NATICID GASTROPOD PREDATION ON TWO GENERA OF MIOCENE PELECYPODS, ARCA AND ASTARTE: STEREOTYPY AND SITE SELECTION

Morphometric data gathered from specimens of the pelecypod genera Astarte (n=501) and Arca (n=520) from the Miocene Calvert Formation of Calvert County, Maryland show that predatory Miocene naticid gastropods may have selected prey from these two genera according to shell size, and then bored holes at specific sites on the shells. The naticid boring locations are more stereotyped as prey size increases, suggesting more specific site selection by naticids as they encounter larger, less easily manipulated prey. In contrast, data indicate no significant relationship between stereotyped placement of borings and naticid gastropod boring diameter. Previous work has shown that boring daimeter is positively correlated with naticid size. However, the larger Arca (mean length » 26.1mm, s=6.5mm) were preyed upon by larger naticid gastropods (mean boring diameter » 3.9mm), with a significant positive correlation between boring diameters and shell size (length and height) (r²=.373 for Arca length, n=57; r²=.334 for Arca height, n=57). Naticid gastropods of varying size preyed on the smaller Astarte (mean length » 20.7mm, s=4mm), with no significant correlation between boring diameter and shell size (r²=.065 for Astarte length, n=161; and r²=.08 for Astarte height, n=161). Thus, it appears that the size of the predator increased with the size of Arca shells, and the relationship between predator and prey sizes may have changed through ontogeny.