2004 Blackwell Publishing Ltd www.blackwellpublishing.com/geb 327
Global Ecology and Biogeography, (Global Ecol. Biogeogr.) (2004) 13, 327­334
RESEARCH
PAPER
Blackwell Publishing, Ltd.Research PAPERSpecies richness, abundance and body size in deep-sea gastropods
Connecting species richness, abundance
and body size in deep-sea gastropods
Craig R. McClain
ABSTRACT
Aim This paper examines species richness, abundance, and body size in deep-sea
gastropods and how they vary over depth, which is a strong correlate of nutrient
input. Previous studies have documented the empirical relationships among these
properties in terrestrial and coastal ecosystems, but a full understanding of how these
patterns arise has yet to be obtained. Examining the relationships among macroecological
variables is a logical progression in deep-sea ecology, where patterns of
body size, diversity, and abundance have been quantified separately but not linked
together.
Location 196­5042 m depth in the western North Atlantic.
Method Individuals analysed represent all Vetigastropoda and Caenogastropoda
(Class Gastropoda) with intact shells, excluding Ptenoglossa, collected by the Woods
Hole Benthic Sampling Program (3424 individuals representing 80 species).Biovolume
was measured for every individual separately (i.e.allowing the same species to occupy
multiple size classes) and divided into log2 body size bins. Analyses were conducted
for all gastropods together and separated into orders and depth regions (representing
different nutrient inputs).A kernel smoothing technique, Kolmogorov-Smirnov test
of fit, and OLS and RMA were used to characterize the patterns.
Results Overall, the relationship between the number of individuals and species is
right skewed. There is also a positive linear relationship between the number of individuals
and the number of species,which is independent of body size.Variation among
these relationships is seen among the three depth regions. At depths inferred to correspond
with intermediate nutrient input levels, species are accumulated faster given
the number of individuals and shift from a right-skewed to a log-normal distribution.
Conclusion A strong link between body size, abundance, and species richness
appears to be ubiquitous over a variety of taxa and environments, including the
deep sea. However, the nature of these relationships is affected by the productivity
regime and scale at which they are examined.
Keywords
Abundance, body size, deep sea, gastropods, macroecology, North Atlantic, productivity
gradient, species richness.
Department of Biology, University of New
Mexico, MSCO3 2020, Albuquerque, NM
87131­0001, USA. E-mail: mcclainc@unm.edu
INTRODUCTION
`Body size is one of the most important axes of biological
diversity.' (Brown & Lomolino, 1998; p. 508)
Exploring how body size influences community structure in
terms of species' abundances and species richness has pervaded
the ecological literature. However, few studies have simultaneously
documented the association between species richness,
abundances, and body size (terrestrial arthropods: Siemann et al.,
1996, 1999; marine gastropods: Fa & Fa, 2002). These findings
have significant implications for how we assess global biodiversity.
Yet the generality of these patterns and the mechanisms
by which they arise have not been fully examined. Here, I focus
on a deep-sea gastropod community over a depth gradient,
which affords the opportunity to test both the ubiquity of these
trends and how they vary over a productivity gradient.
C. R. McClain
328 Global Ecology and Biogeography, 13, 327­334,  2004 Blackwell Publishing Ltd
Research in terrestrial habitats continues to establish the relationships
between abundance, richness, and size trends. Previous
studies support the idea that species richness and population
abundance are related to body size (Warwick & Clarke, 1996;
Blackburn et al., 1997; Gaston et al., 1997; Siemann et al., 1999).
In general, on coarse scales, the relationship between abundance
and size is negative (Damuth, 1981; Cotgreave, 1993; Currie, 1993;
Brown, 1995; Enquist et al., 1998). On local scales, the relationship
is log-normal or log-uniform (Brown & Nicoletto, 1991;
Currie, 1993; Siemann et al., 1996; Warwick & Clarke, 1996). The
relationship between species richness and body size, although
not as thoroughly researched, exhibits both negative (Morse
et al., 1985; May, 1988; Blackburn & Lawton, 1994) and unimodal
trends (Brown, 1995; Siemann et al., 1996; Navarette & Menge,
1997; Siemann et al., 1999). Several theories exist to explain these
relationships including: trade-offs between metabolic efficiency
and reproductive success (Brown et al., 1993), habitat heterogeneity
(May, 1986), self-thinning (Stevens & Carson, 1999),
density compensation (Marquet et al., 1990), extinction and
mobility (Ebenman et al., 1995), fractal distribution of resources
(Ritchie & Olff,1999),and the energetic equivalence rule (Cotgreave,
1993).
Despite the vast size of the deep sea, contemporary research
has only quantified conspicuous macroecological trends in body
size, abundance, and species diversity separately. These trends
typically have been related to depth and correlated environmental
variables.Several studies have described the unimodal trend
between species diversity and depth, in which diversity peaks at
intermediate depths (Rex, 1973, 1981; Etter & Grassle, 1992).
Abundance, however, declines exponentially with increasing
depth (Rex et al., 1990). Many early studies noted size extremes,
gigantism and dwarfism, in deep-sea taxa. Thiel (1975) provided
the first quantitative assessment, documenting that abundances
of the smaller meiofauna decrease less rapidly with depth than
abundances of the larger macrofauna. Additional studies have
produced mixed results (reviewed in Rex & Etter, 1998), with
many reporting increasing size with depth within and among
species (Rex & Etter, 1998; Rex et al., 1999; McClain & Rex, 2001).
METHODS
The individuals analysed in this study represent all vetigastropods
and caenogastropods with intact shells, excluding
Ptenoglossa, collected from the western North Atlantic by the
Woods Hole Benthic Sampling Program (Sanders, 1977). Ectoparasitic
ptenoglossates were eliminated because of their equivocal
taxonomic affinity (Fretter & Graham, 1962; Ponder & Lindberg,
1997) and fundamentally different lifestyle. The remaining dataset
still represents a diverse assemblage, including representatives
of the families Trochidae and Seguenziidae in the Vetigastropoda
and families Buccinidae, Cerithiopsidae, Muricidae, Olividae,
Rissoidae, and Turridae in the Caenogastropoda.
Samples were taken from depths between 196 and 5042 m
(Fig. 1) and contain 3424 individuals, representing 80 species.All
Figure 1 Map depicts positions of samples
used in this study in the North-west Atlantic
Ocean.
Species richness, abundance and body size in deep-sea gastropods
Global Ecology and Biogeography, 13, 327­334,  2004 Blackwell Publishing Ltd 329
material was collected by using epibenthic sleds and anchor
dredges with mesh sizes of 1 mm. The macrofaunal component
was collected on a 0.420-mm sieve (Hessler & Sanders, 1967).
There is considerable uncertainty about the effectiveness of deepsea
sampling, but the molluscs, in general, appear not be as affected
by sampling inconsistencies as are other deep-sea fauna. The
heavy fraction of material collected by the dredge, of which
gastropods with their calcified shells are a part, are less likely to be
excluded due to bow wave compression effects. They are also less
subject to winnowing as the gear approaches the surface than
other groups with lower densities. These observations are supported
by evidence that different sampling gears provide consistent
results for molluscs (Rex et al., 1990). It also important to note
that although epibenthic sleds and anchor dredges provide only
semiquantitative data, as long as relative abundances among size
classes are accurate, the analyses presented herein are permissible.
Previous studies have used a single estimate of size for each
species, whether it is an average or maximum size (e.g. Siemann
et al., 1996, 1999). Partly this reflects the difficulty of measuring
body size for all individuals in an entire community. In addition,
hypotheses of body size are often specific about constraints on
maximum size (McClain & Rex, 2001). However, the choice of
body size metric can have a drastic effect on the observed patterns
(McClain & Rex, 2001). It is also important to consider that
individuals of species within a community are not all the same
size, and size distributions within a species can vary over environmental
gradients, including in the deep sea (e.g. McClain &
Rex, 2001). An individual of a species may actually never reach
the maximum size or only spend a small percentage of its lifespan
at this size. Moreover, indeterminate growers, such as some of the
species considered herein, typically occupy a broad range of size
classes, making a single-size approach inappropriate. Because the
size changes over the lifespan of individual, size­dependent
interactions also vary over the lifespan of animal. It is important
to determine how the use of differing size metrics might influence
the results. I undertook a preliminary analyses using first
a single estimate of size for each species (n = 80) and second,
including each individual separately (n = 3424), i.e. allowing the
same species to occupy multiple size classes. Both methods gave
similar results and so I only present and discuss the results using
the second data set.
Images were obtained by using a Spot RT digital camera connected
to a Nikon SMZ1000 dissection scope. Digital images were
analysed and imaged on a Macintosh G4 using Spot Advanced
software. Biovolume (B) was measured for every individual, such
that B = l * w2
(Siemann et al., 1996, 1999) where l is length and w
is width. Although this calculation is a crude measure of volume,
the error is minor relative to the differences in size among the
specimens. Other methods of estimating shell volume exist (e.g.
Raup & Graus, 1972), but were not used because they include
measurements that require sectioning of shells, which is not
feasible with the smaller species in a sample or with museum
specimens. In addition, the range of shell forms here was not
suitable for the measurement methodology.
The number of species and individuals in a sample were aggregated
into log2 body size bins (size classes are: 1: -3 to -2, range
0.125­0.25 mm3
; 2: -2 to -1, 0.25­0.5 mm3
; 3: -1 to 0, 0.5­
1 mm3
; 4: 0­1, 1­2 mm3
; 5: 1­2, 2­4 mm3
; 6: 2­3, 4­8 mm3
; 7:
3­4, 8­16 mm3
; 8: 4­5, 16­32 mm3
; 9: 5­6, 32­64 mm3
; 10:
6­7, 64­128 mm3
; 11: 7­8, 128­256 mm3
; and 12: 8­9, 256­
512 mm3
). The individuals here range in size from 0.19 mm3
to
501.42 mm3
. Separate analyses were conducted for both orders
(Vetigastrodpoda and Caenogastropoda). I separated the dataset
into three bathymetric regions (continental slope 200­2000 m,
continental rise 2000­4000 m, and the abyssal plain > 4000 m)
that represent high, medium, and low productivity regimes
(Etter & Mullineaux, 2001). Although these regions do not contain
equal depth coverage, they are generally considered to be
biogeographic regions with differing benthic faunas and environmental
variables (Gage & Tyler, 1991). A kernel smoothing
technique was utilized to describe the relationship between
species and individuals among log2 size classes. Deviation from a
log-normal distribution was tested using a Kolmogorov-Smirnov
test of fit.A nonparametric procedure was used because the arbitrary
location and width of classes might influence the results.
Previous studies (Siemann et al., 1996, 1999; Fa & Fa, 2002) have
demonstrated that the relationship between the number of
species (S) and number of individuals (I) can be defined by the
power function S = cI0.5
, derived from a broken-stick model, or
S = cI0.25
from a canonical log-normal distribution, where c is a
constant. Both OLS and RMA regression have been used previously
in the literature to characterize the power function. The
two methods often give different slopes and intercepts (LaBarbera,
1989). Because of the ease of implementation, I use both to
quantify a power function describing the relationship between
number of species and number of individuals within a size class.
RESULTS
When all included species from the western North Atlantic
are combined, there are strong relationships among log S,
log I, and log2 size class (Fig. 2). The number of species and individuals
peak in similar size classes (log S: KS P = 0.0012,
skewness (g1) = 4.67; log I: KS P = 0.0324, skewness (g1) = 4.17).
There is a significant, positive power function between log S
and log I.
These patterns exhibit slight variations among depth regions.
Peak abundances and number of species shift among depth
regions, with the modal size classes being larger on the rise
(Fig. 3). All three depth regions display some evidence of a rightskewed
distribution (Slope: KS P = 0.0238, skewness (g1) = 2.97;
Rise: KS P = 0.3249, skewness (g1) = 2.21; Abyss: KS P = 0.0021,
skewness (g1) = 5.23) and individuals (Slope: KS P = 0.0045,
skewness (g1) = 3.25; Rise: KS P = 0.4363, skewness (g1) = 1.97;
Abyss: KS P = 0.0001, skewness (g1) = 6.5833). An ANCOVA detects
no differences among the slopes between Log I and Log S (Fig. 4,
d.f. = 2.32, F = 1.556, P = 0.2293).
For both taxonomic orders, peak abundance occurs in approximately
similar size classes (Fig. 5, Vetigastropoda: KS P = 0.0217,
skewness = 3.13; Caenogastropoda KS P = 0.0013, skewness =
3.85). However, the curve for log S peaks at a slightly smaller size
class for vetigastropods (Fig. 5, Vetigastropoda: KS P = 0.0014,
C. R. McClain
330 Global Ecology and Biogeography, 13, 327­334,  2004 Blackwell Publishing Ltd
skewness = 4.91; Caenogastropoda KS P = 0.0001, skewness =
3.35).
DISCUSSION
These results add to the growing evidence for a general relationship,
despite differences in taxonomic groups and environments,
between species richness, abundance, and body size. However,
the relationship between the number of species and body size in
North-west Atlantic gastropods in this study is right skewed as
opposed to the log-normal relationships reported for terrestrial
arthropods (Siemann et al., 1996, 1999) and coastal gastropods
(Fa & Fa, 2002). Here, the number of individuals and species are
highest in the third to fifth size classes, similar to tropical molluscs
in New Caledonia (Bouchet et al., 2002), but in contrast to
the intermediate and larger size classes (Roy et al., 2000; Fa & Fa,
2002) reported for some coastal molluscs. The correspondence
in modal size class for the number of individuals and species may
arise because of the strong correlation between the number of
individuals and number of species within a size class.The number
of species scales to the number of individuals as S = cI0.616
, a
finding similar to that of Siemann et al. (1996, 1999) for grassland
arthropods. However, there is some variation, although not significant,
in the exponent among the depth regions. This disparity
in the scaling factor is also seen among taxonomic orders in the
results of Siemann et al. (1996). The modal size class for number
of individuals and number of species also displays some variation
among depth regions.
The right-skewed relationship for the number of species
among size classes at the regional level appears to be ubiquitous
over an assortment of organisms and habitats (reviewed in
Brown, 1995). Numerous models have attempted to explain this
pattern (e.g. Hutchinson & MacArthur, 1959; Brown et al., 1993;
Kozlowski & Gawelczyk, 2002). It is likely that that the right skew
in body size distributions reflect multiple influences. Currently,
the data do not exist to allow direct testing of any proposed
mechanisms by which these relationships arise.
At smaller scales, decomposing the regional relationship into
depth zones, the right-skewed trend begins to break down. In all
three regions the curves represent an intermediate between a
log-normal and a right-skewed distribution. These findings are
consistent with those of Brown & Nicoletto (1991) for terrestrial
mammals (see also Marquet & Cofre, 1999; Bakker & Kelt, 2000).
Their findings demonstrate a movement from a right-skewed
trend at the biome level to a uniform distribution at the patch
level. Although, this study does not examine patch size scales,
a trend away from the right-skewed regional relationship is
observed. The breakdown of the trend at finer scales in other systems
has been attributed to higher spatial turnover in species of
the modal size classes (Brown & Nicoletto, 1991). This suggests
that these modal-sized species occupy narrower ranges and may
be more specialized than their larger or smaller counterparts
(Brown, 1995). For the deep-sea gastropods here, depth range
sizes show no distinctive pattern over body size classes, suggesting
that modal-size species are not more specialized. Although
depth may serve as the important geographic gradient for deepsea
species, it should be noted that depth represents only one axis
of species distributions. Not accounting for latitudinal and longitudinal
gradients, and limited deep-sea sampling, may account
for the lack of a size/range trend.
In most of the deep sea,apart from chemosynthetic communities
(Van Dover, 2000), organisms rely on the import of organic
material from surface production to the sea floor for food.
Energy flux to the benthos displays both latitudinal, depth, and
offshore gradients (Rowe & Pariente, 1992). This nutrient input
controls a species' density, body size, growth, life history, and
Figure 2 (a) Relationship of the log of species
richness (S) per log2 size class where size = shell
volume in mm3
. The line represents a kernel
smoothing curve fitted to the data. Numbers
refertothelog2 sizeclasses(seeMethodsforclass
size ranges). (b) Relationship of the log of
number of individuals (I) per log2 size class.
The line represents a kernel smoothing curve
fitted to the data. (c) Relationship between log
of species richness (S) and log of number of
individuals (I) per log2 size class. The line
represents a fitted regression (OLS:
S = -0.406I0.616
, R2
= 0.719, P = 0.0005;
RMA: S = -0.613I0.726
, R2
= 0.848,P = 0.0500).
(d)Relationshipsbetweenlog of species richness
(S), log of number of individuals (I), and log2
size class.The curve represents the fitted kernel
smoothing curve from A and B.
Species richness, abundance and body size in deep-sea gastropods
Global Ecology and Biogeography, 13, 327­334,  2004 Blackwell Publishing Ltd 331
physiology (Gage & Tyler, 1991). Through the regulation of these
energetic factors, it is feasible that organic input could direct
the equitability of relative abundances among species, species
makeup, and the number of species. Some differences in the distribution
of species and individuals among size classes do exist
among the slope, rise, and abyss, indicating that depth and possibly
the rate of carbon flux to the benthos (Etter & Mullineaux,
2001) is related to how body size, abundance, and species
richness interrelate. At the slope and rise (high and low nutrient
input),a right-skewed trend is observed,but at intermediate levels,
the relationship is log normal. The exact mechanism by which
energy flux into a system could dictate these relationships is still
not completely understood and would be speculative at this time.
However, recent advances in spatial scaling theory have interrelated
the availability of niches, productivity, area, body size,
and species diversity (Ritchie & Olff, 1999; Haskell et al., 2002).
Sampling procedure may have an influence on the distribution
of individuals and species among size classes. It has been
suggested in previous studies (e.g. Currie, 1993) that the
right-skewed relationship exists because of the undersampling of
smaller species. This is especially relevant in the deep-sea where
remote and limited sampling raises questions about the reliability
of detected trends. In most deep sea studies a mesh size of
300­420 m is used to separate the macrofaunal component,
which includes the gastropods in this study. This sieve size often
retains individuals smaller than 300 m mesh. The results of this
study indicate that most individuals are larger than 2.5 mm. This
sieve size aperture adequately samples extremely rare and small
species such as Palazzia planorbis (Rex, 2002). This suggests that
the patterns observed herein are not likely to be an artifact of too
coarse a mesh size.
This is the first time the association between body size,
abundance, and number of species has been characterized in the
deep sea or over a presumed productivity gradient. This work
corroborates the observation, by Siemann et al. (1996), that the
relationship between the number of species and the number of
individuals may be independent of body size. However, it should
be acknowledged that there is some variation in this relationship
Figure 3 Relationship of the log of number
of species (S) and individuals (I) per log2 size
class (mm3
) for slope (200­2000 m), rise
(2000­4000 m), and abyss (> 4000 m).
Numbers refer to the log2 size classes (see
Methods for class size ranges). The lines
represent kernel smoothing curves fitted
to the data.
C. R. McClain
332 Global Ecology and Biogeography, 13, 327­334,  2004 Blackwell Publishing Ltd
between taxa and depth; and it is unclear how body size relates to
this. Although numerous studies have found unimodal trends
between number of species and individuals across size classes
(Siemann et al., 1996 and references within), the present work
adds to the growing body of evidence of a right-skewed relationship.
I tentatively suggest that the variation in the form of this
relationship may in part reflect differences in scales of the studies
or productivity input into the community. Because there are differences
among habitats and taxa, it is interesting that a similar
right-skewed trend in the number of species among size classes is
seen in groups as different as deep-sea invertebrates presented
here and terrestrial mammals (Brown & Nicoletto, 1991).
Hypotheses about how size, abundance, and diversity interact
should, if they are to be general, include factors that apply to
deep-sea assemblages.Numerous studies suggest that one common
underlying factor among all these environments is the strong
influence of nutrient input on community structure (Ricklefs &
Schluter, 1993). The results presented herein indicate that depth,
a correlate of productivity, may affect body size, abundance, and
species diversity. It is likely that the relationship between these
three variables is affected by the overall amount of and variance
in energy flow into the assemblage.
ACKNOWLEDGEMENTS
I thank R. Stevenson, M. Foote, M. Rex, and R. Etter for helpful
comments on the manuscript and research. E. White, A. Allen,
and M. Ernest also provided invaluable insight into this manuscript
and project. I am also grateful to A. Clain and R. Jabbour
for providing some of the size measurements in this study. I also
thank D. A. Fa, A. Clarke, and R. J. Whittaker, who reviewed
and greatly improved this manuscript. This research is funded
by National Science Foundation grants GER-9552836 and
OCE-0135949 to M. Rex.
Figure 4 Relationship between log of species richness (S) and log of
number of individuals (I) per log2 size class. Numbers refer to the
log2 size classes (see Methods for class size ranges). The solid lines
are the fitted OLS regressions (Slope: S = -0.210I0.465
, R2
= 0.9130,
P > 0.0001. Rise: S = -0.027I0.411
, R2
= 0.460, P = 0.0218. Abyss:
S = -0.035I0.364
, R2
= 0.870 P < 0.0001) and dashed lines are the
fitted RMA regressions (Slope: S = -0.242I0.487
, R2
= 0.956,
P > 0.0050. Rise: S = -0.305I0.605
, R2
= 0.678, P = 0.0050. Abyss:
S = 0.072I0.390
, R2
= 0.933, P = 0.0050).
Figure 5 Relationship of the log of number of individuals (I)
and the log of species richness (S) per log2 size class (mm3
) for
vetigastropods and caenogastropods. Numbers refer to the log2
size classes (see Methods for class size ranges). The lines represent
kernel smoothing curves fitted to the data.
Species richness, abundance and body size in deep-sea gastropods
Global Ecology and Biogeography, 13, 327­334,  2004 Blackwell Publishing Ltd 333
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BIOSKETCH
Craig McClain has research interests in characterizing
and understanding the complex biodiversity patterns in
the deep sea from the perspective of the morphological
disparity, body size, biomass, and abundance of deep-sea
organisms.