title for limpet section of the Odyssey
   
  Defenses & predators
 

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.

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  Shell protection
  Shell protection is considered in this section, and topics of ATTACHMENT-STRENGTH PROTECTION, ESCAPE-CRAWLING FROM SEA STARS, PREDATION BY BIRDS, CAMOUFLAGE, and DEFENSIVE CHEMICALS, are considered in other sections. DEFENSES OF KEYHOLE LIMPETS are dealt with separately and include camouflage, mantle response, and (sometimes) aggressive defensive activities of a symbiotic polychaete.
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Research study 1
 

photographs of limpets Lottia limatula and Lottia digitalis after being broken to yield doughnut-shaped shell rings courtesy Shanks & Wright 1986 Oecologia 69: 420
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 photographs of limpets Lottia gigantea and Lottia scabra and Lottia limatula showing impact damage courtesy Shanks & Wright 1986 Oecologia 69: 420
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.

 

 
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 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 photograph of crab Cancer productus courtesy Iain McGaw, University of Nevadaused 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 on attack success by C. productus 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 table of data on mode of attacks on limpets by crabs Cancer productusfollowing results (see table of data).  Of 448 attacks by Cancer productus on equal numbers of 4 treatment groups of limpets over several days in the laboratory, most are of the prying type. Most of these (65%) are on L. persona. Overall, only 26 limpets are killed in 448 attacks, representing a photograph of crab Cancer oregonensis courtesy Ron Long, SFU, Burnabysuccess rate of 4%.

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.

 
Research study 3
 

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

 
Research study 4
 
photograph of dead shell-assemblage in San Juan Island, Washington, courtesy authors

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 below of L. digitalis being attacked) 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 reasons to explain it are unclear, bioerosive evidence suggests that drilled shells are weaker than undrilled ones and do not preserve as well.  Thus, use of this type of taphonomic (fossilization) 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. Yanes & Tyler 2009 Palaios 24: 280. Photographs courtesy the authors.

NOTE  Nucella whelks present in the study area include N. lamellosa, N. ostrina, N. lima, and N. canaliculataphotograph of whelk attacking limpet

graph showing size refuge reached by limpets from predation by whelks

schematic showing drill-hole disposition in limpet shells
graph showing relationship between bore-hole diameters and shell sizes of limpet prey
 
 
Research study 5
 

graph showing relationship between shell height and shell length for 3 species of limpets Lottia persona, L. digitalis, and L. scutumWest-coast limpet species have shells ranging from smooth (e.g., Lottia persona, L. fenestrata, L. scutum, and L. asmi) to heavily ridged (e.g., L. digitalis, L. scabra, and L. pelta). While much has been written theoretically on the protection from durophagous (shell-crushing) predators that may be conferred by shell shape (flatness=hard to grip) and ornamentation (ridging=resistance to compressive forces) of limpets, only a few experiments have actually tested this for west-coast species. Indeed, a recent investigation at Friday Harbor Laboratories of shell morphologies of 3 species L. digitalis, L. pelta, and L. scutum in relation to predation by red-rock crabs Cancer productus reveals no significant survival value of shell size or ornamentation. In fact, of the 3 species tested in the laboratory, the flatter-, smoother-shelled L. scutum actually experiences least mortality (28% in one experiment, versus 79 and 58%, respectively, for ridge-shelled L. digitalis and L. pelta). The crabs are less able to gain purchase with their claws on the flat species than on the taller ridged species (see graph for comparative sizes). Note that these taller species, especially L. digitalis, live higher on the shore than L. scutum, out of easy reach of foraging C. productus except during highest tides. The authors suggest that rather than evolving in response to predation, change in shell morphology in limpets may have been more in response to physical factors in the intertidal environment, such as temperature and wave action. Tyler et al. 2014 J Exp Mar Biol Ecol 451: 9.

NOTE the most common attack strategy of the crabs in the experiment is to pry up the limpets, then eat the flesh. Two other modes of attack, apex crushing and edge crushing, together comprise only 20% of overall mortality in the experiment

NOTE it is unclear why the authors choose to transform the length and height data in the graph logarithmically. These parameters will scale linearly when plotted arithmetically, if that is what is needed for statistical analysis

 
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