CRUSHING PREDATION AND PATELLIFORM SHELL MORPHOLOGY; ARE ALL RIBS CREATED EQUAL?

Predation pressure is often cited as a driving force for morphological adaptations in marine organisms, and crushing predation in particular has had a major role in interpretions of the evolution of gastropod shell morphology (Vermeij 1977, Vermeij et al 1980, 1981). Previous work focused on gastropods with spirally coiled shells (Vermeij 1977, 1993, Vermeij et al 1980, 1981), but little has been done on the selective force of shell crushing predation on patelliform shell morphology (Lowell 1986). While an increase in the expression of armor through the Phanerozoic is linked with increasing selective pressures of crushing predation for spirally coiled gastropods (Vermeij 1993), the question remains as to whether predation has had similar effects on the evolution of armor (radial ribs) in patelliform shells. The effect of crushing-predation by the crab Cancer productus on patelliforms is examined using three common Pacific Northwest species of limpets (Lottia digitalis, L. pelta and Tectura scutum) from San Juan Isl. WA. Feeding trials were conducted with these limpets to assess how handling time and attack success rates varied with 1) presence of radial ribs and 2) shell shape. Attacks were more likely to result in a successful kill if a limpet was ribbed and had a large height/length ratio (Likelihood G=32.6, p<0.0001), however, there was some difficulty differentiating the effects of ribs and height/length. There was a significant correlation between height/length of the shell and the likelihood of a successful attack; when height/length exceeds 0.54, success rate ≥ 50%. Thus, height/length may play a more important role in the likelihood of an attack being successful than the presence of radial ribs. Limpets having a low height/length ratio may have an increased pedal surface area, leading to greater tenacity, an important antipredatory defense for limpets (Lowell 1986). These results illustrate the importance of testable hypotheses which ask explicit biological questions to explore adaptive space, and the avoidance of generalizations based solely on plausibility. This type of result is expected for a structure on which competing selective factors must operate, and radial ribs may play a more important role in preventing dessication and thermal stress (Vermeij 1993) than resisting shell-crushing predation.