Resistance to dislodgement: habitat and size-specific differences in morphology and tenacity in an intertidal snail

An organism's ability to persist on wave-swept rocky shores is determined, in part, by its ability to resist dislodgement by waves. Resistance to dislodgement depends on hydrodynamic forces and on the ability of an organism to resist those forces by adhering to the substratum. In intertidal snails the structure upon which waves act is the shell and the structure that adheres is the muscular foot. We quantified: (1) shell size (defined as the maximum projected surface area, MPSA); (2) shell shape; (3) foot area; (4) maximum force to dislodge a snail in shear; and (5) tenacity (force per foot area required to dislodge) of the herbivorous intertidal snail Littorina obtusata. We compared individuals that were collected from habitats characterized by differences in exposure to waves. Wave-exposed snails were smaller (lower average MPSA). In addition, snails at the wave-exposed site were shorter, and had larger foot area and greater dislodgement force than did protected snails of similar MPSA. The greater dislodgement force at the exposed site was due to larger foot area, not to greater tenacity. In fact, sizespecific tenacity of snails from the protected habitat was greater Within each site, shape changed with size. Although shell width, length and MPSA scaled isometrically, shell height did not; longer shells were relatively short. Nor did foot area scale isometrically with MPSA; larger snails had relatively smaller foot areas. Assuming that MPSA is directly proportional to speed-specific drag, which is reasonable for bluff bodies, we found that dislodgement force was proportional to drag. This result suggests that increasing tenacity compensated for underscaling of foot area. In fact tenacity did increase with foot area, although only significantly so at the wave-exposed site.