DISCREPANCY BETWEEN THE POTENTIAL AND REALIZED TROPHIC POSITIONS OF DRILLING PREDATORY GASTROPODS: ARE NATICIDS ACTUALLY OMNIVORES?

Paleontologists use drilling gastropods as a model system for the study of predator-prey dynamics because predation events–drill holes–are preserved in the fossil record. Drilling gastropods are expected to have a trophic position (TP) equal to 3 (a predator of primary consumers) based on their known ability to drill and consume bivalve prey. The relatively large biological fractionation of nitrogen makes δ15N useful for evaluating TP while the relatively small biological fractionation of carbon makes δ13C useful for tracing dietary sources of carbon. We present field evidence from multiple species and localities in North America that show significant deviation of the realized TP from the predicted TP for drilling gastropods based on stable isotopic analysis of nitrogen and carbon. Both nitrogen and carbon signatures were analyzed in whole body tissues of animals from Long Island Sound (LIS). Taxa collected from LIS include: the baseline proxies Crassostrea virginica (pelagic baseline) and Littorina littorea (littoral baseline), and the predatory gastropods Neverita duplicata, Euspira heros, Urosalpinx cinerea, Nucella lapillus, and Busycotypus canaliculatus. Stable isotopic analysis of nitrogen only was performed on shells of Natica sp., Muricanthus sp., and Conus sp. from the northern Gulf of California. Nitrogen signatures indicate lower than expected trophic positions (TP: 2.3-2.5) for the drilling gastropods Natica sp., N. duplicata, E. heros, U. cinerea, and N. lapillus, but not the whelk B. canaliculatus nor the muricid Muricanthus sp. This result cannot be explained by taxonomy or geography. These TP estimates imply that some drilling gastropods are omnivorous, i.e., feed from multiple trophic levels, or that they have a nitrogen fractionation factor smaller than the experimentally-derived average used in most studies (3.4 per mil). Evidence from the carbon signatures of naticids indicate a reliance on littoral food sources (feeding on some mix of mollusc meat and littoral algae or detritus). Muricids display a strictly pelagic carbon signature. Controlled feeding experiments will be necessary to rule out the possibility of an atypical naticid or muricid fractionation factor.