RELATIONSHIP BETWEEN DRILLING PREDATION AND SHELL MORPHOLOGY OF PATELLID LIMPETS FROM SOUTHWESTERN ENGLAND

Morphology of limpet shells may be influenced by such factors as hydrodynamic forces, desiccation, gill capacity, growth rate and metabolism, territorial behavior, and predation. Relatively little work has been done on the relation of form, function, and drilling predation on limpets. Variations in shell height and length of three species of patellid limpets reflect their zonation on the rocky shores of southwestern England. Flattened smaller Patella prefer middle and lower shore wave-exposed conditions; large, steeply conical forms are dominant higher up near the splash zone. These differences in size, shell shape, and environmental preference may have consequences in their susceptibility to drilling attacks by carnivorous gastropods.

Float specimens of dead Patella were collected from the rocky intertidal of West Looe Beach, Cornwall, in July 2004. Of 188 shells, 31.4% display evidence of predatory damage by drilling gastropods. Drilled specimens contain a total of 79 holes represented by incomplete (34.2%) and successful (65.8%) attacks. To investigate differences in morphology of drilled vs. undrilled limpets, height, length, width, shell thickness, and apical position were measured. Hole size and location on the shell were recorded for drilled individuals.

Limpet shells increase in height and thickness relative to length with growth. Drilling is more frequent on larger (and hence thicker) limpets, but among larger specimens, drilled shells have significantly lower thickness:length ratios. Shells with incomplete drillholes have a greater mean length and thickness:length ratio than shells with complete drillholes. No preference with respect to shell steepness is observed for Patella or steeply conical P. vulgata. Distribution of incomplete vs. complete drillholes varies on the shell exterior, but most holes penetrate the interior on or inside the U-shaped muscle scar. The posterior of the shell contains the highest concentration of complete drillholes, perhaps indicating a preference for areas underlain by energy-rich organs or for optimal stability of the predator during drilling. Thickness and inferior drilling locations may be the cause of failed attacks. Drilling predation appears less important in selection of limpet shell morphology compared to physical stresses or other biotic hazards.