Defenses of limpets include attachment strength, shell, escape crawling, and camouflage (both visual and chemical). Chief predators are crabs, fishes, and sea stars when the tide is in, and birds when the tide is out. There is overlap between defenses and predators. For example, attachment strength is useful against predation by both sea stars and birds, and shells provide protection against both crabs and fishes. For this reason, defenses and predators are intermixed in this overall section.

Laboratory studies on predation on limpets by shore crabs Pachygrapsus crassipes in the Monterey area of California reveal 2 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 4 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).  Chapin 1968 Veliger 11(Suppl): 67.

NOTE limpet “rings” may also arise directly from wave-mediated rock damage, where the shell portion distal to the muscle attachment breaks off and is carried away.  In limpets such as Lottia limatula that are repeatedly broken in this way the shell over the area of muscle attachment may continue to grow, thus creating a ledge around the circumference.  Shanks & Wright 1986 Oecologia 69: 420.

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 4 attack strategies, but none is particularly successful.  These 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 C. magister on 4 treatment groups of these 3 species of limpets (there are 2 size classes of L. scutum: large, 30-40mm; small, 18-23mm) yield the following results. 

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.  Lowell 1986 Biol Bull 171: 577. Photograph of C. oregonensis courtesy Ron Long, SFU, Burnaby, British Columbia.

In the Santa Catalina Islands of California 2 sympatric species Lottia limatula and L. scabra have different behavioural strategies to avoid fast-moving visual predators such as fishes, octopuses, and birds. The former species tends to be active at night and during periods when just awash by the tide; otherwise, they tend to hide in crevices. In comparison, individuals of L. scabra tend to forage from their protective home scars only in daylight when immersed and this behaviour may, at least, minimise potential predation by birds.  Wells 1980 J Exp Mar Biol Ecol 48: 151.

NOTE  Octopus spp. may be major predators of L. limatula, as evidenced by many dead shells bearing drill holes.  The topic of drilling by octopuses is considered in detail elsewhere in the ODYSSEY: LEARN ABOUT OCTOPUSES: FEEDING & GROWTH: DRILLING