INVESTIGATION OF TROPHIC POSITION IN THE SOUTHERN OYSTER DRILL, STRAMONITA HAEMASTOMA, USING STABLE ISOTOPE ANALYSIS

Drill holes found in the fossil record are a valuable tool for reconstructing ecological patterns of predator-prey relationships in past environments. To do so effectively, it is important to study living species of drilling snails and their behavior in modern ecosystems to better constrain fossil interpretations. Previous studies have shown that the southern oyster drill, Stramonita haemastoma, consumes a diet of eastern oysters, mussels, barnacles, (Butler, 1985) limpets, and even polycheate worms where other prey are scarce (Watanabe and Young, 2005). This diet should impart a trophic position (TP) of 3.0 (a secondary consumer or predator) and an intermediate carbon signature between those of the pelagic and littoral primary producers. This study uses the stable isotope signatures of N and C to examine the diet and TP of S. haemastoma from a Florida Gulf oyster reef near St. Teresa, FL. Whole body, soft tissue analysis of δ¹³C and δ¹⁵N were conducted on 13 S. haemastoma specimens, baseline proxy taxa (pelagic: Crassostrea virginica, littoral: Turbo castanea), and other molluscan species. Each specimen was collected live, immediately frozen, and later dissected, dried, and pulverized. All isotopic analyses were run at the stable isotope laboratory at Johns Hopkins University. Lipid correction was done mathematically after (Post et al., 2007). TP values for S. haemastoma ranged from 2.5 to 3.3, with an average of 3.0, suggesting a true predator. Alpha values suggest a mixed diet of both pelagic carbon from prey such as C. virginica and littoral carbon from prey such as limpets, with a larger reliance on littoral carbon sources than predicted. The 3.0 TP of S. haemastoma contrasts with previous work on other drilling gastropods (including the eastern oyster drill Urosalpinx cinerea and the shark eye moon snail Neverita duplicata) which consistently show omnivorous trophic positions of 2.3-2.5. Further study is needed to improve knowledge of the diet of drilling gastropod species given difficultly in attributing individual drill holes to specific predatory taxa in most ecosystems and the wide range of TPs recovered from the drilling gastropod species analyzed thus far. The presence of both predators and omnivores within drilling gastropod clades could have wide-ranging ecological and evolutionary implications.