MORPHOMETRY OF OICHNUS TRACES ON MOON SNAIL SHELLS ALONG A TIDE-DOMINATED SHORELINE, NAHANT BEACH, MASSACHUSETTS

A variety of gastropods and bivalves along the intertidal zone of Nahant Beach, Massachusetts exhibit circular naticid drillholes (Oichnus isp.) produced by a large population of northern moon snail (Lunatia heros). Whereas a small sample (n=15) of L. heros shells collected along a small upper intertidal stretch were dominated by unsuccessful cannibalism (incomplete penetration >80%), an expanded intertidal dataset (n=400) shows this effect on only 6% of the snails. The cross-shore pattern may be explained partially by the preferential landward transport of shells without penetrative boreholes due to their greater buoyancy. A majority (74%) of the unsuccessful predation attempts occurred in shells longer than 25 mm, suggesting the control of shell size and thickness on the success of naticid predation. Successful Oichnus ranged in diameter from 2.77-9.02 mm (mean: 5.75 mm), with non-penetrative traces having similar dimensions of 3.19-8.30 mm (mean: 5.70 mm), suggesting that size of the predator alone does not guarantee successful predation. This is supported by a much larger ratio of Oichnus diameter to shell height in penetrative traces (range: 0.137-0.461; mean=0.274) compared to failed boreholes (range: 0.126-0.332; mean: 0.191). Due to being incomplete, the latter are less countersunk, with 35% having an inward diameter decrease of 25%, compared to 72% for successful predation. These findings have implications for prey selection during L. heros cannibalism, as well as the importance of hydrodynamic processes in shell accumulations on tide-dominated shorelines.