Limpets & relatives Research Study 1
Laboratory studies on predation on limpets by shore crabs Pachygrapsus crassipes in the Monterey area of California reveal two distinctly different attack modes. One involves prying the limpet from the substratum, but another involves an in situ excision of the top part of the limpet’s shell. Experiments with needle-nose pliers, which seem to mimic the action of the crab’s chelae, suggest that there is a weak area or fracture zone that encircles the shell just above the point of attachment of the shell muscles. Of four species of limpets examined in the study, Lottia scutum, L. scabra, L. digitalis, and L. limatula, the last seems most susceptible to this type of predation. Doughnut-shaped shell remains of limpets are common on west-coast shores, suggesting that other crab predators may use the same method (alternatively, shells resulting from other end-of-life events may be broken by wave action as they bounce along the shore).
Shanks & Wright 1986 Oecologia 69: 420
Research Study 2
The most common intertidal predatory crab in the British Columbia/Puget Sound region capable of removing or crushing an adult limpet is the red-rock crab Cancer productus. It has four attack strategies, but none is noticeably better than the others. The four methods are: prying up from a gap at the edge of a shell, laterally sliding the limpet free of its attachment, crushing the shell at its apex, and crushing the shell at its margin. The first technique is used more on flat- or smooth-shelled species where it is hard for the crab to get a grip on the shell (e.g., Lottia scutum, L. persona); the last technique is used on taller, rough-shelled species (e.g., L. pelta). Laboratory studies at the Bamfield Marine Sciences Centre, British Columbia and Puget Sound, Washington on attack success by crabs on four treatment groups of these three species of limpets (there are two size classes of L. scutum: large, 30-40mm; small, 18-23mm) yield the following results shown in the table.
Other observations on a second, but much smaller, potential crab predator Cancer oregonensis show that only the prying method is used, and attacks by this species are unsuccessful. Overall, whether prying is successful or not depends upon the closeness of fit of shell to rock. The author of the study suggests that selection to resist prying forces has been important in the evolution of shell morphology in limpets.
Research Study 3
In the Santa Catalina Islands of California two sympatric species Lottia limatula and L. have different behavioural strategies to avoid fast-moving visual predators such as fishes,
Research Study 4
For whatever reason, nucellid whelks tend not to attack and drill limpets. For example, an analysis at Friday Harbor Laboratories, Washington of an assemblage of dead limpet shells (numbering 1,531) caught up between some large boulders on a beach on San Juan Island (see photograph upper Left) reveals a whelk-caused mortality of only 4% on the limpets Lottia digitalis, L. pelta, and L. scutum. Some interesting details disclosed in the study are: 1) hole diameter correlates positively with limpet-shell size, indicating that larger predators select larger prey (see graph for L. digitalis middle Left); 2) significantly more L. digitalis are drilled (79% of total; see photograph of L. digitalis being attacked on lower Left) than L. pelta (19%) or L. scutum (2%); 3) largest and thickest L. pelta shells are not drilled, suggesting that they have reached a refuge in size (see graph upper Right); and 4) the majority of holes are drilled near the apex of the shell, indicating stereotypical attack behaviour (see illustration lower Right). Although the underlying cause of such small representation of limpets in the diets of whelks is not completely clear, bioerosive evidence suggests that drilled shells are weaker than undrilled ones and do not preserve as well. Thus, use of this type of taphonomic (fossilisation) data may lead to underestimates of actual predation frequencies of whelks on limpets. The study is an interesting one and is sure to generate further research.