Competition between the comber (Serranus cabrilla) and painted comber (Serranus
scriba) at STARESO, Corsica, France.
Anja Sjostrom and Ryan Stephenson
Abstract
Interspecies competition is occurring between two Serranids; Serranus scriba and
Serranus cabrilla at STARESO in the bay of Calvi, Corsica. Normally partitioned by depth,
the two species overlap in resource and habitat use in this area. Our goal was to identify
differences and similarities between the two species, determine the extent to which their
habitats and resources overlap, and establish how they are partitioning them to avoid
constant competition. 37 fish specimen were examined in the lab for stomach content and
jaw morphology. Available habitat and selective usage by the two species was established
by uniform point contact (UPC) and opportunistic surveys noting size, depth, substrate and
cover habitat of 290 S. scriba and 39 S. cabrilla were taken on SCUBA. Our data show the
two species consuming highly similar prey items, and having little difference in jaw
morphology. The two species are partitioning substrate, cover and depth within the study
site. We conclude that competition is occurring at STARESO, and that S. scriba and S.
cabrilla are partitioning habitat to avoid resource overlap.
Introduction
Serranus scriba, the painted comber, and Serranus cabrilla, the comber, are two
species of grouper that coexist in the nearshore rocky reefs off Corsica, France. Though
similar in trophic level and morphology, both species tend to segregate by depth making
co-occurrence uncommon. According to Dr. Pierre Lejeune (pers., Comm), the research
director at STARESO, S. scriba have had a long and stable population in the area whereas S.
cabrilla was present in the area until 1995, when the population became severely depleted.
This deep population of S. cabrilla used to be found in 50-80m depths. In 2000, S. cabrilla
began to reappear in the area. Serranus scriba are found in depths of 0-30m, whereas S.
cabrilla occupy depths of 20-500m (Lythgoe & Lythgoe, 1975). At STARESO, S. cabrilla can
be seen in shallower depths, and overlapping with the local population of S. scriba. Despite
the species normal depth segregation, the two species share similar habitat and dietary
preferences, indicating high potential for interspecies competition (Fasola et al., 1999).
Serranus scriba and S. cabrilla are small fish of the family Serranidae and range
across the Mediterranean and Eastern Atlantic and are true hermaphrodites. Serranus
scriba lives over seagrass beds and rocky reefs and is territorial and solitary. Serranus
cabrilla is the less abundant of the two and is also a solitary species occupying sand
bottoms and rocky reefs (Guidetti, 2000). We know that the diets of the two species are
similar (Lythgoe & Lythgoe, 1975) but we expect that there will be small scale changes in
the diet due to competition. Interspecific competition between such sympatric species has
been well documented in rocky reefs off the California coast (Holbrook & Schmitt 1988;
Larson, 1980) but has not been addressed for these two species. We believe that both
species may be competing for resources and space. In a study in California (Larson, 1980),
one species of rockfish was found to have established a more pronounced dominance in the
overlapping habitats than the subordinate rockfish. This may be present in the competition
occurring between S. scriba and S. cabrilla.
With limited background information on
comparative ecology between S. scriba and S.
cabrilla, the purpose of this study was to a)
determine whether both species are partitioning
resources, and b) whether both species are
partitioning habitat. To answer these questions,
we conducted diet analysis and morphological
comparisons in the lab and used SCUBA to
determine small scale spatial habitat use in the
field. We expect that dietary composition will vary
between the two Serranids species, and that they
will characteristically differ in their utilization of
the Posidonia and rocky reef assemblages for
foraging and shelter at STARESO.
the summer.
Methods and Materials
Species Description
Apparent habitat and diet similarity
between S. scriba and S. cabrilla present a valuable
opportunity to examine resource and habitat
partitioning of sympatric species. Previous work
on Mediterranean rocky slope fish assemblages
indicates the species partition their habitat by
depth (Fasola et al 1997) and generally S. scriba
utilizes Posidonia oceanica meadows more as
habitat than and S. cabrilla. This makes STARESO
an ideal study site, with meadows extending past
the depths that stratify the two species, forcing S.
cabrilla up into the shallows dominated by S.
scriba. Both species are relatively common at
STARESO, where they utilize nearshore rocky reefs
as habitat. Their diets consist mainly of small
crustaceans and other fish (Fasola et al., 1999).
Both species have relatively small home ranges
and are simultaneous hermaphrodites, breeding in
Site description
We gathered all observational data on SCUBA at STARESO field station, Calvi,
Corsica in the northwest Mediterranean Sea (N 42.581795, E 8.725128). We collected data
north and south of Stareso harbor in Posidonia oceanica meadows and rocky reef slopes
between 3 and 14 meters (Figure 1).
Lab Methods
Resource partitioning is occurring between Serranus scriba and Serranus cabrilla.
Gut Content Analysis
To examine if diet composition differs between S. scriba and S. cabrilla, we collected
individuals of both species for gut content analysis by spearfishing. We emptied the
contents of the stomach onto a petri dish marked with a twenty-five point grid. We counted
all contents in a standardized way. At each of the grid’s intersections, prey was identified as
fish, crustacean, mollusk or cephalopod. By tallying all the points for each prey type, the
percent composition that each prey type represented was determined.
Jaw Measurements
In order to test whether jaw morphology is dissimilar in S.scriba and S.cabrilla we
took measurements to provide an illustration of potential prey differences between the two
species. Standard length and width of each specimen was taken prior to gut removal and
analysis. To test for differences in jaw morphology between the two serranid spp. vertical
and horizontal gape measurements were taken. All measurements were recorded in
millimeters.
Field Methods
Serranus scriba and Serranus cabrilla are partitioning habitats and depth in the
mediterranean rocky reefs and posidonia fields.
UPC and Focal data surveys
To examine habitats of both species, we conducted uniform point contact (UPC) and
opportunistic individual focal surveys to quantify comparative habitat features and
resource use due to competition. We conducted surveys to the north and south of STARESO
harbor. To characterize substrate habitat type, UPC counts were taken along 30m transects
placed in areas of habitats known to be occupied by Serranus spp. We used the following
substrate categories: sand, cobble (1-10cm), boulder (rocks larger than 10cm unattached
to substrate), bedrock, and other. Cover layers were grouped in the following manner:
turf=less than 3cm, bushy=greater than 3cm, foliose, erect coralline, encrusting coralline,
gunk=film layer less than 1cm, sessile invert, other or none. To gather available cover and
substrate data for the northern portion of the study area we used nine point quadrats
(0.5m x 0.5m)off a permanent transect line. At 5 meter intervals we ran 30 meter transects
off of the permanent line and placed the quadrat down for sampling at 5 meter intervals on
either side of the transect tape. We counted substrate and cover under each of the
quadrat’s nine points.
To quantify how both species were associating with the different habitats we
conducted opportunistic focal data surveys each S.scriba or S. cabrilla encountered. Each
fish was visually sized to the nearest centimeter and the depth it was sighted at recorded in
meters. We recorded the focal data of each fish by their associated substrate and cover
layer as defined by the UPC categories. Surveys were conducted from October 22th until
October 30th 2012.
Statistical Analysis
Gut content was characterized by the number of diet items in each category and
standardized to total length of the fish. Data were analyzed for similarity (Bray-Curtis
distances) in PRIMER. Horizontal and vertical gapes were divided by the standard length of
each specimen to standardize gape size for each fish. We then used these standardized
gapes to analyse data for similarity (Bray Curtis distances) in PRIMER to determine
similarity in jaw morphology of S. scriba and S. cabrilla. Chi square tests were run to
compare the availability of each substrate and cover type to the observed use by S.scriba
and S.cabrilla. ANOVA tests were run to analyze the available depth range and quantify any
depth stratification between the two species.
Results
We must accept the null hypothesis that the diets of the two species do not differ (Figure 3,
p<0.564). Many of the
replicates from the analyses
ate the exact same food type
as another replicate, which
gives the appearance of a
lesser number of fish. We
must accept the null
hypothesis that there is no
difference in the maximum
vertical and horizontal gape
between the two species
(Figure 3, p<0.964). S.
Figure 3: The similarity in jaw morphology
cabrilla exhibits a strong
(maximum horizontal and maximum vertical gapes
positive association with
divided by standard length) between S. scriba and
bedrock, utilizing it
S.cabrilla. Green triangles depict jaws from S. scriba;
significantly more than the
upside-down blue triangles are from S. cabrilla.
available quantity (Figure 4,
There is no difference between the jaw
p<0.001). They showed a
morphologies of the two species.
strong negative association
with the relatively abundant
Posidonia, with only two out
thirty-nine individuals utilizing
it as habitat (Figure 5, p<0.001).
Negative associations with
boulders reflect the high
abundance of boulders in the
survey area and the low
abundance of S. cabrilla. S.
scriba shows a positive
association with bedrock and
cobble (Figure 6, p<0.001). Both
Figure 2: The similarity in diet composition between
results support the hypothesis
S. scriba and S. cabrilla. Green triangles depict gut
that species distribution will
contents from S. scriba individuals; upside-down blue
differ by substrate type
triangles are from S. cabrilla. There is no detectable
(Figure7, p<0.001). S. scriba is
difference in diet composition between the species.
using Posidonia for habitat with
double the presence it occurs in
the study site. Depth stratification between the species revealed S. cabrilla using
consistently deeper depths than S. scriba (Figure 8, ANOVA pooled variance, p<0.006). We
removed Posidonia as a variable, as only one S.cabrilla individual was sighted over this
cover, as a potential confounding factor and ran the ANOVA again. This generated an even
more statistically significant p<<0.001.
Figure 4a:
Table of
Figure 4
Figure 5a: Table of Figure 5
Figure 8a: Table of Figure 8
Discussion
In order for competition to
be occurring between the fish
there are three important factors
to consider: diet composition,
habitat and depth. If they overlap
in diet and habitat, they must
partition the resources or the
space or they would fight
constantly. In territorial fish like S.
scriba, defense of resources other
than eggs from other species
Figure 8: Average Depth of the two species. S.
indicates the presence of
scriba observations over Posidonia were removed
competition (Grant 1997,
as S. cabrilla did not utilize it and seagrass beds
Kirchshofer 1954). We detected
covered much of the deeper depths.
no dietary difference between the
two species in the study site.
Serranus cabrilla’s ability to break down the chitinase of invertebrate shells exceeds that of
S. scriba, but this difference did not reflect on the diets of the specimens we collected
(Benmouna et al. 1986). The similarity in jaw morphologies indicates no adaptation to
specific prey type. While more accurate assessments of jaw morphology require factors
other than maximum gape, the species feed in a similar fashion (Viladiu et al. 1999). We
hoped to increase the sample size of the fish collected to reveal smaller scale dietary
differences, but this might prove biologically insignificant.
Use of habitat is quite different. Serranus cabrilla associated most commonly with
bedrock whereas boulders were the most prevalent substrate in the study area (Figure 5,
P=0.001). Serranus scriba selected bedrock and cobble the most of the available substrates
(Figure 6, P=0.001). In terms of cover data, S. scriba were consistently using Posidonia
oceanica, whereas the S. cabrilla were avoiding it. This proves interesting in two regards:
one is that they are distinctly partitioning habitat both in terms of substrate and cover
availability. The second is while the S. scriba at times overlap the rocky reef substrate, the
S. cabrilla population are avoiding any association with the Posidonia oceanica (Figure 7,
P=0.001). This further solidifies S. scriba acting as the more dominant species in the area,
and occupying a substantial portion of the available habitat while the S. cabrilla are limited
to subset of that available habitat (Larson, 1980). Although Serranus cabrilla are commonly
found over seagrass, we see very different habitat use at STARESO (Francour, 1997).
The presence of more S. cabrilla with increasing depths was consistent with
literature pertaining to normal depth stratification between the two Serranids (Fasola et
al., 1997; Lythgoe & Lythgoe, 1976). S. cabrilla was consistently deeper than S. scriba, and
even more dynamically so when Posidonia was removed from the ANOVA test. S.cabrilla
also exhibited a higher standard deviation in depth towards deeper habitat, whereas the
S.scriba deviated minimally from their shallower depth ranges. In a survey of rocky reef
fish assemblages in the Mediterranean, the abundance of S. scriba declines as the
abundance of S. cabrilla increases along the depth gradient of 25m (Fasola et al. 1997).
Given that our findings indicate overlap in diet and avoidance of habitat and depth overlap,
the species interactions are quite similar to the black and yellow and Gopher rockfish of
temperate reefs in California. A socially dominant species S. scriba/black and yellow
rockfish forces the other S. cabrilla/gopher rockfish into a subset of the available habitat
(Larson, 1980). In the 1980’s there was a stable population of S. cabrilla in the area which
declined significantly and became nearly non existent in 1995-2000 . The population is
only now re-establishing itself in the area, while the S. scriba have had a consistent and
stable presence in the area for this entire period. It is also noted that there is a population
of S. scriba found at depths between 50-80m, but this deep population disappeared (P.
Lejeune. pers. Comm). This gradual transition of the S. cabrilla back into an overlapping
community as well as the unique situation in the bay of Calvi drive the Serranids into
inevitable ecologically competitive circumstances. Given the morphological similarity of
the sympatric species and prey preferences, they must partition either resources or habitat
and depth, the latter of which we see to be true.
Another interesting visible characteristic of competition is territoriality, of which we
did have several clear encounters. All aggressive displays between the two Serranids
witnessed resulted in S.scriba chasing S.cabrilla from the area of dispute. While we were
not able to quantify this from a statistical standpoint as we did not take data on these
incidences, we can register this as another important component to the ongoing
interspecific competition between the species, and take this as a potential further
indication of the S.scriba dominance in this particular set of ecological circumstances.
We were limited on S.cabrilla data, due to the mere infrequency with which we
encountered individuals. More collected specimen to analyze gut contents as well as more
field observations would have allowed a clearer depiction of the resource and habitat
preferences of the two species. Future studies on the deeper population of S.cabrilla and
the depth ranges of S.scriba would give a more complete ecological picture of the two
species and their interactions. There is still very minimal research on the S.cabrilla and
even less on the interaction of ecological niches of the Serranus spp.
Acknowledgements
We would like to thank Pete Raimondi for his statistical genius, Gaby Keeler for all her help
with transects and fish spearing, Giacomo Bernardi for his incredible fish knowledge and
the TA’s for fishwacking, diving and moral support. Also, the Bio 159 class of 2012.
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