ORIGINAL RESEARCH
published: 03 November 2016
doi: 10.3389/fevo.2016.00125
You Are What You Eat: Stable
Isotopic Evidence Indicates That the
Naticid Gastropod Neverita duplicata
Is an Omnivore
Michelle M. Casey 1* † , Leigh M. Fall 2 † and Gregory P. Dietl 3, 4 †
1
Geosciences Department, Murray State University, Murray, KY, USA, 2 Earth and Atmospheric Sciences, State University of
New York Oneonta, Oneonta, NY, USA, 3 Paleontological Research Institution, Ithaca, NY, USA, 4 Department of Earth and
Atmospheric Sciences, Cornell University, Ithaca, NY, USA
Edited by:
Jordi Figuerola,
Estacion Biologica de Doñana - CSIC,
Spain
Reviewed by:
Juan Carlos Senar,
Natural History Museum of Barcelona,
Spain
Francesca Rossi,
Centre National de la Recherche
Scientifique, France
Elena Angulo,
Estacion Biologica de Doñana - CSIC,
Spain
*Correspondence:
Michelle M. Casey
mcasey5@murraystate.edu
†
These authors have contributed
equally to this work.
Specialty section:
This article was submitted to
Behavioral and Evolutionary Ecology,
a section of the journal
Frontiers in Ecology and Evolution
Received: 03 May 2016
Accepted: 10 October 2016
Published: 03 November 2016
Citation:
Casey MM, Fall LM and Dietl GP
(2016) You Are What You Eat: Stable
Isotopic Evidence Indicates That the
Naticid Gastropod Neverita duplicata
Is an Omnivore.
Front. Ecol. Evol. 4:125.
doi: 10.3389/fevo.2016.00125
Species belonging to the family Naticidae (commonly called moon snails) are important
infaunal gastropod predators found in soft-bottom marine communities worldwide that
traditionally have been thought to prey on other mollusks, giving them the expected
trophic position of a predator (trophic position = 3). Realized trophic position estimates
of the naticid Neverita duplicata from Long Island Sound, however, range between
2.3 and 2.5, indicating omnivory or an anomalously low nitrogen (N) fractionation
factor. To evaluate the likelihood of omnivory, this study presents whole body stable
isotopic analysis of nitrogen and carbon from the soft tissues of laboratory-reared
and field-collected N. duplicata. Experimental organisms were maintained on a diet of
the bivalve prey Mercenaria mercenaria for 1 year. The median N fractionation factor
derived from the experimental moon snails was 3.58‰ thus precluding the presence
of an atypical fractionation factor (substantially lower than 3.4‰). Numerous molluscan
taxa were collected from Long Island Sound in order to evaluate the trophic ecology
of N. duplicata in the context of a natural food web. Evidence from the carbon (C)
signatures of field-collected N. duplicata indicate a reliance on littoral food sources
that is inconsistent with a diet of filter-feeding M. mercenaria, even when calculated
using the species-specific C fractionation factor derived from the laboratory experiment.
Field-collected N. duplicata also show considerable isotopic overlap (N and C) with
grazing Littorina littorea. For these reasons, we hypothesize that N. duplicata feeds on
some combination of benthic primary producers (most likely macroalgae and/or epiphytic
diatoms), carrion, and bivalve/gastropod tissue and discuss the possible identity of plants
consumed.
Keywords: carbon, diet, drilling predation, realized trophic position
INTRODUCTION
Species belonging to the globally distributed marine gastropod family Naticidae—commonly
referred to as moon snails—have traditionally been considered to be predators that drill distinctive,
circular holes in the shells of their bivalve and gastropod prey to gain access to the soft tissues
inside (Carriker, 1981; Kitchell et al., 1981). Laboratory experiments confirm that moon snails drill
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Omnivory in the Moon Snail Neverita duplicate
eight published estimates of marine gastropod 115 N and 113 C
derived from the assimineid species Assiminea japonica and
Angustassiminea castanea (Kurata et al., 2001). These estimates
vary dramatically with food source, from 115 N = −0.76‰ when
fed marsh litter to 115 N = 5.73‰ when fed diatoms and from
113 C = −0.10‰ when fed deposited seston to 113 C = 5.55‰
when fed soil. No data have been published on the 115 N or 113 C
of naticids.
This study uses stable isotopic data to evaluate the possible
causes of the observed discrepancy in the N and C signatures
of N. duplicata in Long Island Sound. Stable isotopic evidence
from a controlled laboratory feeding experiment and the soft
tissue isotopic signature of several mollusk species, including
N. duplicata, collected within the context of a natural food web
will be incorporated to test the following hypotheses: (1) N.
duplicata possesses a N discrimination factor significantly lower
than 3.4‰; (2) N. duplicata engages in trophic omnivory in the
wild; and (3) N. duplicata possesses a high C discrimination factor
that artificially inflates its apparent reliance on littoral C sources
in the wild.
These hypotheses will be evaluated based on the following
predictions: (1) if N. duplicata feeds predominantly on mollusks
in the wild, but has a low N discrimination factor (hypothesis
1), we expect experimental moon snails to have N signatures
statistically indistinguishable from wild-caught moon snails
sacrificed before the start of the experiment (pre-experiment
individuals); (2) if instead N. duplicata is an omnivore in nature
(hypothesis 2), we expect a bivalve-only diet of Mercenaria
mercenaria will result in a 1‰ or more increase in the average N
signature of experimental moon snails relative to pre-experiment
individuals; (3) if N. duplicata’s high C discrimination factor is
responsible for its apparent reliance on littoral C (hypothesis 3),
we expect field-collected N. duplicata to yield dietary proportion
estimates that are predominately pelagic when calculated using
the taxon-specific C discrimination factor derived from the
laboratory experiment; (4) if instead N. duplicata feeds on littoral
primary producers or their secondary consumers (i.e., hypothesis
3 is incorrect), we expect field-collected N. duplicata to continue
to yield predominantly littoral estimates of dietary proportion,
even when calculated using the taxon-specific C discrimination
factor derived from the laboratory experiment.
using a combination of radular scraping and the application of
weak acid produced by an accessory boring organ (Kabat, 1990),
and consume all or most prey tissue (Carriker, 1981; De Angelis
et al., 1985; Kitchell et al., 1986; Visaggi et al., 2013). However,
recent stable isotopic analyses—a powerful tool for determining
dietary sources and complex trophic interactions (Post, 2002;
Layman et al., 2012)—yielded realized trophic positions1 for
the polinicine naticid Neverita duplicata between 2.3 and 2.5 in
Long Island Sound (Casey et al., 2014). The lower than expected
trophic positions reported by Casey et al. (2014) resulted from a
combination of unexpectedly high carbon signatures and lower
than expected nitrogen signatures (Casey and Post, 2011; Casey
et al., 2014). The high C signatures indicate that N. duplicata
relies heavily on littoral C sources (those that grow along the
surface of the seashore). Casey and Post (2011) reported that
between 42 and 100% of the diet of N. duplicata from Long
Island Sound was derived from littoral sources. These values are
inconsistent with a diet of primarily filter-feeding bivalve prey,
which show pelagic (more negative) C signatures due to their
consumption of phytoplankton. In the absence of factors that
can be accounted for methodologically (e.g., isotopic routing,
differential lipid concentration, baseline effects; Casey and Post,
2011 and references therein) the discrepancy in N and C
signatures may be attributed to trophic omnivory, or feeding
from multiple trophic levels (sensu Pimm and Lawton, 1978).
In this case, N. duplicata may be feeding on some combination
of mollusks and littoral primary producers, making them true
omnivores.
Conversely, if N. duplicata is an obligate predator, their low
N signatures and high C signatures might be the result of
taxonomic differences in discrimination factor, or the difference
between the isotopic signatures of source and consumer. Casey
and Post (2011) and Casey et al. (2014) employed average
discrimination factors for N and C. These average fractionation
factors (3.4‰ for N and 0.0‰ for C) were derived experimentally
from multiple taxonomic groups (Minagawa and Wada, 1984;
Post, 2002) and universally applied to all taxa in food web
analyses. Early compilations of discrimination factors targeted
diverse organisms from insects to mammals and yielded average
discrimination factors showing a ∼3.0‰ enrichment in N
signature per trophic level increase (Deniro and Epstein, 1981;
Minagawa and Wada, 1984) and a 0–1‰ enrichment in C per
trophic level increase (Deniro and Epstein, 1978). Increased
taxonomic sampling has revealed greater interspecific variation
in discrimination factors (Vander Zanden and Rasmussen, 2001;
Post, 2002; McCutchan et al., 2003; Vanderklift and Ponsard,
2003; Caut et al., 2009), but provided very little information on
the N discrimination factors (115 N) and C discrimination factors
(113 C) of marine gastropods. To our knowledge, there are only
MATERIALS AND METHODS
Laboratory Experiment
To evaluate hypothesis 1, moon snails from Long Island
Sound (LIS) were collected and fed individuals from one of
their common bivalve prey species to examine the N and C
discrimination factors recorded in their tissue under controlled
feeding conditions. The laboratory food chain was controlled at
all levels from primary producer through primary consumer to
secondary consumer, therefore, N and C signatures were not used
to calculate tropic position in the same manner employed in food
web analyses (see Food Web Analysis Section below).
Forty moon snails (N. duplicata) were collected from Milford,
Connecticut (CT) (41.209660◦ , −73.052572◦ , ± 250 m) on
15 August 2013 during low tide and transported to the
1 The
potential trophic position of predatory moon snails, or the value expected
based on the list of possible trophic interactions for the group (Kling et al., 1992),
is 3.0, though it may be as high as 3.5 in situations where confamilial predation is
common (Carriker, 1951; Kitchell et al., 1981; Chattopadhyay et al., 2014). Realized
trophic position, calculated using both N and C signatures (Post, 2002) often
deviates from potential trophic position as it reflects the identity and proportion
of resources actually used (Kling et al., 1992) given that animals do not consume
every possible food item at all times or in equal proportions.
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invertebrate experiments reported in the literature. For marine
invertebrates, the duration of experiments performing N and C
analyses have ranged from 60 days to 6 months (e.g., Doering
et al., 1986; Rudnick and Resh, 2005; Piola et al., 2006). On
13 August 2014, at the end of the year-long experiment, all
remaining moon snails were frozen for isotopic analysis.
Potential biasing factors were assessed to insure that they
were not influencing the isotopic signatures of post-experiment
moon snails, including: starvation or stress of experimental
moon snails, differences in body size, and isotopic variability
of the laboratory food source. In order to test for any effects
consumption rate may have had on N signature, consumption
(measured as total number of consumed hard clams and average
bi-weekly consumption rate) was plotted against N signature.
A negative correlation between hard clam consumption and N
signature may indicate that high N signatures resulted from
protein catabolism in starving animals rather than from feeding
on hard clam prey. In addition, moon snails were checked daily
for signs of stress that might indicate insufficient nutrition and
possible protein catabolism (e.g., lack of burrowing, retraction of
soft tissues into the shell, discoloration of soft tissue). To ensure
that the largest individuals were not maintaining pre-experiment
isotopic signatures due to slow tissue turnover (indicated by
a negative correlation between body size and N signature;
(Sweeting et al., 2005), whorl diameter (as a proxy for body size)
was regressed against N signature. At the time of each water
change out, one randomly selected hard clam from each hard
clam holding tank was frozen to evaluate the isotopic baseline and
homogeneity of the food source, in terms of both N and C, over
the course of the experiment. These data were used to rule out
shifts in the isotopic signatures of the hard clam food sources as a
potential driver of any observed changes in the N or C signature
of post-experiment moon snails.
Paleontological Research Institution (PRI) in Ithaca, New York
(NY). Neverita duplicata represents the secondary consumer
in the experiment. Fifteen of the moon snails were randomly
selected to include a wide size range and to provide a sufficient
number of individuals to capture variation in N signature. Moon
snail specimens ranged in whorl diameter (width) from 21.1 to
36.1 mm (Table 1). Moon snails were not sexed and represent
a mixture of males and females. On 16 August 2013, 15 moon
snails were placed in separate 37.9 liter, closed-recirculating tanks
to avoid competition among predators for prey resources. Tanks
were filled with LIS sand to a depth of 10 cm, sufficient for both
predator and prey to burrow. Sand was previously sieved, rinsed
with freshwater, and stored dry before use. The remaining 25
moon snails (pre-experiment individuals) were frozen to provide
starting N and C signatures of the experiment. Every 2 weeks
the water in the 15 experimental tanks was changed out with
seawater made from Instant Ocean R . Temperature and salinity
in the tanks was maintained between ∼21–23◦ C and ∼32–34‰,
respectively.
Infaunal bivalves are a common prey for moon snails. The
hard clam Mercenaria mercenaria was chosen as the experimental
prey item because this primary consumer is widely known to
be included in the diet of N. duplicata (Edwards, 1974; Kitchell
et al., 1981; Visaggi et al., 2013) and is easily accessible. The filterfeeding diet of M. mercenaria is reflected in their pelagic (more
negative) C signatures (Casey and Post, 2011). Approximately
2000 individuals (antero-posterior width = 8.1 ± 0.4 mm, and
height = 7.2 ± 0.4 mm; mean ± SD) were obtained from the
Aquaculture Program at the Cornell Cooperative Extension of
Suffolk County, NY. Fifteen were frozen to provide starting N
and C signatures, and the remaining hard clams were kept in six
37.9 liter bivalve stock tanks filled with 10 cm of LIS sand. At the
start of the experiment, hard clams were approximately evenly
divided among the tanks, with about 330 individuals in each. This
number of individuals corresponds to 232 clams/m2 , which is
well below the densities typically found in aquacultural nurseries
(Hadley and Manzi, 1984). When the water was changed out
every 2 weeks, hard clams were fed 250 ml of a prepared algae
(Micro Algae GrowTM ), which serves as the primary producer
at the base of our food chain. The exclusion of littoral primary
producers from the laboratory setup precluded experimental
organisms from having mixed diets (e.g., feeding on multiple C
sources).
Moon snails were fed five hard clams from alternating bivalve
stock tanks every 2 weeks when the water was changed out.
For example, a snail was fed five randomly selected hard clams
from stock tank 1 and then hard clams from stock tank 2 the
following change out. Before new hard clams were added, the
number of drilled hard clams was recorded and any uneaten
hard clams were removed, placed in a holding tank, and not used
again. The number of hard clams consumed by each moon snail
during each 2-week period was recorded. Any snail found dead
was immediately frozen for stable isotopic analysis in order to
minimize the effects of decay on isotopic signature.
To ensure that N and C from the experimental diet had
ample time to be incorporated into the tissue of N. duplicata,
the experiment was run for 1 year, exceeding durations of other
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Food Web Analysis
To evaluate hypothesis 2, that N. duplicata engages in omnivory
in the wild, the isotopic signatures of a variety of bivalve
and gastropod taxa collected by Casey et al. (2014) from
Milford, CT in 2007 and multiple locations within LIS during
the summer of 2008 were reanalyzed. Taxa collected include:
filter-feeding mussels (Mytilus edulis and Geukensia demissa),
grazing periwinkles (Littorina littorea), deposit feeding mud
snails (Tritia obsoleta, formerly Ilyanassa obsoleta), filterfeeding hard clams (M. mercenaria), filter-feeding slipper
limpets (Crepidula fornicata), predatory naticids (N. duplicata),
predatory muricids (Urosalpinx cinerea and Nucella lapillus), and
predatory channeled whelks (Busycotypus canaliculatus). This
food web incorporates all of the common shallow water N.
duplicata prey taxa identified in accumulations of dead mollusk
shells from sediments in LIS by Casey et al. (2014). From west
to east, the 2008 sites include: Rye, NY, Bridgeport, CT, Milford,
CT, Guilford, CT, and Westerly, Rhode Island (Figure 1). When
comparing organisms from multiple sites, isotopic baselines, or
the isotopic signatures at the base of the food web, can vary
widely and bias diet or trophic position estimates. Therefore, we
followed Casey et al. (2014) and used baseline proxies to account
for differences in the C and N signatures of plants at the base
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TABLE 1 | Isotopic signatures, discrimination factors, body size, and consumption rate for experimental Neverita duplicata.
Snail
δ15 N (‰)
δ13 C (‰)
115 N
Whorl diameter (mm)
Shell height (mm)
Total no. clams consumed
Consumption rate◦
LIS 1#
na
na
na
28.0
22.5
109
4.7
LIS 2
13.8
−8.7
3.60
27.8
23.5
95
4.1
LIS 3+
13.2
−8.4
3.02
22.8
20.8
62
4.1
LIS 4
13.3
−10.0
3.10
23.6
18.8
110
4.8
LIS 5
13.9
−9.1
3.69
24.6
21.5
108
4.7
LIS 6∧
13.3
−8.1
3.11
27.7
22.4
70
5.0
LIS 7
14.3
−8.9
4.12
28.0
23.8
107
4.7
LIS 8
12.8
−8.4
2.53
25.8
22.6
70
3.0
LIS 9
13.2
−9.1
3.01
24.7
21.6
63
2.9
LIS 10
13.5
−8.8
3.25
21.1
18.9
114
5.0
LIS 11
13.8
−8.6
3.56
30.5
27.4
57
2.5
LIS 12*
15.4
−6.9
5.19
30.0
26.1
51
3.9
LIS 13a
14.3
−6.3
4.05
34.2
29.0
56
3.3
LIS 14†
14.2
−7.5
3.96
35.1
22.6
43
3.1
LIS 15**
15.4
−7.4
5.19
36.1
35.6
0
0.0
∆15 N, nitrogen discrimination factor.
◦ Consumption rate, average number of clams eaten every two weeks.
# Individual died during the 12th month of the experiment. Tissue completely decayed (8/13/14).
+ Individual died during the 8th month of the experiment (3/31/14).
∧ Individual died during the 8th month of the experiment (3/21/14).
*Individual died during the 6th month of the experiment (2/28/14).
a Individual died during the 9th month of the experiment (4/18/14).
†
Individual died during the 8th month of the experiment (3/18/14).
**Individual died during the 7th month of the experiment (2/28/14).
of the food web (Post, 2002; Casey and Post, 2011). Carbon
and N signatures of field-collected N. duplicata and their relative
position within the greater LIS food web were used to identify
or exclude potential plant food sources. Published isotopic values
were obtained from the literature for LIS upland land plants and
marsh grass detritus (Peterson et al., 1985)2 and compared to the
C signatures of field-collected moon snails. In the context of the
present study, the isotopic data were used to evaluate the trophic
position of N. duplicata within a natural food web.
Mollusk soft tissue was removed from the shell, cleaned of
the digestive tract, dried at 40◦ C, and ground to a powder
using a porcelain mortar and pestle or cryogenic grinder.
Laboratory samples were analyzed on a ThermoFinnigan MAT
253 Continuous Flow System with Elemental Analyzer at
the University of Kansas’ W. M. Keck Paleoenvironmental
and Environmental Stable Isotope Laboratory (K-PESIL). The
standard deviation of replicated standards was 0.14‰ for δ13 C
and 0.40‰ for δ15 N (K-PESIL). All isotopic signatures are
expressed in standard per mil notation. The standard reference
for C is Pee Dee Belemnite and for N is the atmosphere.
Because lipids tend to be depleted in 13 C relative to whole
body or bulk diet compositions, the C isotopic signatures of
experimental and field-collected specimens were lipid corrected
using the C:N ratio method of Post et al. (2007). Trophic
position of field-collected specimens was calculated using the
equations and baseline proxies of Casey and Post (2011). Two
end-member mixing models accounted for the baseline by
using the C signatures of baseline proxy taxa and target taxa
to calculate the proportion of dietary C derived from pelagic
primary producers (α), which was then incorporated into the
calculation of trophic position (Post, 2002; Casey and Post, 2011).
The effect of N discrimination factor on the trophic position
estimates was assessed by comparing trophic position calculated
using the 3.4‰ published estimate of average N discrimination
factor (Minagawa and Wada, 1984; Post, 2002) and the N
discrimination factor observed in the laboratory experiment.
Estimates of α were used to evaluate mixing of C sources in fieldcollected specimens. To evaluate hypothesis 3, estimates of A were
calculated twice, first assuming that 113 C = 0, and a second
Analytical Methods
Isotopic signatures of C and N from experimental specimens
were analyzed in the same manner used by Casey et al. (2014)
to allow direct comparisons to be made. Whole-body samples
were analyzed to preclude any differences in isotopic signatures
due to compositional differences between tissues or isotopic
routing, which complicate stable isotope studies where wholebody analysis is impractical or impossible (Schwarcz, 1991,
2002), and allow direct comparison with previously published
isotopic values of drilling gastropods (e.g., Casey and Post, 2011;
Casey et al., 2014). Isotopic routing, or the routing of dietary
components with different isotopic signatures to separate body
compartments such that isotopic signatures of individual tissues
do not reflect the bulk diet of the organism, is particularly
prevalent when dealing with potential omnivores (Layman et al.,
2012 and references therein).
2 Although these values were not collected at the same time as our study organisms,
given the extreme isotopic dissimilarity between upland land plants or marsh
grass and the rest of the food web under study, they are likely reasonable
approximations.
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FIGURE 1 | Map of Long Island Sound. Symbols show sample locations of field-collected (from Casey et al., 2014) and experimental specimens.
time using the average value of 113 C measured in the laboratory
experiment.
signatures of moon snails sacrificed before the experiment
ranged between 12.8 and 14.0‰ with a median value of 13.1‰
(Figure 2B), whereas the N signatures of post-experiment moon
snails ranged between 12.8 and 15.4‰ (Table 1) with a median
value of 13.8‰ (Figure 2B).
Statistical Methods
To test for significant differences in central tendency of isotopic
signatures and estimates of discrimination factors, the MannWhitney U (Wilcoxon) test, a non-parametric test of difference
in median, was performed. The non-parametric Kruskal-Wallis
H test for multiple comparisons was applied when comparing
medians of more than two groups. Non-parametric tests were
used because the distributions of isotopic signatures and
estimates of discrimination factors each failed a Shapiro-Wilk
test for normality making parametric tests for difference in mean
unsuitable. The relationships between N isotopic signature and
prey consumption (total number consumed and average number
consumed in a 2-week period) and N signature and body size
were evaluated using Pearson Product Moment Correlation tests.
Use of the less powerful, non-parametric, rank-based Spearman
correlation test did not change any patterns of significance. All
statistical analyses were run using PAST (Hammer et al., 2001).
Carbon
Carbon signatures of experimental N. duplicata varied
between −6.3 and −10.0‰ (Table 1). The difference in
median C signature between pre- and post-experiment moon
snails was not significant (Mann-Whitney U = −1.73, n1 = 9,
n2 = 14, p = 0.08). The median C discrimination factor (the
difference in δ13 C between post-experiment moon snails and the
mean bivalve signature) was 1.90‰.
Check for Laboratory Artifacts
None of the potential biasing factors evaluated could account
for the N pattern we found in the experiment. Experimental
moon snails exhibited no signs of stress (e.g., retraction into
shell, discoloration, discharge of fluids, abnormal burrowing
behavior). The N signature of post-experiment moon snails was
not significantly correlated with the total number of hard clams
consumed (r2 = 0.00, n = 14, p = 0.99) or the bi-weekly
consumption rate (r2 = 0.00, n = 14, p = 0.99). Baselinecorrected N signature of post-experiment moon snails was
significantly, positively correlated with body size, measured as
whorl diameter of the shell in mm (Supplementary Image 1; r2 =
0.48, n = 14 p = 0.01).
Minimal variation was evident in the isotopic signatures of
the hard clam food source used in the laboratory experiment
throughout the duration of the year-long study. There was
no significant difference in median δ15 N among hard clams
sacrificed during the first half of the experiment (August 2013–
January 2014, median = 10.26‰, n = 9) and those sacrificed
during the second half of the experiment (February 2014–August
2014, median = 10.11‰, n = 8) (Mann-Whitney U = −0.24, n1
= 9, n2 = 8, p = 0.81). There was, however, a small, significant
RESULTS
Laboratory Experiment
Nitrogen
Nitrogen discrimination factors of post-experiment moon snails
ranged between 2.53 and 5.19‰ (Table 1) with a median of
3.58‰ (Figure 2A). The 115 N estimates of pre-experiment
individuals ranged between 2.57 and 3.75‰ with a median
of 2.89‰ (Figure 2A). The post-experiment median 115 N
represented a statistically significant increase of 24% (MannWhitney U = −2.30, n1 = 9, n2 = 14, p = 0.02) relative to
the median 115 N of wild-caught moon snails sacrificed before
the experiment. This result was robust in spite of the removal
of the two post-experiment moon snails expressing the highest
115 N (Table 1) that died before the conclusion of the experiment
(Mann-Whitney U = −1.95, n1 = 9, n2 = 14, p = 0.05). Nitrogen
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Omnivory in the Moon Snail Neverita duplicate
FIGURE 2 | Box plots showing median, standard deviation, minimum, and maximum nitrogen discrimination factors and nitrogen signatures. (A)
Median nitrogen discrimination factor of moon snails sacrificed before the start of the experiment (Pre-experiment) and moon snails sacrificed after the year-long
experiment (Post-experiment). Discrimination factor calculated as nitrogen signature of moon snails minus average M. mercenaria nitrogen signature. (B) Nitrogen
signatures of experimental M. mercenaria including two outliers, pre-experiment moon snails, and post-experiment moon snails.
difference in the median δ13 C between hard clams sacrificed
during the first half of the experiment (median = −10.64‰) and
hard clams sacrificed during the second half of the experiment
(median = −10.13‰) (Mann-Whitney U = −2.84, n1 = 9, n2
= 8, p = 0.005). There were no significant differences between
hard clams from different holding tanks in terms of median N
signature (Kruskal-Wallis, H = 1.71, n1 = n2 = 3, n3 = 11, p =
0.43) or median C signature (Kruskal-Wallis, H = 0.10, n1 = n2
= 3, n3 = 11, p = 0.95). Carbon and N signatures of hard clams in
the laboratory were very similar to field-collected M. mercenaria
from Westerly (Supplementary Image 2).
were not significantly different from one another (Kruskal-Wallis
H test, H = 0.52, nRye = 4, nBridgeport = 17, nMilford′ 07 = 12,
nMilford′ 08 = 10, p = 0.91). Baseline-corrected N signature of N.
duplicata showed a weak but significant, positive correlation with
whorl diameter, as a proxy for body size (Supplementary Image
1, r2 = 0.28, n = 45, p = 0.008). Maximum baseline-corrected N
signatures occurred in the middle of the body size range.
Carbon
Neverita duplicata collected from LIS had the largest range of
C values of any taxon analyzed (Figures 3, 4). The range of
N. duplicata C signatures (measured as the difference between
site-specific maximum and minimum) was large: 2.8‰ at Rye,
5.7‰ at Bridgeport, 3.7‰ and 4.5‰ at Milford (years 2007 and
2008, respectively), and 4.1‰ at Guilford. The maximum N.
duplicata δ13 C recorded was −5.8‰ (Guilford); the minimum
N. duplicata δ13 C recorded was −16.5‰ (Milford in 2007). The
C signatures of N. duplicata frequently overlapped with those
of epiphytic grazers (L. littorea), omnivorous deposit-feeders
(T. obsoleta), and the inferred naticid prey (M. mercenaria)
(Figures 3, 4). The percentage of N. duplicata diet derived from
pelagic C sources (α) varied between 0% (total reliance on
littoral C sources) and 100% (total reliance on pelagic C sources).
The published C signatures of upland land plant detritus from
New England (Peterson et al., 1985) was δ13 C = −28.6‰, far
more negative than the range of C signatures of field-collected
N. duplicata. The median value of α calculated using the C
discrimination factor from the experimental results (1.90‰) was
α = 0.40, or 60% littoral, with 40.8% of moon snails (n = 20)
deriving more than two thirds of their diet from littoral sources
(α ≤ 0.33).
Isotopic Food Web Analysis
Nitrogen
In spite of an east to west increase in the isotopic baseline
for N, the relative position of N. duplicata within the food
web remained relatively constant across localities (Figure 3)
and sampling years (Figure 4). No N. duplicata were found at
the Westerly locality. The range of N. duplicata N signatures
(measured as the difference between site-specific maximum and
minimum) was low: 1.2‰ at Rye, 3.0‰ at Bridgeport, 1.5‰ and
2.5‰ at Milford (years 2007 and 2008, respectively), and 0.5‰ at
Guilford. Overlap was evident in the N signatures of N. duplicata
and those of epiphytic grazers (L. littorea), deposit-feeders (T.
obsoleta), and to a lesser extent, their inferred filter-feeding hard
clam prey (M. mercenaria) (Figures 3, 4). Published N signatures
for upland land plants (δ15 N = 0‰) and marsh grass detritus
(δ15 N = 4‰) from New England (Peterson et al., 1985), were
much lower than those of N. duplicata.
Using a N discrimination factor of 3.4‰ (Minagawa and
Wada, 1984; Post, 2002), median trophic position of N. duplicata
ranged from 1.9 at the Guilford locality to 2.3 at the Milford
locality in 2007; individual trophic positions ranged from a
minimum of 1.8 to a maximum of 2.5. Median trophic position
for the Guilford site was significantly different than all other sites
(Kruskal-Wallis H test, H = 13.67, nRye = 4, nBridgeport = 17,
nMilford′ 07 = 12, nMilford′ 08 = 10, nGuilford = 6, p = 0.01), which
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DISCUSSION
The significant increase in the median N signature of postexperiment moon snails relative to pre-experiment moon snails
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Omnivory in the Moon Snail Neverita duplicate
FIGURE 3 | Whole-body nitrogen and carbon signatures of field-collected mollusks sampled during the summer of 2008. (A) Rye, NY. (B) Bridgeport, CT.
(C) Milford, CT. (D) Guilford, CT.
2011; Casey et al., 2014). In fact, the median difference in
N signature between post-experiment moon snails and the
average value of hard clam prey was 3.58 ± 0.79‰ (median
± standard deviation), higher than the 3.4‰ average of Post
(2002), thus refuting hypothesis 1 that N. duplicata has a
below average N discrimination factor when feeding on M.
mercenaria.
Field evidence further supports omnivory based on the littoral
nature of N. duplicata’s C signatures. The nearshore, molluscan
food web from LIS (Figures 3, 4) shows an inclined isotopic
baseline, typical of both marine and lacustrine ecosystems
(Post, 2002; Casey et al., 2014), in which pelagic primary
producers display substantially lower (more negative) δ13 C and
slightly lower δ15 N signatures than littoral primary producers
(inclined solid line, Figure 4). The inclined isotopic baseline
accentuates the pattern of lower-than-expected trophic positions.
The difference in N signatures between littoral and pelagic
primary producers at the base of the food web means that a
N. duplicata with a more negative C signature (greater reliance
on pelagic C sources) has a higher trophic position than a
N. duplicata with a less negative C signature (greater reliance
on littoral C sources), even if both individuals have the same
δ15 N value. In addition to lower-than-expected δ15 N values,
N. duplicata has surprisingly high (more littoral) C signatures,
which is inconsistent with a diet dominated by filter-feeding hard
clams (e.g., M. mercenaria, Figures 3, 4). This pattern persists
when dietary proportion is calculated using the taxon-specific
113 C obtained from the laboratory feeding experiment, thus
refuting hypothesis 3.
FIGURE 4 | The Milford, CT, food web as sampled during the summer
of 2007. Dashed line indicates expected N. duplicata nitrogen signatures if
they are predators (average baseline proxy value + 3.4‰). Inclination of the
dashed line follows the inclination of the isotopic baseline as measured by
proxy taxa (solid line).
(Figure 2) indicates that preying exclusively on M. mercenaria
for 1 year represented a change in diet relative to that consumed
in the wild. This result supports hypothesis 2 that N. duplicata
from LIS feeds as an omnivore under natural conditions.
There is no evidence that N. duplicata has a N discrimination
factor lower than the 3.4‰ average used in previous studies
of drilling gastropod stable isotope ecology (Casey and Post,
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Omnivory in the Moon Snail Neverita duplicate
number of prey consumed. Furthermore, experimental hard
clam prey yielded N and C signatures similar to field-collected
con-specifics from LIS (Supplementary Image 2), and did not
change or exhibit any abnormalities through the course of
the experiment in a way that would explain the observed
increase in average N or C signature of experimental moon
snails.
One potential limitation of this study is the fact that the sex of
each post-experiment moon snail is not known. This precludes
the evaluation of differences in δ15 N signatures between the
sexes due to isotopic routing during the allocation of resources
toward reproduction (del Rio et al., 2009). However, because
fertilization of eggs occurs internally in N. duplicata (Hanks,
1960) the only way that the estimated N signatures could
have been biased by reproductive output was if egg collars
were produced by females. As no egg collars were observed in
the experimental tanks during the course of the experiment,
all reproductive tissues likely remained within the body of
experimental moon snails. The whole body tissue analysis
employed in this study captured the bulk isotopic signatures of
both male and female moon snails, including their internally
stored reproductive tissues. Thus, whole body isotopic analysis
precluded any isotopic routing in reproductive tissues from
substantially biasing the isotopic signatures measured during this
experiment.
The differential rates at which tissues are built or maintained
with constituents from a new diet, or tissue turnover rates, can
result in lingering effects of the previous diet’s isotope ratio
(McCutchan et al., 2003; Sweeting et al., 2005). Fast-growing
juveniles incorporate new nutrients into their body more quickly
than slower growing adults (Hentschel, 1998), meaning that
isotopic differences between individuals of varying sizes may
be an artifact of nutrient turnover rates (Rossi et al., 2004).
In the absence of data on rates of tissue turnover in marine
gastropods, experimental moon snails were maintained on a
hard clam diet for a full year as a precaution, longer than
many marine invertebrate diet-switching studies that typically
last weeks to 6 months (e.g., Doering et al., 1986; Rudnick and
Resh, 2005; Piola et al., 2006). A slow tissue turnover rate would
bias against seeing a shift in isotopic signatures even if the natural
diet of N. duplicata was different from their experimental diet.
Whereas, slight differences in tissue turnover rates may account
for the variability of N and C signatures recorded, the fact
that a significant increase in N signature was observed largely
negates the role of turnover rates as a source of experimental
bias. In the absence of viable alternative explanations for the
observed increase in median N signature of post-experiment
moon snails, the experimental results refute hypothesis 1, that
moon snails have a below average 115 N, and support hypothesis
2, that N. duplicata feeds on some type of plant material in
the wild.
Finally, the high variability of moon snail isotopic signatures
supports an omnivorous diet. Although not necessarily true of
all omnivores, high intra-specific variability in diet (in terms of
both N and C signature) is consistent with omnivory as individual
diets of omnivores are often highly variable due to differences in
foraging behaviors, prey preferences, handling capabilities, or the
spatial heterogeneity of food sources to which they are exposed
(Griffen and Griffen, 2014).
Taken together, these results are surprising—given the
extensive body of research that has been conducted on the
predatory behavior and prey preferences of N. duplicata
(Kitchell et al., 1981)—and likely have broad ecological
implications (see Appendix 1). However, multiple alternative
explanations for the observed pattern—artifacts related to
stress or food source abnormalities and explanations provided
by the biology or behavior of naticids—must first be ruled
out before accepting a revision of N. duplicata’s trophic
ecology.
Alternative Explanations
Laboratory Artifacts
Several factors that affect isotopic signatures were controlled for
methodologically in our experiment (e.g., lipid corrections to C
signatures and the use of whole body tissue analysis to prevent
impacts from tissue-specific effects or isotopic routing). Several
other factors known to influence isotopic signatures—tissue
decay of dead experimental organisms, predator starvation and
protein catabolism, abnormalities or temporal variation in the
isotopic signature of the experimental prey, reproductive output,
and size-dependent differences in rates of tissue turnover—were
not controlled for in our experimental design. However, each of
these factors can largely be eliminated as a source of potential
bias.
The highest N discrimination factor estimates were observed
in two moon snails that died before the conclusion of the
experiment (Table 1; both individual’s 115 N = 5.19‰). One of
the moon snails (LIS 12) had been feeding but died before the
conclusion of the experiment and was frozen within 36 h after
death to prevent decay of tissues. It is unclear whether short
periods of decay (less than 36 h) would lead to significantly
enriched N signatures (Payo-Payo et al., 2013). LIS 15 was the
other high N discrimination factor moon snail. This individual
was the largest moon snail in the study and did not feed
(likely because the prey were too small). Tissues of starving
animals show steady enrichment in 15 N as lean body mass
decreases due to catabolism of the body’s own proteins and
preferential excretion of light N (Gannes et al., 1997 and
references therein). It is worth noting, however, that omission
of these two moon snails does not change the significance of
the observed increase in median 115 N. It is unlikely that the
other experimental moon snails were starving or experiencing
stress that would cause any unexpected alterations of the N
signature as they consumed large numbers of prey (Table 1).
The N signature of post-experiment moon snails showed no
correlation with either the number of hard clams consumed
or the average biweekly rate of hard clam consumption, which
indicates that the high N signatures are not the result of the
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Isotopic Food Web Analysis
Several factors may affect the N and C signatures of fieldcollected moon snails, including cannibalism, changes in diet
with growth (ontogenetic niche shifts), some physiologic or
metabolic process unique to predatory marine gastropods, or
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Omnivory in the Moon Snail Neverita duplicate
evaluated by comparing N. duplicata with other field-collected
marine gastropod predators. The channeled whelk, Busycotypus
canaliculatus, is a gastropod predator that preys on bivalves by
wedging its prey’s shell open using the lip of its own shell. The
channeled whelk shows congruence between its potential trophic
position (3) and realized trophic position (2.9) (Supplementary
Table 1; Figure 4). The realized trophic position of the channeled
whelk thus confirms that lower than expected trophic positions
of N. duplicata are not characteristic of all gastropod predators
(although they may be present within another family of drilling
gastropods; see Appendix 2).
To evaluate the possibility that a higher than expected C
fractionation factor biased results (hypothesis 3), field-collected
samples were evaluated using the estimates of 113 C derived in
our laboratory experiment. The pattern of higher-than-expected
(more littoral) C signatures may be partially explained by the
laboratory C discrimination factor of + 1.90 ± 0.99‰ (median
± SD). This +1.90‰ C discrimination factor is in contrast to
the 0.0 to + 1.0‰ difference between source and consumer cited
in most compilations of average C discrimination factor (Vander
Zanden and Rasmussen, 2001; Post, 2002; McCutchan et al., 2003;
Vanderklift and Ponsard, 2003; Caut et al., 2009). Incorporation
of the laboratory C discrimination factor (1.9‰) into calculations
of α for moon snails from the field yielded a distribution of
α that is not consistent with a diet of predominately filterfeeding (pelagic-sourced) prey or assimilation of undigested
phytoplankton present in the gut of filter-feeding prey, thus
refuting hypothesis 3. The littoral nature of these C signatures
is surprising and cannot be explained by the unintentional
inclusion of detritus or undigested food from the gut in the
processed isotopic samples, given the extremely small digestive
tract of N. duplicata (Strong, 2003). Nor are the N signatures
of N. duplicata high enough to be consistent with a diet that
includes prey that derive C from littoral sources, e.g., littoral
grazers such as the periwinkle, L. littorea, or omnivorous depositfeeders such as the mud snail, T. obsoleta (see dashed line in
Figure 4). In the absence of viable alternative explanations for
the observed trophic position and littoral C signatures, the field
data support hypothesis 2, that N. duplicata feeds as an omnivore
in nature. For this reason, we hypothesize that N. duplicata feeds
on some combination of benthic primary producers, carrion, and
bivalve/gastropod tissue.
higher than expected C fractionation factor. As was the case
for laboratory artifacts, each of these factors can be largely
eliminated from this study. Neverita duplicata is known to
be a frequent con-specific or con-familial cannibal (Carriker,
1951; Kitchell et al., 1981; Dietl and Alexander, 1995). If
cannibalism was prevalent among the LIS specimens sampled,
cannibalistic moon snails would have increased N signatures
relative to those of other N. duplicata, rather than lower
than expected N signatures, and trophic position greater than
3. According to Chattopadhyay et al. (2014) cannibalism is
more common in large-bodied individuals than small-bodied
individuals. Although there is a weak but significant, positive
correlation between body size and baseline-corrected N signature
for field specimens (Supplementary Image 1), none of the N.
duplicata sampled has a trophic position greater than 3 that
would indicate cannibalism.
Life-history omnivory, or switching between plant and animal
food sources at different life stages, is likely to affect isotopic
signature. Neverita duplicata has a pelagic larval stage that feeds
on microalgae (Hanks, 1960). As all specimens of N. duplicata
sampled from the field were 17 mm in whorl diameter or larger
(Supplementary Image 1), or approximately 9 months to 1
year in age (Hanks, 1960; Edwards and Huebner, 1977), it is
very unlikely that any individuals retained veliger stage isotopic
signatures that could explain the omnivorous δ15 N values
measured in this study. It is important, however, to distinguish
between discrete post-metamorphosis or larval diet changes
and the more gradual ontogenetic niche shifts documenting
changes in preferred bivalve and gastropod prey with increasing
naticid size. Many naticids alter their prey preferences as
they increase in body size to favor larger individuals and
larger taxa (Kabat, 1990; Clements and Rawlings, 2014). The
weak, positive correlation between field-collected N. duplicata
baseline-corrected N signature and body size (Supplementary
Image 1) may suggest that changes in bivalve and gastropod
prey preference affect N signature (i.e., represent a gradual
ontogenetic niche shift). As discussed above (see Laboratory
Artifacts Section), a positive correlation between body size and
N signature may be explained by the size-dependent nature
of isotopic turnover in tissues (McCutchan et al., 2003; Rossi
et al., 2004; Sweeting et al., 2005). The isotopic signature
of prey items (be they bivalves or plants) change seasonally,
therefore isotopic differences in moon snails of varying sizes
may reflect the variable rates at which large and small moon
snails incorporate this seasonally variable prey into their tissues
rather than a difference in diet. Therefore, more evidence will be
necessary to tease apart the effects of size-dependent differences
in nutrient incorporation rate from those of ontogenetic changes
in prey preference. Even if gradual ontogenetic niche shifts
do occur in N. duplicata, they do not explain why trophic
position is not greater than 2.5 in the field-collected moon snails
presumably feeding on bivalve and gastropod prey. Nor are
gradual ontogenetic niche shifts consistent with the fact that
the highest trophic position estimates do not correspond to the
largest individuals.
The presence of a physiologic or metabolic process that
may bias the N signature of marine predatory gastropods was
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Possible Identity of Plants Consumed
Our hypothesis that N. duplicata includes plant material in its diet
is not unprecedented within the Naticidae. Using microscopic
gut content analysis, Bernard (1967) noted that post-settlement
juveniles of the polinicine naticid Euspira lewisii from Vancouver
Island, British Columbia ate plant material both in the wild
and under experimental conditions (but see Pedersen and
Page, 2000). At first, juvenile E. lewisii consumed epiphytic
diatoms living on the macroalgae Ulva sp., and eventually
grazed on the macroalgae itself (Bernard, 1967). Consumption
of both epiphytic diatoms and Ulva sp. is consistent with the
isotopic signatures of field-collected N. duplicata discussed above.
Bernard (1967) further observed that E. lewisii transitioned to
a drilling habit during their 5th or 6th month of life, likely
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Omnivory in the Moon Snail Neverita duplicate
future dietary experiments or natural history observations on N.
duplicata.
coinciding with the seasonal disappearance of Ulva sp. during the
late winter to early spring. By contrast, Pedersen and Page (2000)
found that the presence of the algae Ulva sp. was not sufficient
to induce metamorphosis in E. lewisii and did not observe
juveniles consuming algae in the laboratory. Similarly, Hanks
(1960) found that juveniles (4–6 weeks post-metamorphosis) of
N. duplicata collected from Barnstable Harbor, Massachusetts,
did not consume an unidentified macroalgae under laboratory
conditions. The observations of Hanks (1960) and Pedersen and
? are inconsistent with our isotopic data and the observations of
Bernard (1967). These conflicting results highlight the need for
additional, careful feeding observations and experiments (both
field and laboratory based) with N. duplicata.
One way to identify potential plant species suitable for future
N. duplicata feeding experiments is to examine the diets of other
marine snails with similar C and N signatures in the food web
analyzed herein3 . The isotopic signature of N. duplicata shows
considerable overlap with mud snails and periwinkles, both of
which have well-documented diets. Mud snails are opportunistic
scavengers that primarily eat benthic diatoms through depositfeeding (Wetzel, 1977; Connor and Edgar, 1982) but have been
known to consume fish and crab carrion (Scheltema, 1964;
Curtis and Hurd, 1979; Feller, 1984), the sea lettuce Ulva lactuca
(Giannotti and McGlathery, 2001), detritus-associated bacteria
(Wetzel, 1977; Curtis and Hurd, 1979), and even the egg cases and
recently hatched juveniles of its competitor Cerithidea californica
(Race, 1982). Although the diet of T. obsoleta appears to be
analogous to that of N. duplicata because both feed from multiple
trophic levels, N. duplicata shows no signs of detritus within the
digestive tract (pers. obs.) making it highly unlikely that they act
as deposit-feeders.
The periwinkle L. littorea, also isotopically similar to N.
duplicata, is a grazer that feeds primarily on ephemeral,
foliose green algae, such as U. lactuca and Enteromorpha
intestinalis (Watson and Norton, 1985), and epiphytic diatoms,
including Melosira nummuloides, Ulothrix implexa, and the
ciliate Vorticella spp. (Sommer, 1999). Whereas, some members
of the genus Littoraria farm fungus on the stalks of the cordgrass
Spartina alterniflora in salt marsh habitats (Silliman and Newell,
2003), the periwinkles analyzed herein belong to the genus
Littorina and were collected exclusively from cobbles located in
sand flat habitats. Littorina littorea and N. duplicata both possess
taenioglossan (rake-like) radulae (Carriker, 1981; Steneck and
Watling, 1982) suitable for feeding on meat and a wide variety of
algae, including tough, leathery, or coralline forms (Steneck and
Watling, 1982) and epiphytic diatoms (those living on the surface
of other plants). It thus seems likely that N. duplicata may also
feed on epiphytic diatoms or macroalgae for which its radula is
well-suited. For these reasons, epiphytic diatom and macroalgal
species eaten by L. littorea make compelling candidates for
Recommendations
The results from this study can be used to generate
recommendations for future research on the ecology of N.
duplicata. The plants incorporated into the diet of N. duplicata
could be taxonomically identified using additional methods:
(1) Immunoassays of N. duplicata and potential food items
could be conducted to assess the presence of very broad or
very narrow categories of diet items in the gut depending on
the antisera developed, e.g., benthic diatoms vs. the benthic
diatom Melosira nummuloides (see Feller, 1984). (2) Radiometric
C labeling experiments are another promising means of
detecting the presence of specific littoral primary producers
in the diet of N. duplicata (see Wetzel, 1977). (3) Genetic
methods, including DNA barcoding, could be used to identify
aquatic plant species present in gut contents (see Saunders
and McDevit, 2012). (4) Laboratory experiments measuring
growth and survival of organisms maintained on herbivorous
vs. carnivorous vs. mixed diets could be used to differentiate
facultative from obligate omnivory (see Curtis and Hurd, 1979).
The individuals from these feeding experiments could also
be used to evaluate the potential decrease in N. duplicata’s N
discrimination factor associated with a change in diet quality
(specifically the effect of decreased diet quality—lower C:N
ratios—associated with consuming plants (Bearhop et al., 2004;
del Rio et al., 2009), although the application of the average
115 N value (3.4‰) is likely appropriate for future trophic
studies of moon snails where the identification of omnivory,
rather than the high resolution estimation of trophic position, is
the goal.
CONCLUSIONS
Stable isotopic evidence from a laboratory experiment and field
collections indicate that the naticid N. duplicata is an omnivore.
This new stable isotopic evidence indicates that N. duplicata
likely feeds on benthic diatoms or littoral marcoalgae in addition
to molluscan prey and scavenged carcasses. Neverita duplicata
shows a wide range of N discrimination factors when fed a diet
of known composition, the median of which is 3.58‰. Increases
in δ15 N signature of N. duplicata fed exclusively M. mercenaria
for 1 year cannot be explained by lab effects, predator starvation,
differential reproduction, or peculiarities in the hard clam food
source. The heavy reliance of field-collected N. duplicata on
littoral C sources supports the omnivory interpretation. Lowerthan-expected N signatures in field-collected moon snails cannot
be explained by cannibalism, taxonomic prey preference, or the
presence of ontogenetic niche shifts. Whereas preliminary results
from other predatory gastropod taxa (e.g., B. canaliculatus)
indicate that omnivory is not ubiquitous among predatory
marine gastropods, the spatial and taxonomic extent of this
pattern is not yet known. The prevalence of omnivory among
drilling gastropods will be key to exploring potential ecological
implications of omnivorous behavior.
3 Due
to the diffusion resistance of CO2 in water, littoral plants are unable to fully
express their fractionation factor as they are essentially utilizing a finite pool of C.
Because their C isotopic signature is driven primarily by this boundary layer effect
and not taxon-specific differences, it is impossible to distinguish between different
species of littoral primary producers in N. duplicata’s diet using isotopes because
their C signatures would be too similar.
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Omnivory in the Moon Snail Neverita duplicate
AUTHOR CONTRIBUTIONS
ACKNOWLEDGMENTS
MC, GD, and LF conceived and designed the experiment. GD
and LF performed the experiment. MC analyzed the isotopic
samples and resultant data. All authors contributed to the
writing of the manuscript. MC drafted Figure 1. LF drafted all
other figures.
We would like to thank the following: Tom Butler, Annalee
Tweitmann and Steve Durham (Cornell) for help running the
laboratory experiment; Gerry Olack (Yale), Greg Cane (KPESIL), and John Pollak (Cornell) for laboratory assistance;
Gregg Rivara (Cornell Cooperative Extension) for supplying M.
mercenaria prey; Daniel Casey, Joanna Wolfe, Úna Farrell, and
Emily Einstein, for help with field collections; and EA, FR, and JS
for thoughtful reviews.
FUNDING
This project was funded by the Geological Society of America
Graduate Student Research Grant, the Paleontological Society
Richard Osgood Student Research Award, the SUNY Oneonta
Faculty Research Grant Program, and the SUNY Oneonta
Individual Development Awards Program.
SUPPLEMENTARY MATERIAL
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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
The reviewer EA and handling Editor declared their shared affiliation, and
the handling Editor states that the process nevertheless met the standards of a fair
and objective review.
Copyright © 2016 Casey, Fall and Dietl. This is an open-access article distributed
under the terms of the Creative Commons Attribution License (CC BY). The use,
distribution or reproduction in other forums is permitted, provided the original
author(s) or licensor are credited and that the original publication in this journal
is cited, in accordance with accepted academic practice. No use, distribution or
reproduction is permitted which does not comply with these terms.
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APPENDIX 1: ECOLOGICAL IMPLICATIONS
could be a factor in explaining the occasional deviation of naticid
prey preferences from predictions based on cost-benefit analysis
(e.g., Kowalewski, 1990; Anderson et al., 1991).
Lastly, temperature has been demonstrated to shift the diet
preference of omnivores toward the increased consumption of
plants when temperatures are high and toward the increased
consumption of meat when temperatures are low for both
copepods (Boersma et al., 2016) and fish (Behrens and Lafferty,
2007). This pattern is expressed seasonally as well as latitudinally
(Boersma et al., 2016) and is likely driven by the increase in
performance derived from eating meat that only manifests at
low temperatures (Behrens and Lafferty, 2007). If this pattern
holds true for N. duplicata, one would predict lower trophic
positions during the summer months than other times during the
year, and lower trophic positions in the warmer environments
experienced by individuals in Florida—the southernmost part
of N. duplicata’s geographic range along the Atlantic coast
of the United States—than cooler environments experienced
further north, such as the Long Island Sound sites examined in
this study. Additional research on the seasonal and latitudinal
patterns of trophic position within N. duplicata is needed,
but if the degree of omnivory is found to be temperature
dependent, an increased reliance of N. duplicata on plants in
the southern part of its range may at least partially explain
Kelley and Hansen’s (2007) unexpected latitudinal pattern of low
drilling frequencies for low-latitude molluscan assemblages from
Florida.
The previously undocumented omnivory of N. duplicata likely
has broad ecological implications at both local (individual and
population level) and regional scales. Although no study has
previously investigated the implications of omnivory in moon
snails, analogy with other omnivorous organisms may serve as a
useful guide for future research. For example, at the individual
level, the benefits that many predominately carnivorous insect
taxa gain by eating plants is known to be highly context-specific.
Consuming a particular species of plant may be detrimental,
slightly beneficial, or very beneficial to an omnivore depending
on the amounts and identities of other foods recently consumed
(Eubanks and Denno, 1999; Coll and Guershon, 2002). In
an analogous way, we hypothesize that the benefits of plant
consumption experienced by N. duplicata may include: the ability
to persist in habitats after prey have become scarce or habitat
quality has decreased; a reduction in search time when prey are
scarce; a reduction in interspecific competition; and a reduction
in cannibalism. It is impossible to know which of these benefits
may come into play for N. duplicata without more data on the
environmental context of and the exact plant resources in the
diets of omnivorous moon snails.
At the level of the population, N. duplicata omnivory could
decouple predator-prey dynamics. We know that populations
of omnivorous insects frequently reach higher densities when
plant resources are available (Coll and Guershon, 2002; Eubanks,
2005), which leads to increased predation intensity and smaller
prey populations in some cases (e.g., Eubanks and Denno, 1999;
Harmon et al., 2000). However, the degree of prey suppression
can vary based on differences in the persistence of omnivores at
low or no prey abundance, the effects of plant feeding on per
capita prey consumption and the food preferences of omnivores,
and the effect of plant feeding on the dispersal and distribution of
omnivores (Eubanks, 2005). For example, Cottrell and Yeargan
(1998) found that predation by the lady beetle Colemegilla
maculata on the eggs of Helicoverpa zea decreased in the presence
of corn pollen, leading to a reduction in predation intensity in
spite of an increase in predator density. In contrast, Harmon
et al. (2000) found that the effects of C. maculata’s decreased per
capita consumption of H. zea eggs in the presence of dandelion
pollen was out-weighed by the concurrent increase in predator
density, leading to an overall increase in predation pressure
where supplemental plant food was present. It is thus premature
to generalize from these observations what might happen to
the intensity of predation, commonly indexed by the frequency
of drill-holes in prey shells (Kelley and Hansen, 2003), if N.
duplicata is an omnivore. Prevalent omnivory could increase
drilling frequency in some situations and decrease it in others. In
addition to changes in prey suppression, the nutritional quality
of one food type may be altered by the ingestion of another
food type, causing the prey preferences of omnivores to appear
suboptimal (Eubanks, 2005). If omnivory proves to be true for N.
duplicata, the subsequent alteration of bivalve nutritional quality
Frontiers in Ecology and Evolution | www.frontiersin.org
APPENDIX 2: TROPHIC POSITION OF THE
MURICIDAE
The trophic positions of muricid drilling gastropods, U. cinerea
and N. lapillus, ranged between 2.3 and 2.5 in LIS (Supplementary
Table 1; Figures 3, 4). Though unforeseen, frequent omnivory
among predators should not be surprising given the high
prevalence of omnivory in marine environments (Long et al.,
2011 and references therein) and the lack of empirical evidence
for defined trophic levels among predatory taxa (Thompson
et al., 2007). In fact, Thompson et al. (2007) characterized the
portion of food webs above the level of herbivores as a “tangled
web of omnivores.” Data on the trophic position of muricids is
limited, however, N signatures derived from the organic residue
of Muricanthus sp. shells from the Gulf of California indicate
that not all muricid taxa show lower than expected N values
(Fall et al., 2011). As suggested in this study on N. duplicata, an
unusual fractionation factor could explain the low N signatures
present in some muricid species. Additional analyses, such as the
laboratory feeding experiment discussed herein, will be necessary
to rigorously evaluate alternative explanations for the observed
low trophic positions of U. cinerea and N. lapillus. Due to their
dissimilar C signatures (Figures 3, 4), it is unlikely that U. cinerea
and N. lapillus consume the same epiphytic diatoms or benthic
macroalgae thought to be eaten by N. duplicata even if these
muricids turn out to be omnivores.
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