STABLE ISOTOPIC ANALYSIS REVEALS OMNIVORY IN THE NATICID GASTROPOD NEVERITA DUPLICATA: PART 2, MODERN FOOD WEB ANALYSIS

Laboratory feeding experiments conducted to evaluate omnivory in the naticid gastropod Neverita duplicata refuted the hypothesis that this species has a below average nitrogen discrimination factor that could explain previously reported low trophic positions. This result was surprising given the extensive body of research on the predatory drilling behavior and prey preferences of N. duplicata. To further investigate the trophic ecology of N. duplicata in the context of a natural food web free from potential laboratory artifacts, whole body stable isotopic analysis of nitrogen and carbon from the soft tissues of numerous molluscan taxa from Long Island Sound were analyzed. Field-collected N. duplicata yielded low trophic positions (avg. TP = 2.2) indicating that they feed from multiple trophic levels. Evidence from the carbon signatures of field-collected N. duplicata and baseline proxies indicate a reliance on littoral food sources (those that grow along the seafloor) that is inconsistent with a diet of exclusively filter-feeding bivalves (which feed predominately on phytoplankton). Nitrogen signatures of field-collected N. duplicata are too low to indicate feeding on primary consumers that rely on littoral primary production (preying on detritivores or algal grazers). In fact, N. duplicata show considerable isotopic overlap with the grazing periwinkle, Littorina littorea, and the deposit feeding/omnivorous mud snail Tritia obsoleta, but show no evidence of detritus in the digestive tract. Although it is impossible to distinguish between different littoral carbon sources isotopically, it seems likely that N. duplicata feeds on epiphytic diatoms or macroalgae for which its radula is well-suited. Other carbon sources can be excluded due to their unique isotopic signatures, including marsh grass detritus, upland land plant detritus delivered by rivers, and phytoplankton. For this reason, we hypothesize that N. duplicata feeds on some combination of benthic primary producers (most likely macroalgae and/or epiphytic diatoms) in addition to carrion, and bivalve/gastropod tissue. While the full geographic extent of omnivory within N. duplicata is not yet known, the presence of omnivory could have wide-ranging ecological and evolutionary implications.