INFLUENCE OF INTRAVALVE VARIATION IN THICKNESS ON NATICID BOREHOLE STEREOTYPY

Patterns of borehole positions of predatory naticid gastropods on prey shells may be influenced by prey morphology (i.e., size, shape, ornamentation, valve thickness, skeletal minerology) and habit, but rigorous testing of any relationship between borehole stereotypy and valve thickness is wanting. This investigation uses landmark analyzes combined with a nearest neighbor test and detailed valve thickness “mapping” on >500 naticid-drilled shells among 12 New Jersey clam species to address the issue of valve thickness influencing interspecific differences in borehole stereotypy. Bored shells of each species were sorted into anterior-posterior size classes, as well as right vs. left valve initially. The Clark-Evans dispersion index, a nearest neighbor test, was performed on borehole patterns plotted compositely for each size-valve class where n = 5 minimally. The index is a ratio, R, of actual vs. expected nearest neighbor mean distances between boreholes, standardized for borehole density, which is statistically testable (normal deviate z). Theoretically, R may vary between 0 (coincidence of all boreholes) and 2.15 (hexagonal patterns to composite borehole distributions). Valve thickness profiles were taken from the beak-to-ventral margin of sectioned shells from each size class. These radial profiles span from the anterior to the posterior margin, capturing thickness variations in naticid-bored valve areas such as the pallial sinuses (if present), adductor muscle scars, umbo, and lunule-escutcheon. R values were plotted on V, the averaged coefficient of variation of valve thickness for each size class. Preliminary results indicate that variation in valve thickness is correlated with borehole stereotypy. Stereotypy is high (low R) in species with the greatest intravalve thickness variation (high V). Boreholes are localized where the valve is thinnest (e,.g., Spisula solidissima, Mercenaria mercenaria). Boreholes are less concentrated (high R) on species or size classes with more uniform thickness (lower V)(e.g., Mya arenaria, Astarte castanea). Comparison of similar sized and shaped species (e.g., M. mercenaria vs. Pitar morrhuana) revealed greater stereotypy in the species with greatest intravalve thickness variability (M. mercenaria).