Latitudinal trends in naticid predation on Anadara ovalis (Bruguière, 1789) and Divalinga quadrisulcata (Orbigny, 1842) from New Jersey to the Florida Keys

Predation indices commonly calculated to quantify the interaction between drilling naticid gastropods and their bivalve prey include boreholesite stereotypy, predator-prey size selectivity, preysize class preference, size refuge from predation, prey effectiveness (PE) and predation intensity (PI; synonymous with drilling frequency or incidence of drilling in the literature). The indices were calculated for samples of the blood ark Anadara ovalis (Bruguière, 1789) (six sites; n = 2023, collectively) and the cross-hatched Iucine Divalinga quadrisulcata (Orbigny, 1842) (five sites; n = 2597, collectively) from New Jersey (Virginian molluscan subprovince) to the Florida Keys (Caribbean molluscan province). Borehole sites are concentrated on the ventral margin in New Jersey and the dorsal margin in South Carolina and Florida on valves of D. quadrisulcata, with a North Carolina site showing a bimodal distribution for the umbos and the ventral margin. This latitiudinal shift in borehole-site stereotypy is attributed to a different suite of naticid species drilling D. quadrisulcata in the northern vs. southern populations. Euspira heros, (Say, 1822) and Neverita duplicata (Say, 1822) drill the cross-hatched Iucine north of the Chesapeake Bay, whereas Sinum perspectivum (Say, 1831), Polinices lacteus (Guilding, 1834), and Naticarius canrena (Linnaeus, 1758) and other naticid species drill populations of the cross-hatched Iucine in the Outerbanks and further south. No such latitudinal shift in stereotypy is evident for A. ovalis; complete boreholes are invariably concentrated on the exposed umbos of the hairy periostracum-veneered, semi-infaunal blood ark at all sampled latitudes. No size refuge from predation is evident for D, quadrisulcata at any latitude as the largest size classes are bored at each sample site. In contrast, A. ovalis has a size refuge at each locality, with the largest unbored specimens at least 10 mm greater than the largest bored specimen. Nevertheless, maximum size of a bored valve does not increase or decrease across the latitudes. Prey size selectivity by naticids shows no pattern from New Jersey to Florida for either bivalve species. Correlations between predator size, indexed by the outer borehole diameter (OBD) in the valve, and prey size (length or width) are significant for only three of 11 samples combined for both species. Prey effectiveness increases for D. quadrisulcata from north to south, suggesting that this bivalve prey more frequently escaped from foot-envelopment by the smaller southern naticids (< mean OBD). In contrast, incomplete boreholes are very infrequent on A. ovalis, which is a sluggish, semi-infaunal burrower that cannot elude the enveloping naticid foot. Although possibly complicated by taphonomic biases, as is PE, predation intensity increases for both species from the Virginian Subprovince to the Caribbean Province. In northern latitudes, the copious, large surfclam Spisula solidissima (Dillwyn, 1822) is the preferred naticid prey. But south of the Chesapeake, naticids increasingly drill D. quadrisulcata and A. ovalis as part of the alternative bivalve prey to the vanishing surfclams.