrower to the East). Burrow numbers much lower, seemingly as wind stronger. Slope
also too steep for burrows to be permanent.
D. 8m wide, even steeper. Choked with dead trees suggesting recent erosion inland- made data
collection impossible. Interestingly many crabs, but only large individuals with few small burrows
evident.
E. Shallow, 11-14m wide. No crabs were seen and few burrows except at beach top, (mostly small to
medium sized). Few scattered pyramids.
F. Shallow, 18m wide. Large berm near top where slope was downwards towards vegetation. Long
line of pyramids just above swash limit and a few on berm. Many males were visable building,
although few juveniles were seen. Beach was good to sample however unfortunately too far from
site of stay.
G. 80m wide. Start of bird colony. No burrows for first 25m, only found near second berm away from
bird colony and towards foreshore. Few mounds just above swash limit. The further from the sea
was a clear pattern of increasing burrow size, but at very low densities.
H. Very flat 100-175m wide area until berm then sharp drop to sea. Flat devoid of burrows, either
due to bird colony or distance from sea.
I.
60m wide with few burrows. Mainly small with the occasional large. Bird colony on beach so
sampling impossible.
J.
15-20m wide (by this time high-tide). Considerable throwing patterns evident around burrows
and many large/small burrows.
3.4 Mapping burrows in a ‘point pattern’
Using photographs to map burrow positions (Heywood and Edwards, 1961; Henson, 1961)
was not feasible. Mapping burrows in a ‘point pattern’ gave the best opportunity for
detailed analysis of spatial distribution, (Diggle, 2003; Illian et al, 2008). The method enables
one to distinguish between the two ‘negative/positive’ forms of aggregated dispersion,
(Dale, 1999). Such data analyses form an entire subsector of statistics, used extensively in
phytosociological studies, (Dale, 1999, Kenneth and Looney, 1985).
Beach A was chosen for sampling. Similar studies have used a single large transect, (Lighter,
1977). However four replicate transits of beach A were thought necessary to compensate for
local beach variation.
3.5 Data collection for transects of Beach A
Transect width was chosen as 10m so that few nearest neighbours would be outside the
transect. Transects were plotted with a compass and strings perpendicular to the sea at
equal distances along the beach.. These began 1.5 m from the vegetation and continued
until the HWM was reached, figure 3. This was as very few burrows were dug in the
foreshore and these were all temporary.
Figure 3: Diagram of beach and the different zones. Four 10m wide transects placed at intervals of
205m across beach (approximately 680m in length) in the foreshore. Red box shows approximate
area when activity of crabs was measured.
The first transect was divided into and mapped in 1m2 quadrats and x,y coordinates for each
burrow were noted. The largest and smallest diameters of entrances were measured with
vernier callipers as an indirect measurement of resident crab carapace length; a strong,
linear relationship consistently found in Ocypode including in O. ceratophthalma, (Fellows,
1966; Lighter, 1977; Chan et al, 2006) and O. cordimana, (Moss and Mcphee, 2006).
Direction of burrow entrance, steepness, (Vertical, sloped or coiled) were also recorded.
Emergence holes, (Barrass, 1963) and abandoned burrows were precluded. Distinction
between burrows of O. cordimana and O. ceratophthalma, was difficult as expected (Seike
and Nara, 2008). Further notes such as presence of oval shaped mouth and pyramid were
recorded and beach characteristics including berms, vegetation and beach slope .
The first transect was fully mapped following but did not span the entire zone due to
mapping time constraints.
Figure 4: Diagram showing the method for transect 1 where contiguous quadrats were mapped. As
for all transects it was positioned in the foreshore of the beach 1.5m from vegetation and ending
at the upper limit of the swash. This was done three times. Nearest neighbours outside the
transect area were counted to negate the problem of delimited area of study (Dale 199; Sinclair
1985)
There is a general recognition that Ocypode burrows are inhabited by one individual, (Milne
and Milne, 1946; Wolcott, 1978; Quijón et al, 2001) so density was first estimated for each
zone (berm or flat) by counting the number of active burrows, a method used frequently in
ghost crab studies, (Barros, 2001; Lucrezi et al, 2008; Yong and Lim, 2009). Randomly
selecting a sample of burrows using numbered flags, (Pielou, 1969) was found to be
unfeasible. Therefore, using a calculator, random coordinates were chosen by finding an X
and a Y coordinate along the transect’s length and width. From this point the nearest
‘reference’ burrow was found and the distance between measured to calculate its
coordinates, figure 5, a method known as T-square sampling, (Waite, 2000). The distance
and compass direction to five nearest neighbours were measured, 5 being so that different
spatial scales could be measured, (Thompson, 1956; Dale, 1999
Using the distances and direction from the reference burrow, the coordinates of the
neighbouring burrows could be calculated using triangulation, figure 4. This method was
independently designed for this study however is in fact used similarly by botanists, (Dale,
1999) although accuracy has been found to be somewhat low, compared to the method
used to plot reference burrows, (Mosby, 1959). So that a better comparison could be made with
transect 1, this same method was applied to the mapped data (random selection of reference
burrows and their neighbours), although using a buffer zone of 1m. This set of data will be referred
to as transect 1a.
3.6 Mapping of Nearest Neighbours for transects 2,3 and 4
The three replicates transects were sampled using a ‘short-cut’ nearest neighbour technique
(Diggle, 2003) and mapped by triangulation, Figure 5.
Figure 5: Mapping of transects 2, 3 and 4. a) Position on beach, b) Up close detailed view.
B is the 10m string plotted across the transect at a randomly found height, in which a random
width, C, can be found. A is the nearest burrow to C, and P the distance between. The coordinates
(X, Y) for burrow A are thus (X)= C and (Y)= B +/-P. Trigonometry was used to calculate the
coordinates (x,y) for the five nearest neighbouring burrows such as E, using; A’s coordinates,
direction to neighbours, α (measured as the degrees off N), and the distance, Q, to A.
B
C
a)
b)
Figure 6: Calculation of coordinates, (x,y) for neighbouring burrows, (examples). The corresponding
orientation of the right angle triangle for each neighbour meant different equations were needed
(see table 2 for entire list) depending on the number of degrees of N.
A) y= Y-Q(COS(α+64))
x= X-Q(SIN(α+64))
B) y= Y+Q(SIN(α-26))
x= X-Q(COS (α-26))
C) y= Y+Q
x= X
D) y= Y+Q(COS (α-116))
x= X+Q(SIN(α-116))
Table 2: Trigonomic equations used to calculate neighbouring burrow coordinates, (x,y), where Y
and X are the coordinates for the reference burrow, and α is the direction to the neighbouring
burrow, measured in degrees off North (N).
Degree’s off N
to neighbouring
burrow
0°
0° - 25°
26° - 115°
116°
117° - 205°
206°
207° - 295°
296°
297° - 360°
y
X
Y-Q(SIN26)
Y-Q(COS(α+64)
Y+Q(SIN(α-26))
Y+Q
Y+Q(COS (α-116))
Y
Y-Q(SIN(α-206))
Y-Q
Y-Q(COS(α-296))
X-Q(COS(26))
X-Q(SIN(α+64))
X-Q(COS (α-26))
X
X+Q(SIN(α-116))
X+Q
X+Q(COS(α-206))
X
X-Q(SIN(α-296))
3.7 Activity data collection
A section of beach A, (red boxed area in figure 3), was patrolled every hour in a total of 9
slots throughout the day, from 9:15 to 12: 15 and 15:15 to 18:15. The number of adult O.
cordimana and O. ceratophthalma and undetermined juvenile ghost crabs were noted in
three areas: the backshore, the foreshore and the transition zone where vegetation grows in
a sandy area for approximately 20m inshore. The same walk was made through each area, in
the same time period. Throughout the study this area was being pruned of Casuarina
bushes, an activity that appeared to expose large amounts of detrital food for O.
ceratophthalma but not O. cordimana.
3.7 Observations of adult O. ceratophthalma
At night, adult (fully horned and pigmented) O. ceratophthalma came out of their burrows en
masse and moved into the transitory zone between the beach and inland area to forage .
Densities appeared particularly high, (5 crabs m-2), possibly due to Casuarina pruning activities
which appeared to unearth food for the crabs. Individuals, away from their burrows, could be
caught with fishing nets, and handedness and sex ratios examined, (growth and sex differences were
examined by Drabik-Hamshare, 2010). Capture occurred in the inland area where activity area was
measured, figure 3, and although not truly random, crabs were chased until caught so as to ensure
no bias towards slower crabs. No juvenile Ocypode were found in this area. Marking carapaces
allowed re-caught individuals to be identified; although the population was so open and re-caught
individuals too few for a mark, release, recapture analysis. Crabs sex, carapace and horn colours
were noted and horns, large and small chelipeds and carapace width and diameter were
measured using vernier callipers, figure 7, for around 12 individuals each night.
Figure 7: Example of how author measured crab chelae. Measurements of horns were from the top
of the eye to horn tip and carapace width was measured from the widest points and length was
measured from between the crabs eyes.
4. Analysis of Data
The analysis of data collected from burrows held three assumptions; that ‘a single burrow
represents the position of a single crab’, ‘burrow diameter represents the carapace length of
inhabitants’, and that ‘burrows represent the position of crabs active such that a sample of
the population frozen in space and time’. These are considered valid for reasons given in
previous sections.
Transects usually took two days to cover, and mapping (once formulae had been corrected)
revealed few errors suggesting new holes were seldom made or lost during the timescale of
the data collection. Many holes were partly narrowed in one dimension so that the widest
diameter was used to indicate size of carapace (Area=(Widest burrow diameter/ 2) 2 x π) as
used by Barrass, (1963).
4.1 Population distribution1 and density
The size distribution of burrows in each transect zone was plotted using burrow diameter.
Any individuals sampled twice were excluded. The numbers of juveniles and adults between
transects were analysed using a chi square test of independence. O. ceratopthalama and O.
cordimana generally reach sexual maturity is 33mm (Haley, 1973; Lighter, 1977), and 30mm,
(Jackson et al, 1991) in carapace length respectively. Therefore burrows less than 30mm
wide were considered to be dug by non-sexually mature ‘juveniles’ and those 30mm or
greater by mature ‘adults’. As in reality, burrow size is not accurate to the mm, ( Fellows,
1966; Lighter, 1977; Chan et al, 2006), these represent only approximate estimations.
1
The term ‘population’ is used in the results section, however it is important to remember this study examined
two populations collectively due to lack of discrimination between O. cordimana burrows and O.
ceratophthalma burrows and that this term is therefore somewhat redundant.
4.1.1 Population density
Density was estimated from the number of active burrows across the beach. Although
dispersion patterns were examined, no account was taken of environmental gradients across
beaches, such as in moisture, (Shuchman and Warburg, 1978), exposure, (Quijon et al, 2001)
and sand packing, (Hughes, 1966).
A kruskal-wallace test was used to compare the medians of densities recorded in specified
beach zones, found to be non-normally distributed with a kolmogorov-smirnov test,
(P<0.05). The number of ghost crabs on Beach A was then estimated using the average.
4.1.2 Crab activity
Chi squared test goodness of fit tests were used on analyses examining the frequencies of
crabs. Tests were conducted using a Yates correction and only carried out when expected
values were 5 or greater.
4.2 Dispersion pattern of Ocypode burrows
Evans, (1952) suggests that different tests for non-randomness can under certain
circumstances give conflicting results, such as those described by Grieg-Smith, (1983).
Therefore more than one test of non-randomness should always be used, with further tests
then carried out as needed, (Ludwig and Reynolds, 1988).
4.2.1 Frequency distributions among quadrats
One can compare the frequency distribution of burrows amongst quadrats to that expected
of a Poisson distribution, i.e. the null hypothesis, complete spatial randomness, (CSR). If
random, the mean to variance ratio should approximate one, (1 ≈ S2 / ). Significant
deviations suggesting non-randomness, (the alternate hypothesis), were tested by
calculating the index of dispersion ID, which is approximately distributed as
2.
The critical
limits used were, 0.95 and 0.05, and degrees of freedom, n-1, (Southwood and Henderson,
2000)
Where:
is the number of events at i= 0, 1, 2, 3, 4, 5 burrows 1m-2
f is frequency and n is the sample size
is the mean
SE is the standard error
The second method is to calculate expected frequencies according to a Poisson distribution
and use a Chi square goodness of fit to test for a significant deviation from observed (O)
distributions. A Poisson is particularly applicable if the density per quadrat is low as was
found in this study, (Kenneth and Looney, 1985).
The chi square value was calculated using:
Where expected frequencies (E) =
For transect 1, complete mapping allows such analyses to be carried out on contiguous
quadrats, (Dale, 1999). Such an analysis is very valuable to a study on a point pattern, (Dale,
1999; Diggle, 2005) and the area fully mapped in the other transects was calculated by
plotting circles centred on each primary burrow with the radius stretching as far as the fifth nearest
neighbour. Caution must be taken however as individuals rather than quadrats were selected
randomly and gaps are likely to be under-sampled if burrows were severely aggregated. Therefore
transect 1a was compared with 1 for differences which would suggest this method had a bias.
4.2.2 Nearest Neighbour Analysis
To analyse the degree of a spatial pattern at different scales, one can plot mean square
variance against quadrat (block) size to identify scales where aggregation or regularity is
occurring, (Kenneth and Kershaw, 1973; Dale, 1999). Only in transect 1 would such an
ANOVA be possible however. Ideally such analyses should be used on multiple samples to
see if the observed pattern was a chance occurrence, (Kenneth and Looney, 1985). Nearest
neighbour analyses form a useful alternative, commonly used in studies short of time and
manpower, (Clark and Evans, 1954; Dale, 1999) and are particularly useful to studies upon
burrowing animals, (Hairston, 1959). Whether consistencies with the above analyses occur
can also be examined, (Southwood and Henderson, 2000).
For this study Thompson’s statistic, was used, (Thompson, 1956), an extension of Clark and
Evans, (1954). This enables the dispersion pattern to be perceived over a larger area, as well
as more accurately, (Southwood and Henderson, 2000). This analyses was valid as the
density was measured directly for the entire transect and the requirements of “individuals in
a continuum”, (Pielou, 1969) was considered fulfilled. Due to small sample sizes, (>30), the
correction factor suggested by Clark and Evans, (1954) was used, (n-1).
4.3 The relationship between crab size and distance to neighbours
Space can be viewed as a resource and if limited become the object of competitive
interactions, (Lighter, 1977). A regular dispersion would indicate that Ocypode individuals
are maintaining a distance between each other, due to negative interactions such as
territorial behaviour. If the distance between borrows depends on the size of the burrows
in question, this might indicate …….a despotic ideal distribution REFERNCE?
4.3.1 Size of burrow effect on nearest neighbour
For each transect, regressions were conducted on size of burrow and distance to nearest
neighbours. A kolmogorov-smirnov test found the data to be non-normal, (P<0.05). From a
variety of transformations, a log transformation yielded the smallest K-S value and the
largest probability that data does not deviate from a normal distribution. This result was
therefore used. However, given that two transects had skewed as opposed to normal
distributions, results should be treated with caution.
To take into account the impact of neighbour’s size, a second regression was done on
average size of a burrow and nearest neighbour and the distance between them. Although
again the residuals of two transects were non-normally distributed, according to a K-S test,
(P>0.05), transformations did not improve normality and were subsequently not used. Both
regressions were also conducted on the collated data from transects 2, 3 and 4 as these had
similar densities and dispersions.
Finally the mean distance to the first three nearest neighbours was found for juveniles and
adults and compared using a student’s t-test to see if adults have statistically greater
individual spacing even if nearest neighbour distances did not increase linearly with size of
crab.
4.4 Dispersion of different juveniles and adults
A final analysis was also conducted separately on the dispersion of small, (>2.0cm), medium,
(2.0 to 3.99cm) and large individuals, (3.0 to 3.99cm). This was achieved using the mapped
transect and comparing their distribution amongst quadrats to a Poisson distribution using a
goodness of fit. Data from all transects were also combined in a test for ‘segregation’,
(Pielou, 1961), between juveniles and adults. The procedure used is usually one to test for
association between two species and in this case compared the observed frequencies of
nearest neighbor pairings of juveniles and adults to the expected when the null hypothesis
of independence is assumed, (Waite, 2000), in a typical chi square contingency table.
Pielou’s coefficient of segregation was also found (S), where 1 indicates complete
segregation between juveniles and adults, 0 un-segregated and -1 that pairings between
species/types occur in isolated pairs (full association):
5. Results
5.1 Beach morphology
The beach was generally divided into zones of berms which are deposits of beach material
forming the active shoreline at high tide, or by storms. These are separated by flats, although
there were differences in the position and width of these, as well as differences in height
from sea level, Figure 5.1. The high tide mark was indicated by the third berm, however the
HTSM, and thus the upper intertidal zone was unknown, but likely to be indicated by the
second berm, (Short, 1996). The width of the foreshore varied with the tides and along the
beach, between 4 and 10m, and the backshore varied between 35.5 and 38.5m.
Figure 5.1: Cross sections, (a), and aerial view of transects, (b), showing slopes and berms.
Transects 2, 3 and 4 were measured up to the HWM (the highest point of the swash zone or
foreshore, as shown by a steeper slope and blue).
a)
Transect 1
did not
reach the
HWM
HWM and
maximum
limit of
swash
Transect 4
Transect 3
Transect 2
Transect 1
b)
HWM
Transect 1
HWM
Transect 2
HWM
Transect 3
HWM
Transect 4
Distance from top of transects/beach (m)
5.2 Population Distribution
The nearest neighbor method recorded around half of the total transect areas and burrows,
suggesting results should amply represent the total transect, table 5.1. The population
distribution of burrows between transects are not identical but do show similarities, (figure
5.2). Although transect 1 lacks data for crabs near the H.W.M., which according to a linear
relationship found by Drabik-Hamshare, (2010) is likely to have mostly juveniles, a bimodal
shape is apparent in each transect. A large peak is observed in the number of crabs
approximating 1-2cm in carapace length, (which tended to be unpigmented) and a smaller,
less consistent peak in mature crabs approximating 3-5cm. The proportion of non-sexually
mature crabs and sexually mature crabs, table 5.1, was significantly different between
transects, (X2cal= 21.31 > X2tab=16.27, P=0.001, d.f= 3, Chi square test of independence).
Table 5.1 Percentage of transects sampled and juvenile/ adult ratio
% of
burrows
sampled
%
transects
mapped
Juvenile/
adult
ratio
Transect 1
100%
100%
2.8
Transect 2
47%
55%
1.3
Transect 3
59%
46%
1.6
Transect 4
53%
56%
4.0
Figure 5.2 Relative population distributions of Ocypode crabs (O. ceratopthalama and O.
cordimana combined) determined indirectly from maximum burrow diameters
Frequency (%)
Maximum burrow diameter (cm)
5.3 Density of burrows
Overall, density appears to increase along the beach, figure 5.3. This is very slight for the first
three transects, from 0.60 to 0.67 crab m-2, but sharply increases at transect 1 at 0.95 crab
m-2, table 5.2. Although density was not measured here up until the HWM, higher densities
were consistent along the first 20m that were measured. The use of differing methods
necessitates caution when comparing densities, however despite perhaps more detailed
burrow counts in the first transect, (counts per quadrat rather than counts per zone), higher
densities did seem apparent in the field on transect 1 and burrows were easy enough to spot
in both methods.
Table 5.2 Active burrow (crab ) density of both Ocypode ceratophthalma and Ocypode cordimana
on four transects
Transect
No. burrows
Density
area
(number
of burrows
Inside transect Outside transect*
(m2)
per m2)
Transect 1
200
190
0
0.950
Transect 2
385
91
33
0.665
Transect 3
355
113
21
0.647
Transect 4
360
101
33
0.597
* Neighbours found outside transect
Figure 5.3 Changes in density of Ocypode burrows along the approximately 680m beach
5.4 Population estimate
The use of burrows has been shown to be an accurate estimation of density and population
of Ocypode crabs, (Warren, 1990; Barrass, 1963; Buffer and Bird 2002; Barros, 2001). If the
average backshore width is used to estimate the area covered by the population, (24820m2),
then the estimated population size is 17,740 Ocypode individuals based on the average
density of 0.7418 burrows m-2. However the standard error is high, (±3946 crabs, P=0.05)
and unlikely to be a good estimate, given the change in density along the beach. Overall it is
clear however that on beach A alone, the ghost crabs numbered in their thousands, although
it is unclear if one species predominated. A daily mark capture release on foraging adult O.
ceratophthalma failed to provide a estimate of their numbers due to high immigration and
emigration rates and of a total of 80 caught, only 7 were re-catches, (see Drabik-Hamshare,
2010).
5.5 Zonal distribution
Densities changes were therefore examined in the mapped transect, (1) figure 5.4. No clear
increase or decrease along the beaches cross-section is observed, although density was
highest either side of the upper berm.
The density in different zones (berms and flats) also did not consistently increase or
decrease towards the sea, with different patterns seen between transects. The variation
within zones was also larger than that between zones, figure 5.5. A kruskal-wallis test
accordingly found no significance between the median densities of each zone, (X2cal= 2.78<
X2tab= 9.49, P=0.05), although as zones were at different heights and distance from the sea
their affect on abundance may have been confounded.
Figure 5.4 Density of Ocypode crabs per m2 across beach cross section to illustrate zonal
distribution
H.W.M.
Figure 5.5 Differences in median density of Ocypode burrows between beach zones and
interquartile range
5.6 Burrows types
Burrow type did not appear to be associated with a given distance from the sea as expected,
figure 5.6, with large variation found between transects. Vertical burrows were the only type
found at <22m from the H.W.M. 9% of burrows were coiled, but only 80% of these
appeared to be dug by mature crabs (>30mm burrow diameter). The ratio of right or left
turning coiled burrows did not differ significantly from 1:1, (X2=0.681, <0.05, chi square
goodness of fit), similarly to the 1:1 ratio found in caught individuals, (See Drabik-Hamshare,
2010). Sloping burrows were dug both by juveniles (60%) and mature crabs, (40%) whilst
vertical burrows were mostly dug by small juveniles, (84% >2cm in width).
Figure 5.6 Frequencies of burrow types at different distances from the H.T.M expressed as
fractions of total with standard deviation showing large variation between transects
Frequency of burrows
(expressed as fraction of total)
Coiled burrows
Sloped burrows
Vertical burrows
Distance from H.T.M
5.7 Crab activity
Overall twice as many mature O. ceratophthalma than O. cordimana were observed active
during the day, (see table 5.3 for significant daily differences), with unidentified juveniles
always seen in low numbers, mostly due to their crypsis and tendency to remain near
burrows. Juveniles appeared to be more diurnal than adults, peaking around 3:00. Mature
Ocypode were mostly nocturnal; O. cordimana activity peaked later than mature O.
ceratophthalma, around dusk, and at night was seen in large numbers particularly inland. O.
ceratophthalma activity gradually peaked earlier from 16:15 to 14:15 in both the beach and
the transition where their burrows did not reach, possibly relating to the shifting high-tide,
(see appendix 7.7 for daily totals and tidal changes). Very few juveniles or mature O.
cordimana were observed in the transition zone, figures 5.8 and 5.9, although many O.
cordimana were observed there later at night. In O. ceratophthalma however, no significant
difference in numbers were observed between the two zones2, figure 5.8, and the overall
difference between O. cordimana and O. ceratophthalma was not significant on the beach,
figure 5.7, (X2= 0.0105, P<0.05, chi square goodness of fit). Daily variations appeared to be
affected by weather as cloudy days coincided with peak activity. The 3 rd day was a full moon;
and unusually low numbers of mature Ocypode in the evening occurred although this was
likely a factor of heavy rain.
Table 5.3 Total recorded crabs each day and significant difference between using Chi
squared test for goodness of fit
Day 1
2
Day 2 Day 3
Day 4
Day 5
Day 6
Day 7
Chi square goodness of fit only possible on data collected 4:15 onwards due to low expected frequencies, (>5),
however at times tested, P<0.05, (see appendix, 7.10).
Total
Observed O. ceratophthalma
81
O. cordimana
46
Expected
same for both
63.5
species (1:1 ratio)
X2
Chi Square
9.65**
126
112
119
42
23
32.5
132
39
85.5
128
50
89.0
206
76
141.0
144
52
98
0.83
5.57* 50.58*** 34.19*** 59.93*** 43.18***
*P=0.05, **P=0.01, ***P=0.001
Figure 5.7 Average activity recorded during the day for O. ceratophthalma and O.
cordimana on the beach
Figure 5.8 Average activity recorded during the day for O. ceratophthalma and O.
cordimana in the transition zone
Figure 5.9 Average activity recorded during the day for juvenile Ocypode in both the beach
and transition zone
859
398
628.5
-
5.8 Dispersion pattern: using frequencies in quadrats
Transects maps are shown in colour plate 5.10. Maps f) and g) show the quadrats fully
mapped using the nearest neighbor method. The densities calculated from these quadrat
samples were found to be very different to the actual densities measured, Table 5.4. This is
the opposite to the expected inflation of densities under the identified potential bias of
quadrats selection. As a different result is concluded when the mapped data from transect 1
is sampled and mapped quadrats found under the nearest neighbor method, (transect 1a);
that the quadrat data is indeed biased, even if not in the way expected seems likely. This
combined with the slight error in plotting coordinates by triangulation of angles, (Mosby,
1969), means that only transect 1 where contiguous quadrats were mapped is considered
reliable although the others are nonetheless indicated below. Although the use of nearest
neighbour data mapping by triangulation to extract quadrat frequency data potentially
forms a way of quickly collecting data for a usually laborious point pattern analysis, it seems
that due to non random quadrat selection, this method may only prove useful in allowing a
map of samples to be constructed, 5.10 a) to e).
Similar to Pielou’s (1959) hypothetical dispersion pattern, the measured dispersion patterns
differ between the two dispersion methods, demonstrating the need for a nearest neighbor
analysis of dispersion. Even so transect 1 which is considered reliable is indicated by the
index of dispersion, table 5.5, and the Poisson goodness of fit, table 5.6, (in bold), to be
significantly regular, (P=0.05 and P<0.005 respectively), and the deviation from CSR is
demonstrated seen in figure 5.10a.
Table 5.4 Number of both Ocypode ceratophthalma and Ocypode cordimana burrows in quadrats
Transect
1
1a
2
3
4
No. of
burrows (in
quadrat
sample)
190
55
78
65
88
Number of fully mapped
1m2 quadrats
Inside
Outside
transect
transect
200
0
47
1
159
50
139
18
179
53
sample density
(burrows 1m -2)
1.146
0.373
0.414
0.379
Actual
density
(burrows
1m -2)
0.950
0.950
0.665
0.647
0.597
Table 5.5 Index of dispersion for Ocypode burrows across four transects.
Transe
ct
Variance
(S2)
Mean
( )
1
1.143
0.95
Varianc
e to
mean
ratio
(S2 / )
1.203
Index of Degree
dispersio s of
n (ID)
freedo
m (n1)*
239.47
189
tabulate
tabulate
d 2
when
p= 0.05
d 2
when
p= 0.95
232.91
167.36
Regular
1a
1.000
1.146
0.872
41
47
64.00
32.27
Random
2
0.422
0.277
1.523
284.80
208
219.91
156.37
Regular
3
0.334
0.411
0.812
141.34
156
205.78
144.49
Clumped
4
0.425
0.361
1.178
273.19
231
268.53
197.74
Regular
* Note, n is the total no. of samples (quadrats) whether inside or outside desingated quadrat areas.
Distributi
on
Table 5.6 Goodness of fit test for observed burrow distribution among quadrats compared to that
expected under a Poisson distribution.
Transect 1
Transect 1a
Transect 2
Transect 3
Transect 4
Degrees of
freedom
(categories)
2
1
1
1
1
No. of
quadrats
200
55
78
65
88
Calculated
10.5690
1.41642
0.40928
1.20023
0.890838
2
Probability
level (p)
0.005
0.234
0.522
0.273
0.345
Distribution
Regular
Random
Random
Random
Random
Figure 5. 10 Frequency distribution of burrows in 1m2 quadrats (dark gray bars) compared to
expected following a Poisson (or random) distribution, (light gray line).
Frequency of quadrats
a) Transect 1
c) Transect 3
b) Transect 2
d) Transect 4
4
Number of burrows in 1m2 quadrats
5.9 Dispersion of Ocypode burrows: using nearest neighbour distances
Maps of the nearest neighbor clusters are shown in 5.10, a) to e). The nearest neighbor
analysis, shows that dispersion did differ at different scales. At a fine scale, of the first
nearest neighbor distance (average 82cm), the burrows were significantly regular, in
agreement with the results above (transect 1), suggesting the quadrat size chosen was the
correct size for the density of burrows encountered, (Dale, 1999). At a greater scale, of the
second nearest neighbor, on average 119cm away, and of the third nearest neighbor, on
average 147cm away, the burrows tend towards a significantly clumped distribution, table
5.7.
Table 5.7: Distribution based on extension of Clark and Evans, (1954) nearest neighbour analysis,
(Thompson, 1956)
Transect
1a
Transect
2
Transect
3
Transect
4
Scale of 1st nearest neighbour Scale of 2nd nearest neighbour
Scale of 3rd nearest neighbour
R1
Critical Distribution
R2
Critical Distribution
R3
Critical
Distribution
limits
limits
limits
1.235 >1.209,
Regular
1.167 >1.145,
Regular
1.115 <1.118
Random
P=0.05
P=0.05
>0.882,
p=0.05
1.254 >1.224,
Regular
0.800 <0.845,
Clumped
0.794 <0.845,
Clumped
P=0.05
P=0.05
P=0.01
1.333 >1.275,
Regular
0.816 <0.848,
Clumped
0.821 <0.845,
Clumped
P=0.05
P=0.05
P=0.01
1.363 >1.275,
Regular
0.749 <0.749,
Clumped
0.749 <0.845,
Clumped
P=0.05
P=0.01
P=0.01
5.10 Dispersion pattern of different size categories and mature males
Although burrows collectively were dispersed regularly, when separate size categories were
examined, the distribution of small burrows in quadrats (>2.0cm) did not significantly deviate from
expected assuming CSR, whilst burrows 3.0cm or larger did significantly deviate from CSR with more
burrows clumping than expected, table 5.8 and figure 5.10b. The observed frequencies of nearest
neighbour pairings between juveniles and adults were fewer than the expected frequencies under
the null hypothesis of independence, table 5.9, suggesting segregation between juveniles and adults,
(S=0.722). However this difference was not significant, (X2cal= 0.722 < X2tab = 3.84, P=0.05) and
therefore the null hypothesis was accepted.
Table 5.8 Dispersion pattern of burrows in three size categories using Poisson goodness of fit test
Degrees of
No. of
Calculated X2
Probability level Dispersion
freedom
burrows
small
1
106
1.819
0.177
Random
(>2.0cm)
medium (2.0
1
37
4.0382
0.044
Clumped
to 3.99 cm)
large (>4.00
1
47
8.1194
0.004
Clumped
cm)
Table 5.9 Goodness of fit test for segregation between juveniles and adults (S= Pielou’s coefficient
of segregation).
A=
observed Expected
chi square
relative freq assuming A and B
juveniles
occur independently of each
B=
other
adults
AA
41
38.54
0.0555
A
0.602151
AB
15
17.46
0.6865
B
0.397849
BA
23
25.46
0.4708
A'
0.688172
BB
14
11.54
0.1854
B'
0.311828
Total
93
93
1.3982
S=
0.722
Unfortunately only 2-3 of the burrows sampled in trasnsects 2-4 and 13 out of 190 burrows on
transect 1 were coiled. Therefore a separate analysis of the dispersion of these was not possible,
despite the potentially clumped distribition of half the burrows, occurring in one area near the
H.W.M on transect 1, figure 5.10. However as some coiled burrows, (section 5.6), and many oval
shaped burrows were dug by crabs smaller than 30mm, using these featrues to identify ‘copulation’
burrows does not seem reliable. Evidence of a nearby pyramid is likely to be the best indicator of
sexually mature males, however identifying the associated burrow is difficult due to the distance
between. Nearby burrows to pyramids are nonetheless shown in green, figure 5.10.
Figure 5.10 Position of coiled burrows (in red) amongst the rest of the population, (blue)
5.13 Impact of burrow and therefore crab size upon distance to nearest neighbours
Despite the finding that crabs were digging burrows in a regular distribution, suggesting negative
interactions, crab size, (measured as burrow diameter) did not appear to have an effect on individual
spacing, table 5.9. This lack of correlation was clear, figure 1.10, and despite the invalidation of
normality in transects 1 and 4, it is believed this result is reliable considering the clear lack of
correlation depicted in plotted data, figure 1.10.
Table 5.9 Effect of log burrow size on log average distance to three nearest neighbours
Transect
1
Transect
2
Transect
3
Transect
4
Combined
Regression results
Normality
plot
**F1,22= 0.45,
P=0.510
*F1,19= 0.12,
P=0.736
*F1,22= 0.00,
P=0.944
**F1,22= 0.84,
P=0.368
*F1,91=0.17, P=0.685
skewed
K-S test results
log burrow
log av. Distance to
diameter
NN
P>0.150
P>0.150
normal
P<0.010
P>0.150
normal
P<0.010
P>0.150
skewed
P<0.030
P>0.150
normal
p<0.010
P>0.150
Figure 5.10 Example of lack of correlation between log burrow size and log average distance to
nearest neighbours, (see appendix for other transects)
Even if the size of the nearest neighbour was taken into account, no effect of size of burrows was
found upon distance between them, table 5.9. Although two transects showed higher F values,
(transects 3 and 4), they each suggested opposing correlations, figure 5.12. Lastly, when burrows
were divided into immature and mature sizes, no difference in the mean distance to the first three
nearest neighbours were found, figure 5.11. The mean nearest neighbour distances of juveniles and
adults are shown in figure 5.12. No statistical difference was found, (T= -0.390, P=0.70, d.f=91).
Table 5.10 effect of combined burrow size of two neighbouring burrows on the distance between
them
Regression results
Assumptions
of normality
met
K-S test results
log average burrow
Distance to NN
diameters
Transect 1 F1,22= 0.48, P=0.494
normal
P=0.444
Transect 2 F1,19= 0.06, P=0.811
skewed
P<0.010
Transect 3 F1,22= 3.31, P=0.082
normal
P<0.010
Transect 4 F1,22= 4.45, P=0.075
normal
P<0.030
Combined* F1,91=0.01, P=0.918
skewed
P=>0.150
*only transects 2,3 and 4 with similar densities and dispersion patterns
P>0.150
P=0.047
P<0.010
P=0.132
P<0.010
Figure 5.11 Interval Plots of mean distance to nearest neighbour for burrows dug my mature and
immature crabs (burrows >30mm and 30mm or greater, respectively).
95% CI for the Mean
Mean distance to nearest neighbour, (cm)
140
130
120
110
100
immature
mature
Figure 5.11 Combined burrow size of nearest neighbours against the distance between them.