You Are What You Eat: Stable Isotopic Evidence Indicates That the Naticid Gastropod Neverita duplicata Is an Omnivore

Michelle M Casey,Gregory P Dietl,Leigh M Fall

doi:10.3389/fevo.2016.00125

Michelle M Casey, Gregory P Dietl + Show 1 more

Open Access

https://doi.org/10.3389/fevo.2016.00125

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Abstract

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 one 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 experiments. 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 ecological implications of potential naticid omnivory.

Highlights

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)
The laboratory food chain was controlled at all levels from primary producer through primary consumer to secondary consumer, N and C signatures were not used to calculate tropic position in the same manner employed in food web analyses
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

Summary

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). 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

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