THE ROLE OF SPINOSE ORNAMENT IN PREDATOR DETERRENCE: THE BIVALVE ARCINELLA, PINECREST (PLIOCENE) OF FLORIDA

The highly spinose shells of the sessile, epifaunal bivalve Arcinella cornuta, from the Plio-Pleistocene deposits of Florida, provide evidence of disruption of stereotyped attacks by boring gastropod predators. Records of attacks by naticid and muricid gastropods have frequently been preserved as borings created through combined physical and chemical assault on the shells of their putative prey. Naticid attacks demonstrate stereotyped patterns developed over evolutionary time. Naticids are highly selective with regard to prey size and boring site, optimizing net energy return. Most boreholes in A. cornuta are circular in plan view and larger than 2mm in diameter, suggesting they were created by naticids. Size and site selectivity, however, were not pronounced in attacks on A. cornuta. Borehole diameter (indicating predator size) did not correlate with prey size. Although selection of the boring site was nonrandom, successful attacks occurred in several regions of the prey shell and employed several strategies. 32.5% of successful borings occurred between spine rows within the shell sector that covers the posterior adductor muscle, and 24.6% pierced the muscle itself. Conversely, 17.5% of successful borings occurred at the depressed lunule, anterior of the beak of each valve. This is the thickest region of the shell, but the only region without spines. There was no preference with regard to left or right valve.

This disruption in stereotyped behavior may have occurred, in part, because the spines of the prey animal masked its true size and impeded access to preferred boring locations. Larger predators tended to bore medium-sized prey, often at the lunule. Borings in that region were, on average, more than 1mm greater in diameter than those occurring in other regions of the shell. This may indicate that larger predators were unable to recognize optimally-sized prey and successfully bore between spine rows, and thus were forced to abandon preferred boring sites and settle for a thicker, but less ornamented region of the shell. The spines likely disrupted the efficiency of attacks and provided A. cornuta a "virtual size refuge" against larger naticid predators.