STABLE ISOTOPIC ANALYSIS REVEALS OMNIVORY IN NEVERITA DUPLICATA: PART 1, LABORATORY FEEDING EXPERIMENTS

Species belonging to the family Naticidae are important gastropod predators found in soft-bottom marine communities world-wide and are commonly studied by paleontologists interested in drill-holes as preservable records of predator-prey interactions. Naticids (commonly known as moon snails) have traditionally been thought to prey on other molluscs, giving them an expected trophic position (TP) of a predator (TP = 3). Previous stable isotopic evidence shows lower than expected nitrogen signatures and trophic positions for the drilling gastropod Neverita duplicata (avg. TP: 2.2) (Casey et al., 2014). These TP estimates suggest that N. duplicata may be omnivorous (i.e., feed from multiple trophic levels) or have a nitrogen fractionation factor lower than the experimentally derived average used in most studies (3.4 per mil). Laboratory feeding experiments were conducted to evaluate if N. duplicata has a lower nitrogen fractionation factor than 3.4 per mil. Neverita duplicata (N = 40) were collected from Long Island Sound (Milford, CT) in August 2013. Fifteen randomly selected N. duplicata were kept in separate 37.9 liter tanks until August 2014. The remaining moon snails (pre-experiment individuals) were frozen to provide a starting nitrogen signature of the experiment. Moon snails were fed the filter-feeding bivalve Mercenaria mercenaria, and the bivalves were fed prepared algae. To ensure that tissue turnover was not a concern, moon snails were fed the prey item for a full year. Results from the experiment indicate that N. duplicata does not have a lower nitrogen discrimination factor. The median value of pre-experiment individuals is 2.89 per mil, whereas the post-experiment individuals median value is 3.58 per mil – a significant increase of 24%. The high nitrogen values are not the result of laboratory artifacts. There is no correlation between nitrogen values and number of bivalves eaten or average bi-weekly consumption rate. Nitrogen signatures of M. mercenaria prey remained consistent throughout the experiment and are similar to field con-specifics. These results suggest that N. duplicata may engage in omnivory in the wild. The next step is to clarify the diet of N. duplicata by examining it within the context of a modern food web.