INVESTIGATION OF OMNIVOROUS TROPHIC POSITION IN THE DRILLING GASTROPOD, UROSALPINX CINEREA, USING STABLE ISOTOPE ANALYSIS

Drill-holes found in the fossil record are an important tool, frequently used to study the ecological patterns of the past. It is therefore important to gain a better understanding of the role that extant drilling snails play in modern ecosystems. Although traditionally thought to be a predator, which would show a trophic position of 3.0, data from several specimens of the muricid Urosalpinx cinerea from Long Island Sound revealed trophic positions between 2.3 and 2.5, suggestive of a more omnivorous diet. This study addresses the generality of this surprising result by examining a U. cinerea population from Wilmington, North Carolina. Preliminary whole body, soft tissue stable isotope analysis of nitrogen and carbon was conducted on five U. cinerea specimens. The isotopic baseline for the study area was calculated using proxy taxa, including the filter-feeding ribbed mussel Geukensia demissa for the pelagic baseline and the grazing marsh periwinkle Littoraria irrorata for the littoral baseline. All isotopic analyses were run at the University of Kentucky Stable Isotope Geochemistry Laboratory. Trophic position for these U. cinerea specimens ranged from 2.4 to 2.9. The trophic value at the top of the range, 2.9, is close enough to 3.0 to be considered predatory. Working hypotheses to explain a trophic position lower than 3.0 in U. cinerea include: trophic omnivory driven by plant consumption, or a lower-than-average nitrogen discrimination factor. Although no studies on the nitrogen fractionation factors of muricids currently exist, the shell-drilling naticid Neverita duplicata from Long Island Sound has recently been demonstrated to have a normal nitrogen fractionation factor and omnivorous isotopic signatures. The difference between the trophic ranges of the two locations may indicate that U. cinerea have a more predatory diet, consuming more meat, in North Carolina than in Long Island Sound. However, further work is needed to confirm that these values reflect dietary differences and not a below average nitrogen fractionation factor.