Why do Tanganyikan gastropods have patchy distributions? Abundance, Diversity,
and Species Distribution in relation to Abiotic and Biotic Parameters
Students: Meredith Barrett, Alexis Bashonga Bishobibiri , Joseph Catron
Mentor: Dr. Ellinor Michel
Introduction
Lake Tanganyika is recognized as a biodiversity “hotspot” due to the large abundance and diversity of
organismsAmong the ostracods, cichilid fish, and thiarid gastropods, the majority of these species are
endemic (Coulter, 1991). The lake is geologically old, tectonically active and the shoreline has diverse
substrate types, which may have contributed to the distribution and evolutionof the endemic species. Our
study aimed to test whether the gastropod assemblages, which are known to be patchy in the rocky littoral
zone, correlate with physical variables.
The deepest of Tanganyika’s three basins reaches 1500 meters making it the second deepest lake in the
world. The tropical location minimizes seasonality and allows the waters to remain stratified, resulting in
anoxic waters below about 100m depth. The benthic fauna, such as the rock-dwelling gastropods we focus
on in this study, are limited to a narrow littoral zone, and cannot disperse across the open lake. Abiotic and
biotic factors such as substrate, shoreline lithology, and productivity may affect distribution patterns. Some
species display unpredictable patchy distribution patterns, which exhibit irregular and inconsistent patterns
of residence in various habitats. The understanding of these patterns, especially those of allopatry and
sympatry, illuminates essential evolutionary processes and interactions that have occurred throughout the
history of the lake.
Past studies indicated that many species of Lavigeria, which comprise a species flock that dominates the
rocky benthos in diversity and abundance, have highly patchy distributions, This is particularly evident in
the Kigoma region (Michel et al., 2003), where the shoreline has been relatively well sampled. However,
previous research on distributions provided qualitative data but was quantitative at a limited number of sites
(e.g. France & McIntyre, 1998; Sekandende, 2000; Zorich, 1999). This study examined patchiness with
more complete spatial distribution data at regular intervals on the rocky substrates along the coast from the
southern limit at Mzungu Beach (7 km south of Kigoma) to Mwamgongo, north of Gombe Stream National
Park (30 km from Kigoma). We collected qualitative data of gastropod species presence and absence, as
well as quantitative data on abundance. This provided site species richness and biomass information. In
addition, we assessed several other physical parameters such as algal biomass, site lithology, substrate
composition, site aspect, and site bathymetry to test for possible correlates with the observed patchiness.
The null hypothesis states that these parameters do not correlate with gastropod distributions. Previous
studies suggest that the patchy distribution may indicate metapopulation dynamics among the endemic
gastropods (Michel, personal communication, 2003; Powers, 2000). Ultimately, these data could lead to a
greater understanding of the evolutionary processes that generated the endemic diversity within the
Lavigeria species flock.
Objectives
Our objectives were too expand spatial distribution data on presence/absence, abundance, and diversity of
gastropods in the Kigoma region of Lake Tanganyika, and to determine the effects of several physical
parameters at sites along the shore of the Kigoma region on species abundance, diversity, distribution, and
biomass of littoral gastropods.
Methods
Survey Site Selection
We sampled twelve sites in the Kigoma region. We recorded the GPS latitude longitude coordinates at
each site; from south to north these were Mzungu Beach (S 04 55.044’ E 029 35.855’), Jakobsen’s Beach
(not recorded) , Pundamilia (no recorded) , Katabe North (S 04 53.397’ E 029 36.725’), Hilltop (S 04
53.241’ E 029 36.842’), Luansa (S 04 52.522, E 029 37.217’) , Kalalangabo 3 (S 04 55.044’ E 029
36.255’), Kalalangabo 1 (S 04 50.289’ E 029 36.635’), Lemba (S 04 49.635’ E 029 36.255’), Katongwe (S
04 49.635 E 029 36.083), Mitumba (S04 37.977’ E 029 37.898’) , and Mwangango (S 04 36.960’ E 029
38.290’) (Fig. 1). We selected survey sites based on shoreline aspect and substrate type at each location.
Shoreline aspects of southwest, northeast, and west-facing sites were chosen between Mzungu Beach and
the northern limit of Gombe Stream National Park by examining a map of the lake shoreline. The study
attempted to sample equidistant sites along the shoreline and to search for noticeable areas of transition.
Natural boundaries delimit this biogeographic region by the Luiche River in the south and the long sandy
shoreline extending past Ujiji to the Malagarasi and the sandy littoral regions north of Mwammgongo
(north of Gombe Stream National Park). This region will be referred to as the Kigoma region. We ensured
that we collected data for sites that replicated the variables in question. For example, for shoreline aspect
we made sure we had several sites that were southwest-facing, as strong wave action from the direction of
the prevailing winds and potential upwelling are likely physical variables that affect gastropod distribution.
Moreover, the sites at Mzungu Beach, Kalalangabo 1, Jakobsen’s Beach, and Mwangongo provided
quantifiable replication of previous data collected by McIntyre and France (1998).
After this selection, the study conducted a survey of the area by snorkel to determine location of suitable
gastropod habitat. The study focused on “hard and stable” substrates at five meters due to previous findings
that species density and diversity measure highest on cobbles and boulders at this depth (Powers, 2000,
Michel 1994). Site endpoints were established based on aspect and substrate changes, and GPS points were
recorded for future reference.
Physical Variables
We recorded the following qualitative observations at each site: shoreline lithology, wind and wave
direction, shoreline aspect, and benthic characteristics. Shoreline lithology was characterized by the type of
dominant rock outcroppings and grain size (boulder/ cobble/ pebble). Compass bearings perpendicular to
the shoreline determined aspect as a degree west or east of North. Nathan Stewart snorkel-surveyed every
site and recorded qualitative depth and substrate transitions. He compiled this data into complete shoreline
and benthic maps (Appendix A). The study also conducted a depth survey from the shore extending
outward along a transect line of 50 meters. Depth readings given by a hand-held depth sounder provided
depth profile data at 10-meter increments from shore. Three readings were recorded, averaged, and later
translated into a graphic depth profile representation.
Species Presence/Absence
We determined the presence of gastropod species with linear depth “sweeps” or searches from 20 meters to
1 meter, and horizontal sweeps at 5 meters by SCUBA. Search time was standardized, with approximately
45 minutes spent on each sweep. Each pair of divers attempted to collect representatives every gastropod
species present along the transects by searching in all substrate types, including on the sides of boulders
and cobbles, underneath cobbles, and within sand and fine sediments. Observational notes of substrate
transitions from 20 meters to 1-meter depths were recorded, as well as species’ habitat and depth
preferences. The sampled representatives from these thorough searches were identified later with the aid of
hand lenses, binocular microscopes and literature (Brown, 1996; West et al. 2003; Michel et al. 2003;
Michel & Todd in prep.). For species that have been identified as new, or requiring further taxonomic
work, we have used the open nomenclature of Michel & Todd, in prep and West et al., 2003. .
Gastropod Abundance
We quantified gastropod abundance by collecting all individuals within each of five 1m2 quadrats at each
site. Quadrats were constructed of rope and sand bags. Each quadrat maintained a consistent 1m2 area
because of careful, taut placement over rocky substrate. Replicated quadrats were placed at 5 meters depth,
approximately 10 meters apart. All gastropods visible with careful searching from above were collected and
preserved for later counting and measuring. This search did not permit overturning of rocks or sifting of
sand, as this would create variable sampled areas. Each sampler drew a quadrat sketch and recorded
substrate percent composition. The percentage of fine sediments, sand, pebbles, cobbles, and boulders was
estimated for each quadrat based on the Wentworth Scale (Table 1). These estimates were later combined
to achieve an average substrate composition description for each site.
Gastropod Sizes, Predation and Site Biomass
In the laboratory, each snail from the quadrat samples was identified, measured for height, width, and shell
lip thickness in millimeters (Socci, 2001). We also recorded the number of shell scars on each individual as
an indication of survived predation attempts. All shells from the quadrats and sweeps were cleaned by
maceration and dried for museum collections.
Total gastropod biomass was calculated for each site in grams/meter2. Average biomass/shell was
calculated from the formula 0.2208e0.996 x shell height (mm) (from Socci 2001) and the values were
summed by number of individuals/site. Algal Biomass, Total Organic Sediments, and Total Inorganic
Sediments
The study measured chlorophyll a concentrations for a comparative algal biomass proxy, as well as the total
organic and inorganic sediments from cobbles near the quadrat sites. Collectors chose cobbles of
approximately 50 to 100 millimeters with flat, upward-facing surfaces. A standard Nalgene sample bottle
cap was placed over cobbles at 5 meters depth by snorkeling. Cobbles were then carefully carried to the
surface by the snorkeler, who held the cap tightly to the cobble. At the surface, cobbles were scrubbed
around the bottle cap using wire brushes and rinsed to remove both attached and non-attached substances
from the cobble. The collector then removed the cap and rinsed non-attached algae and sediments into a
collection tub using lake water. The sample was then brought to a known volume in a graduated cylinder.
The remaining attached substances on the rock were scrubbed with wire brushes and collected by rinsing
the algae from the rock into a graduated cylinder. The sample was then brought to a known volume. The
study conducted 8 replicated samples at each site. Collected samples were kept cool and shaded in an
icebox. In the laboratory the samples were filtered on 47 millimeter Pall-Gelman filters for chlorophyll a
and ash-free dry mass analysis. Chlorophyll a was extracted over twenty-four hours in 90% ethanol. Ashfree dry mass was determined by combustion of the filter for 3 hours at 550 degrees Celsius. The
combined ash weight from non-attached and attached sediments was calculated to determine the total
organic and inorganic sediments. Collectors ran all samples through the spectrometer at wavelengths of 665
and 750 nanometers, and acidified the samples using 0.1 M HCl. The total chlorophyll a concentration was
calculated by combining the spectrometer values of the non-attached and attached samples.
Results
Species Presence/Absence
At the twelve sampled sites we catalogued a total of twenty species, nine of which were Lavigeria (Table
2). Most Lavigeria species and other gastropods exhibited a patchy distribution. L. cornata, L. sp AA, L. sp
m, L. sp A*, and Reymondia minor were found at only a few sites, while Vinundu westae, Lavigeria
grandis, Lavigeria nassa and Reymondia horei were widespread (Table 2). Based on qualitative data
collection, the relative abundance and habitat preference of each species was determined (Table 2). In
support of Powers (2000), most species did prefer hard substrates such as boulder and cobble. Only L. sp
A*, L sp. AA, Anceya giraudi, Paramelania damoni, Paramelania“minor”, and Syrnolopsis were found on
sand or fine sediment.
N.B. – later analyses of our designated “L. sp A” suggests that it is actually a population of small L. sp. W
or a new species, but not L. sp. A as in Michel et al. 2003. We will refer to it here as L. sp. A* to be
concordant with our field notes and keep its identity clear.
Gastropod Abundance
Abundance varied significantly between sites in the Kigoma region (Figure 2). Hilltop exhibited the
greatest snail abundance with almost double the number of individuals of the next most abundant site,
Kalalangabo 1. Luansa had the third greatest number of individual snails (Figure 2). When the data was
subset to include only Lavigeria species Katabe North was the site of greatest abundance, followed by
Hilltop and Kalalangabo 1 (Figures 3 & 4). Lavigeria species abundances alone is are shown in Figure 4,
and can be interpreted by subclade membership. The Lavigeria nassa/grandis/coronata clade has at least
one, sometimes all three, representatives at every site in the region. The Lavigeria sp W/J/A* clade
occurred more patchily, appearing at scattered sites throughout the region. The Lavigeria sp L/AA/M clade
occurred narrowly only in the central section of the region (although other representatives are known from
around the lake, Michel et al.2003).
Gastropod Diversity
From the quadrat sampling, gastropod abundances and species richness data was compiled for each site.
We used Simpson’s D and Fisher’s Alpha diversity indices to compare diversity between sites in the
Kigoma region (Figure 5). Both of these indices incorporate abundance, thus producing a measure of
evenness, but they compensate for sample sizes differently. Both indices show large differences in
diversity among sites (Figure 5). Simpson’s D indicated Kalalangabo 3 to be most diverse, with Jakobsen’s
and Kalalangabo 1 to be the second and third most diverse, respectively (Table 4). According to this index,
the highest diversity occurs in the central section of the Kigoma region (north of Kigoma Bay). Mzungu
Beach, Luansa, and Pundamilia were the least diverse sites. Eveness did not vary highly between sites
(Table 4).
According to Fisher’s Alpha, Jakobsen’s Beach is the most diverse site, followed closely by Kalangabo 3
(Table 4). The least diverse sites include Luansa, Mzungu Beach, and Mwangongo. According to this
metric diversity varies between sites – there is no clinal increase, rather sites contrast in their diversity.
Furthermore, the two indices provide different assessments of diversity. Between Pundamilia and Hilltop
diversity increases according to the Simpson’s D and decreases according to Fisher’s Alpha. Both drop in
diversity between Kalalangabo 3 and 1. Diversity increases slightly between Lemba and Katangwe with the
Simpson’s D, yet decreases with the Fisher’s Alpha. In general, Fisher’s Alpha diversity decreases from
Lemba to Mwangongo. With one small increase at Katangwe, the Simpson’s D also follows a similarly
decreasing trend in diversity after Kalalangabo 3 when heading north in the Kigoma region.
Gastropod Biomass
The trend in total gastropod biomass at each site paralleled the abundance and diversity patterns, with clear
differences between sites (Figure 6). Sites with the greatest abundance also showed the highest biomass.
Physical Parameters: Shoreline Aspect, Substrate Composition, Algae, and Sediment At all twelve sites, we
collected data on physical variables (Table 3). Spatial data included underwater site maps and depth profile
plots. The shoreline aspect, shoreline lithology, and average percent substrate composition at each site were
compared with species richness, density, and diversity indices (Table 3). Each site is ranked relative to each
other in terms of these quantitative values (Table 4). All sites exhibited one of the following shoreline
lithologies: Kigoma quartzite, conglomerate, or Manyovu Red Beds, all of which are characteristic rock
formations in the Kigoma region. Beach rock was also found at numerous sites. The grain size did differ
between sites from monolithic rock cliffs to boulders or cobbles.
Shoreline Aspect
We determined compass bearing perpendicular to the major trend of the shoreline, providing facing
direction for each site. For this analysis we simplified these data into general directions (southwest,
northeast, west) to test for trends. These aspect groupings were compared with both abundance and
diversity indices to determine if aspect was correlated with significant variation between sites (Figures 7
and 8). The total abundance of individuals was also calculated cumulatively for the sites in each aspect
grouping. Dividing by the number of sites per aspect produced the average value. There were three
northeast-facing sites, five southwest-facing sites, and four west-facing sites. When compared, the
northeast-facing sites exhibit a greater abundance. However, the p-value test of 0.341095 indicated no
significant difference between sites in terms of abundance. Each aspect grouping was also compared using
the Simpson’s D and Fisher’s Alpha. Both indices exhibited higher values for the northeast-facing sites.
These differences were shown to be significant in their p-value of 0.04795 and a strong trend with a value
of 0.05235.
Substrate Percent Composition
Although the study focused on rocky substrates as suggested by Powers (2002), substrate composition did
vary between quadrats and between sites. The substrate compositions of each quadrat were averaged to
achieve a single substrate composition for each site (Figure 11). This data was later quantified as habitat
richness per site, with each grain size composition of value greater than 10% counting as a significant
substrate within each composition. For example, a site with 20% boulder, 30% cobble, 30% pebble, 15%
sand, and 5% fine sediments would count as a heterogeneous habitat with a value of 4. The study then
compared the differences in these substrate compositions to species density and diversity indices per site.
Species abundance increases with increasing habitat richness, but both diversity indices show a decreasing
diversity with increasing habitat richness (Figure 9). The R-squared values indicate that these trends may
be uncorrelated.
Algae and Sediment
Chlorophyll a concentration, a proxy for algal biomass, was determined for each site. Total organic and
inorganic sediments were also collected and calculated for each site. These data showed some variability
between sites (Figure 10). The only site with a large difference in inorganic sedimentation was
Mwangongo. Total organic sedimentation was highest at Katabe North and Luansa. The chlorophyll a
concentrations were mostly similar between sites, but were slightly lower at Kalalangabo 3 and Hilltop.
Gastropod biomass, abundance, and diversity indices (Simpson and Fisher’s Alpha) were correlated to algal
biomass, organic sediments, and inorganic sediments using Pearson R correlation (Table 5). None of the
correlations showed strong relationships.
Scarring
Crabs have been shown to prey upon snails (Rosales, 2000). Snails exhibiting scars have survived predation
attempts. The number of scars may be a good reflection of predation intensity in a region. The average
number of scarred snails at each site was compared to gastropod biomass and diversity (Simpson’s D and
Fisher’s Alpha) using a Pearson’s R correlation. No strong correlation trends were observed.
Discussion
Species Sympatry and Allopatry
The Kigoma Region is a good representative of an independent area for gastropod diversity and
distribution. Species diversity and abundance decrease on both ends of the region, which may delineate the
region from the surrounding shoreline (Figure 5). Northern sampling in the Gombe Stream National Park
area exhibited decreased species abundance and diversity. More sampling south of Kitwe Point would
further establish the southern extension of the region.
Our results support the earlier description of patchy distributions for the endemic gastropods in this region.
Several Lavigeria species are patchy or exist at only one site. These species include L sp AA, L. sp L, L. sp.
W, L. sp A*, L. sp M, L. sp J, and L. coronata. Based on a molecular phylogeny (Michel, 2000) we
examined the distribution patterns of distinct Lavigeria clades. L. coronata shares a clade with L. nassa and
L. grandis. These species are largely sympatrically distributed. Using the sympatric occurrence criterion to
verify species distinctness in morphologically closely similar species (Genner et al., 2004) we are able to
address the differences in the Lavigeria species in the Kigoma region. For example L. coronata,by cooccurring and being readily distinguishable from L. grandis and L. nassa, is verified in its distinct species
status (as was indicated by morphology and genetics, Michel 2000). In the L. sp L/M/AA clade, L. sp AA
and L. sp M are allopatric, L. sp L and L. sp M are sympatric at one site, and L sp. L and L. sp AA are
sympatric at one site (Table 2). These data show that L. sp L and L. sp M are in fact all differentiable
through a distributional criterion (as well as morphology), and not allospecies (Genner et al., 2004). L. sp L
and L. sp AA are not found sympatrically, however they differ unmistakably in morphology, thus do not
present any difficulties for taxonomy. The allopatry of L. sp AA and L.sp M does not conclude definitively
that they are separate species, thus these await genetic testing. Identification of the L. sp AA specimen
found at Kalalangabo 3 was verified (E. Michel, pers. comm.), showing this species occurs, albeit rarely, at
this site. L. sp AA and L. sp A* are allopatric. In the L. sp J/W/A clade, L. sp J and L. sp W are sympatric at
two sites, therefore supporting their separate species status. L. sp A* and L. sp W, although difficult to
distinguish from one another, were verified to sympatrically occur at one site, Lemba, also indicating that
they are distinct species. L. sp A* and L. sp J did not occur sympatrically so their separate species status is
unverified in this study. In general, however, our detailed sampling revealed a number of cases of
sympatric occurrence of readily separable species, supporting the division of Lavigeria into many tighly
morphologically differing species.
Abundance, Biomass, and Diversity Patterns
Patchy species distribution patterns contributed to variable patterns in species abundance, biomass, and
biodiversity. Physical parameters of each site could not accurately predict these patterns. Site areas that
appeared comparable to other sites varied widely in species composition. For example, Lavigeria cornata
was found at rocky outcroppings in the south and was not found in similar habitats until much farther north
at Kalalangabo 3. The presence of L. coronata terminates in the middle of a long stretch of seemingly
suitable rocky habitat. There appeared to be no habitat or substrate change at the boundary, yet the
transition was sharp. Both juvenile and adult L. cornata were found close to the transition area. These
patchy patterns of unexplained boundary points could be true for other species as well, and may support the
possibility of metapopulations. A species sweep of the entire shoreline could discover other unique patterns
in species patchiness.
The variation in biomass matched the fluctuations in abundance of sites from south to north. Field
observations showed that sites with higher abundance of gastropods tended to exhibit smaller sized
individuals. One suggestion proposed that gastropod biomass remains relatively constant across sites—
some sites contain small numbers of larger-sized individuals and others show greater numbers of smallersized individuals. The data did not support this proposition, as biomass levels approximately matches
species abundance. Those sites with the highest abundance, such as Hilltop, Katabe North, and
Kalalangabo 1, also had the greatest biomass. Biomass was not constant between sites of the Kigoma
region.
Physical Parameters: Shoreline Aspect, Substrate Composition, Algae, and Sediment
Our initial hypotheses suggested that the physical variables of a site would affect gastropod diversity,
abundance, and distribution. Our correlation tests revealed that of all the variables tested, only shoreline
aspect correlated significantly with diversity.
Coulter (1991) states that longitudinal north-south waves contribute the most forceful wave energy in Lake
Tanganyika. One could predict that sites with southwest-facing aspects would undergo the greatest wave
energy (there were no south facing shorelines), and therefore snail abundances or diversity would differ
from sites facing west or northeast. The northeast-facing sites remain much more protected from the
pounding wave action of intense directional waves from the southwest that occur during the dry season.
Westward facing sites undergo wave action, but less than southwest-facing sites. The diversity showed a
significant correlation with a northeast facing aspect. Whereas both abundance and diversity values were
higher in northeast-facing sites, only diversity proved to be statistically different between aspect-grouped
sites. These waves could affect sites in numerous ways, including the transportation of fine sediments to
deeper regions. Aspect may also affect light availability, the stability of substrate, and the presence of
predators.
Comparisons of sites in terms of organic and inorganic sediments, algae abundance, and substrate
variability led to no clear explanation for patchy distribution. The data indicated a slight correlation
between high abundance of gastropods and low algal biomass.These data might support the proposal that
Tanganyikan benthic grazers and algae community are highly co-adapted and a large biomass can be
supported by fast regenerating, low-biomass algal producers. None of these variables, with the exception of
aspect, independently correlates with gastropod distribution. Different combinations of certain parameters,
however, may contribute to these patterns. Multivariate analysis could expose emergent relationships
between physical variables related to gastropod species distribution and abundance.
The rocky sites we surveyed could be fairly isolated from each other from a gastropod’s perspective, which
might influence the distribution patterns we found. . In many cases our rocky habitats were separated by
sandy stretches that could act as barriersto the rocky substrate species we assessed. We found few snails in
sandy areas, which they would have to cross to disperse as adults. A number of authors have shown that
this type of habitat isolation has affected diversification of cichlids in Lake Tanganyika (e.g. references in
Coulter 1991), however this idea fails to explain the patchiness of gastropod species. Many species are
found in alternating or distant sites, which suggests that either dispersial has occurred in the past with only
occasional successful colonization or subsequent extinctions. This pattern suggest that the many of the
endemic gastropods exisit in metapopulations.
The available habitat area may also influence species diversity and distribution. MacArthur and Wilson
(1967) established the population biology theory that larger habitat areas maintain more species. This study
chose to focus on rocky substrates because that is where the abundance and diversity of gastropods is
highest. Rocky habitats of larger area may then harbor higher abundance and diversity of gastropods.
Unfortunately, this study did not directly document the total length of hard substrate available at each site,
and there remained unsampled areas between sites. However we did test for a species-area effect, we did
attempt a preiliminary test for a species-area effect based on map-based inference of known rocky and
sandy sections of shoreline. We measured the lengths of rocky habitat and then compared with the
abundance and diversity of species found in that region. We found a strong correlation between area of
rocky habitat and diversity (Figure 11). Larger areas exhibited much higher levels of diversity. This result
warrents further testing, perhaps with manta board and SCUBA surveys of all substrate and species
transitions along the 30 kilometers of shoreline.
This alone does not complete the picture for the observed variations in species distributions. The overall
richness in the Kigoma region from our surveying indicates a total of 20 species. Our sweeps did not
sample deep-water or extremely shallow habitats as effectively as possible. Typhobia and Neothauma taxa
were likely missed in deep water, while Stormsia, Syrnolopsis, or Reymondia could have been overlooked
in shallow waters. Further sampling would expand the data.
Conclusions
This project demonstrated a patchy distribution of gastropods, particularly of Lavigeria species, in the
Kigoma region of Lake Tanganyika. Physical parameters at each site, such as substrate composition,
shoreline lithology, organic and inorganic sediments, and algal biomass failed to correlate with gastropod
abundance, diversity, or distribution. Shoreline aspect did, however, correlate with diversity. More
protected northeast-facing sites exhibited the highest levels of diversity. Both abundance and diversity
values varied widely within the Kigoma region. We found that gastropod biomass does not remain constant
within the region, but rather increased as gastropod abundance increased. Gastropod sampling did lead to
more concrete distributional data for the narrow range endemic Lavigeria species and allowed us to
comment on sympatric criteria for species distinction. Many possibilities exist to expand upon this project
in the future. More thorough sampling could determine if a relationship exists between rocky habitat area
and species diversity. Multivariate statistical analysis of this project’s data could uncover emergent
combinations of physical factors that affect gastropod distribution. The lack of correlations between
physical parameters and distribution calls for a further examination of the possibility of metapopulations. L.
coronata would present an interesting case study, as its highly patchy distribution and abrupt population
boundaries present an unpredictable pattern. Additional shoreline sweeps could also determine if other
gastropod species exhibit similarly unexplained population limits. We found extremely large numbers of
dead shells at each site. A comparison of present and past species may better help to understand how
populations change over time. A study examining juvenile snail presence would also determine if certain
habitats create more effective juvenile survivorship locations. The expansion of GIS mapping of the area
(e.g Gathogo, 2001) would also aid in understanding the complexities of the region, and provide a more
interactive display of present data.
Acknowledgements
We would like to thank Michelle Olsgard for her muscles, brains, and AMAZING!! perspective of the
world, Nate Stewart for his Swahili, his hard work, and daily energetic smiles, and Ellinor Michel for
always keeping us thinking. Her inputs to this project were monumental. Thanks to the NSF for making the
Nyanza Project happen, and to the MacArthur Foundation and the Field Museum of Natural History for
supporting ABB’s participation in the Nyanza Project. We would like to thank the Mercury for her great
little 5 horse power motor and the Achilles for not really having a weak spot. Kwaheri na Usiku mwema.
References
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Tables and Figures
Substrate Type
Grain Size
Fine sediments (Silt and clay)
.00025-.062 mm
Sand
.062-2 mm
Pebbles
2-64 mm (.08-2.52 in)
Cobbles
Boulders
64-256 mm (2.52-10 in)
256-4096 mm (10.1-161.3in)
Table 1: Substrate classification by grain size. Modified Wentworth Scale, after Lane, et al., 1947,
Trans American Geophysical Union, v. 28, p. 936-938
Figure 1: Kigoma region map with all sites labeled
Genus
Species1
Relative Abundance2
Lavigeria
nassa
Very abundant and
widespread
Abundant at proximal sites
(KLG3…), absent elsewhere
grandis
coronata
species A
Abundant at KLG3, JKB,
Absent elsewhere
Somewhat abundant at KTG,
Habitat
Preference3
Boulder and
Cobble
Boulder
Site Occurrence4
All
66.7
Boulder
KLG1,
KLG3,PUN,LEM,KTG,JKB, MIT,
LUA
KLG3, JKB
Sand/Fine
LEM, KTG
16.7
1
Gastropods found in the Kigoma region of Lake Tanganyika during quadrat, depth, and horizontal sweeps.
Relative abundance determined from quantitative quadrat data and observation.
3
Habitat Preference compiled from observation and quadrat substrate composition data.
4
Gastropod site occurrence determined during sweep and quadrat data collection.
5
Occurrence frequency= (Number of sites where a species occurred/12) x 100
2
Abundance Key:
Very Abundant
Abundant
Somewhat Abundant
Occurrence
Frequency5
100
Rare
Very Rare
Absent
Site Key:
Kalalangabo 1=KLG1 Lemba=LEM Katangwe=KTG Mitumba=MIT Mzungu=MZU Mwangongo=MWA Jakobsens= JKB
Punda Milia=PUN Kalalangabo3=KLG3 Katabe North=KATN Luansa=LUA Hilltop=HTP
16.7
rare elsewhere
species J
species L
species M
species W
species AA
Very Abundant at HTP, Rare
to Absent elsewhere
Somewhat abundant at
KLG13, Rare to absent
elsewhere
Somewhat abundant at HTP,
Rare to absent elsewhere
Abundant at KLG1,MZU,
Rare to Absent elsewhere
Very rare at KLG3, Absent
elsewhere
Anceya
giraudi
Somewhat abundant on sand
Spekia
zonata
Vinundu
westae
Abundant on shallow rock,
Rare at 5m
Abundant and widespread
under rocks
guillemei
Reymondia
horei
minor
Paramelania
damoni
“minor”
Syrnolopsis
Bridouxia
Sediment, and
Pebble
Cobble, Boulder,
and Pebble
Shallow Cobble,
Under Rock
Shallow Cobble,
Under Rock
Cobble
Sand/Fine
Sediment, and
Boulder rarely
Sand/Fine
Sediment
Shallow Boulder
MZU, HTP, PUN, JKB,KATN
41.7
KLG1, HTP, KLG3, LEM, MIT,
LUA
50
HTP, PUN, KATN,
25
KLG1,MZU, HTP, KLG3, LEM,
JKB
KLG3
50
66.7
Rare at 5m, Abundant deeper
at MWA
Abundant on rock, especially
deeper
Very abundant at HTP under
rock, Absent elsewhere
Absent at 5m, Somewhat
abundant at depth
Boulder
KLG1, PUN, LEM, KTG, KATN
MWA, MIT, LUA
KLG1, MZU, LEM, KTG, KATN,
LUA
KLG1, MZU,HTP, KLG3, PUN,
LEM, KTG, JKB, KATN, MWA,
MIT, LUA
PUN, KTG, JKB, MWA
Boulder, Under
Rock
Under Rock
KLG1, HTP, KLG3, PUN, LEM,
KTG, JKB, KATN, LUA
HTP
75
Deep Sand/Fine
Sediment, and
Boulder
MZU, HTP, KATN, MWA, LUA
41.7
Rare at 5m, Abundant
elsewhere
Rare at 5m rock, Somewhat
abundant in sand
Rare, Somewhat Abundant
under rocks
Sand/Fine
Sediment
Sand/Fine
Sediment
Under Rock
KLG3, LEM, KTG, JKB, KATN,
MIT, LUA
MWA, MIT, LUA
58.3
25
HTP, JKB, KATN, MIT, LUA
41.7
Under Rock
MIT
8.3
Cobble, Under
Rock, Boulder
Stormsia
Table 2: Species found table
NE facing
sum abun/sq meter
W facing
SW facing
Figure 2: Site vs. abundance (all species)
Mwangongo
Mitumba
Lemba
Punda Melia
Site by Aspect
Kalalangabo 3
Hilltop
Jakobsen's
Katangwe
Kalalangabo 1
Luansa
Katabe North
Hilltop Values Divided
by Tw o to Enable
View ing of Other Sites
Mzungu
Total Abundance (Total Individuals/m)
Total Gastropod Abundance At Different Aspects in the Kigoma Region
50
45
40
35
30
25
20
15
10
5
0
8
50
100
33.3
8.3
Figure 3: Site vs. abundance of Lavigeria species
Lavigeria Species Abundance
60
50
40
2
Indiv/ m 30
20
Sites (South to North)
Figure 4: Site vs. abundance of Lavigeria by clade
Mwangongo
Mitumba
Katangwe
Lemba
Kalalangabo 1
Lwanza
Kalalangabo 3
Hilltop
Katabe North
Jakobsen's
Punda Melili
0
Mzungu
10
L. sp A
L. sp L
L. sp AA
L. sp M
L sp W
L. sp J
L. coronata
L. grandis
L. nassa
Gastropod Species Abundance per Site in the
Kigoma Region
35
L. nassa
L. grandis
Raymondia horei
L. coronata
Vinundu guiellmae
L. sp J
L. sp W
Anceya giradi
Vinundu westae
L. sp M
Bridouxia
Paramelania minor
L. sp AA
L. sp L
L. sp A
Species Abundance (Individuals/m
30
25
20
15
10
Mwangongo
Katangwe
Lemba
Kalalangabo 1
Kalalangabo 3
Lwanza
Hilltop
Kalabe North
Punda Malia
Jakobsen's
Mzungu
0
Mitumba
5
Figure 5: Site vs. species abundance
L. nassa
L. grandis
Raymondia horei
L. coronata
Vinundu guiellmae
L. sp J
L. sp W
Anceya giradi
Vinundu westae
L. sp M
Bridouxia
Paramelania minor
L. sp AA
L. sp L
L. sp A
Mzungu
1.2
0
0
0
0
0
11.6
0
0.2
0
0
0
0
0
0
Jakobsen's
3.2
1.4
1.2
4
0.6
0.6
0.6
0.4
0
0
0
0
0
0
0
Punda Melili
7.2
0
0.2
0
0
0
0
0.2
0
0
0
0
0
0
0
Kalabe North
30.8
0
1
0
0
11
0
0.2
2
9
0
0
0
0
0
Hilltop Lwanza Kalalangabo 3
25.2
24.4
7.2
0
0
3.6
0.8
0
1.4
0
0
2
0
0
0
35.6
0
0
0
0
0.8
0
0
0
8.8
1
3
22
0
0
0.4
0
0
0
0.6
0.8
0
0
0.2
0
0
3.8
0
0
0
Kalalangabo 1
10.6
7.4
1
0
0
0
13
1
1
0
0
0
0
6.6
0
Lemba
7.8
7.4
2.8
0
0
0
0.2
0
0.4
0
0
0.2
0
0
0.2
Katangwe Mitumba
4.4
4.8
1.6
0.2
0
0
0
0
0
0
0
0
0
0
0
0
1.2
5.6
0
0
0
0
0.4
0.8
0
0
0
0
3.8
0
Mwangongo
3.8
0
0
0
0
0
0
4.6
0.8
0
0
0
0
0
0
300
Gastropod Biomass at Sites in Kigoma Region
250
200
Biomass (g/m2)
150
100
50
M
Mitumba
Katangwe
Lemba
Kalalangabo 1
Kalalangabo 3
Lwanza
Hilltop
Kalabe North
Punda Melili
Jakobsen's
Mzungu
0
Figure 6: Biomass per site
Average Cumulative Species Abundance of Sites
Grouped by Aspect in the Kigoma Region
Average Individuals
Per Aspect
30
25
20
15
10
5
0
SW
NE
W
SW
Figure 7: aspect vs. abundance
NE
Shoreline Aspect
W
Diversity by Aspect in the Kigoma Region
5
P value for Difference of Simpsons
Index Between Aspects=0.04795
4.5
Average4Species Diversity Indices
P value for Difference of Fisher's Index
Between Aspects=0.05235
3.5
3
2.5
2
1.5
1
0.5
Average Simpson's Diversity Index
0
SW
NE
Average Fisher's Diversity Index
W
Aspect
Figure 8: aspect vs. diversity
Gastropod Diversity and Habitat Richness in the
Simpson's Diversity
Kigoma Region
Diversity Indices
5
4
3
2
1
0
0
2
4
Substrate Heterogeneity
Figure 9: habitat richness vs. aspect and diversity
6
Species density (Individuals/m2)
100
90
80
70
60
50
40
30
20
10
0
6
Index
Fisher's Alpha
Diversity Index
Species density
Linear (Fisher's Alpha
Diversity Index)
Linear (Simpson's
Diversity Index)
Linear (Species
R2 = 0.1039
R2 = 0.0657
R2 = 0.0207
Total Non-Ogranic Sediments, Total Organic Sediments, and Benthic
Cholorphyll A Concentration on Survey Sites in the Kigoma Region
Mwangongo
Mitumba
Katongwe
Lemba
Kalalangabo
3
Luansa
Hilltop
Katabe N
Jacobsen's
S
Pundamilia
80
70
60
50
40
30
20
10
0
Total Non-Organic (mg/cm^2)
Total AFDM (mg/cm^2)
TOTAL Chl A (ug/cm^2)
Figure 10: site compassions of sediments, algal biomass, and total organics
C o m p a r is o n o f R o c k y S u b s tr a te to S p e c ie s
D iv e r s ity
Fishers Diversity Index
3
R 2 = 0 .8 6 6 8
2 .5
2
1 .5
1
0 .5
0
0
1
2
3
4
5
E s tim a te d R o c k y Ha b ita t D is ta n c e S c a le d to 1 c m
Figure 11: Area vs. diversity of sites along the Kigoma region
6
Table 3: Site information table
Site
Mzungu
Jakobsen's
Pundamilia
Katabe North
Hilltop
Luansa
Kalalangabo 3
Kalalangabo 1
Lemba
Katangwe
Rich
Ness
(Spe
cies/
6
Species
Density
(indiv/m2)
Simpson’s
Diversity
Index (D)
Evenness (E)
D/# species
Fisher’s
Alpha
Diversity
Index
21.67
1.242281682
0.414093894
0.65
80% Boulder
10% Cobble
11
12
4.663212435
0.582901554
2.48
75% Boulder
10% Cobble
8
11.8
1.170900693
0.292725173
1.12
Conglomerate cliff, Kigoma quartzite cobbles 15% Boulder
40% Cobble
10
54
2.52318981
0.420531635
1.09
Kigoma Quartzite Cobbles
Conglomerate Cliffs
10 % Boulder
25 % Cobble
10
92.8
3.499129874
0.583188312
SW
Kigoma quartzite boulder, beach rock
80% Boulder
5% Cobble
11
26
1.132859633
0.377619878
0.548
NE
Kigoma Quartzite Boulders
80 % Boulder
19% Cobble
9
22.6
5.442211055
0.604690117
2.299
SW
Manyovu Red Bed Cliffs
Kigoma Quartzite Cobbles
Kigoma quartzite boulders/cobbles and
beach rock
70% Boulder
10% Cobble
8
40.67
4.306712963
0.615244709
.548
90% Boulder
8% Sand
10
19
2.917879082
0.416839869
1.74
SW
Kigoma quartzite pebbles, Manoyovu
Red Bed Cliff
83% Boulder
2% Pebble
9
11.4
3.423603793
0.684720759
1.32
W
Kigoma quartzite cobble and boulder,
beach rock
11.4
2.359477124
0.589869281
0.98
W
Kigoma Quartzite outcrop, gravel and
sand beach
10
85% Boulder
10% Coarse sand with dead shells
6
90% Boulder
9.2
2.335540839
rank:9
0.778513613
rank:1
0.719
rank:9
As
pect
Lithology
Substrate Composition,
5m
SW
Kigoma Quartzite Cobbles
Conglomerate cliffs
30 % Boulder
30 % Cobble
NE
Kigoma quartzite boulder and cliff
W
Kigoma quartzite boulders
and rock faces
SW
NE
W
Mitumba
Mwangongo
10% Fine Sediments
0.973
Site
1
2
3
4
5
6
7
8
9
10
11
12
Simpson’s
Diversity
Kalalangabo 3
Jakobsen's
Kalalangabo 1
Hilltop
Katangwe
Lemba
Katabe North
Mitumba
Mwangongo
Mzungu
Punda Malia
Luwanza
Fisher’s Alpha
Evenness
Jakobsen's
Kalalangabo 3
Lemba
Katangwe
Punda Malia
Katabe North
Mitumba
Hilltop
Mwangongo
Mzungu
Kalalangabo
Luwanza
Mwangongo
Katangwe
Kalalangabo
Kalalangabo 3
Mitumba
Hilltop
Jakobsen's
Katabe North
Lemba
Mzungu
Luwanza
Punda Malia
Table 4: Rankings of sites by diversity indices and evenness
Comparisons
Inroganic Sediments to Biomasss
Chl A Concentration to Biomass
Organic Sediment to Biomass
Organic Sediment to Abundance
Inorganic Sediment to Abundance
Chlorophyll A to Abudance
Organic Sediment to Fisher's Diversity
Inorganic Sediment to Fisher's Diversity
Chl A Concentration to Fisher's Diversity
Organic Sediment to Simpson's Diversity
Inorganic Sediment to Simpon's Diversity
Chl A to Simpon's Diversity
Pearson r square Correlation
0.243
0.756
0.137
0.073
0.22
0.163
0.117
0.04
0.043
0.125
0.071
0.0032
Table 5: Correlations between various sediment factors and biotic factors