ARE MODERN PREDATION STUDIES ON BEACH COLLECTED MOLLUSKS SUITABLE ANALOGS FOR THE FOSSIL RECORD OF PREDATION? THE FIDELITY OF PAIRED BEACH AND SUBTIDAL DEATH ASSEMBLAGES FROM LONG ISLAND SOUND

Recent beach death assemblages are frequently used to quantify patterns in drilling predation at various scales (regional, latitudinal). The resulting patterns are often compared to those obtained from fossil assemblages, even though the majority of fossil assemblages preserve subtidal environments and the taphonomic agents at work are known to be different. To evaluate the effects of differential taphonomy on drilling frequency and community fidelity, we sampled the Long Island Sound molluscan death assemblage subtidally and supratidally near Westport, CT. The subtidal sample was collected in approximately 7.6 m water depth. The supratidal sample was collected at low tide along east and west beach at Sherwood Island State Park, which are both low energy with dominant wave heights < 1.0 m. Taxonomic similarity was measured using the Jaccard-Chao index (JC) and rank-order agreement was measured using Spearman’s rho. Beach samples collected from the strand line showed higher taxonomic similarity (JC_{west beach} = 0.77, JC_{east beach} = 0.79) than beach samples collected from the subtidal zone (JC_{west beach} = 0.36, JC_{east beach}= 0.21). West beach had higher fidelity of rank-order abundance in spite of smaller sample size than east beach. Beach samples (both east and west) showed lower species richness than the dredge sample (beach richness = 19 for east and west beaches versus dredge richness = 24). This pattern persisted after rarefaction.

Drilling frequencies were calculated separately for selected prey taxa in dredge and beach samples, but were impossible to compare due to issues of differential prey abundance. The most commonly drilled dredge taxa, the false quahog Pitar morrhuanus and the quahog Mercenaria mercenaria, were extremely rare in the beach samples (n = 1 individual and n = 34 individuals, respectively). Likewise, the most commonly drilled beach taxa, the softshell clam Mya arenaria and the eastern oyster Crassostrea virginica, were rare in the dredge sample (n = 2 individuals and n = 28 individuals, respectively). These preliminary data indicate beach death assemblages are biased representations of the subtidal predatory environment and potentially are not suitable for comparison with fossil assemblages.