THE EFFECTS OF LIMPET MORPHOLOGY ON PREDATION BY ADULT CANCRID CRABS

Although limpets are important prey for many crab species, and limpet populations may be regulated by crab predation, little is known about the effects of limpet shell morphology on crab predation. Morphological adaptions to predation are well documented for gastropods, and identification of limpet morphologies that decrease vulnerability to crab predation could be useful in exploring the role of predation in spatial variation of limpet abundance and diversity. Alternatively, shell shape often tracks environmental variables, and can aid in paleoenvironmental reconstruction.

To identify limpet shell morphologies that decrease vulnerability to adult Cancer productus, laboratory feeding trials using Pacific Northwest limpets (Lottia digitalis, L. pelta and L. scutum) were conducted. The following morphological variables were examined: a) large size (i.e., increased tenacity and shell thickness, and exceeding maximum gape of crab chelae), b) shell ornament (increased shell strength and/or handling time), and c) low-spired geometry (difficulty attaining purchase). Morphologies reducing vulnerability should result in increased survivorship or longer grappling times (increasing cost-benefit ratio or likelihood of predator disruption). Crabs may also utilize different attack strategies based on prey morphology, with varying success across strategies. Binomial multiple logistic regression was used to determine which of these variables contribute to the outcome of attacks.

Although mortality varied between species, no relationship between prey size and increased survivorship was observed. Contrary to the expectation that radial ribs resist predation, individuals with smooth morphologies experienced lower mortality. Furthermore, the presence of shell ornament was the only significant explanatory variable in predicting mortality. As species possessing high-spires and ridges may typically occur high in the intertidal where predation risk due to crabs is relatively lower, shell ornament is likely an adaption to physical factors such as thermal stress, and does not appear to be antipredatory for limpets. Strong shell ornament in fossil limpets appears more likely to indicate high-intertidal conditions, rather than strong predation, and may be useful as a paleoenvironmental indicator.