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.

Dunce-cap limpets Acmaea mitra often have growths of encrusting algae, including red alga Hildenbrandia sp. and crustose coralline alga Pseudolithophyllum sp., that completely obscure their shells.  Since the limpets are often found on these substrata, the growths may provide visual camouflage from predators such as fishes and crabs, or from chemical-sensing predators such as sea stars.

Note that in none of the 4 limpets shown here does the host appear to promote in any way the attachment of the various organisms featured, nor would it have any mechanism to prevent them from attaching. So, their presence, and thus their purported camouflaging effects, are adventitious.

Note also that in the Lottia pelta example, it is quite probably that the mass of limpets, however useful as camouflaging while the limpet was alive, ultimately led to its death though increasing drag forces in waves.

Are there any specific investigations of chemical camouflage in wet-coast limpets?  In the Bodega-Bay region of northern California the surfgrass Phyllospadix scouleri hosts several species of gastropods, including the limpet Lottia paleacea and the predatory sea star Leptasterias hexactis.  Lottia paleacea only lives on surfgrass; hence, is stenotopic.  Adaptations for surfgrass life include narrow precise-fitting shell, strong foot-clamping, ability to subsist on the plant’s epithelial layers as food, and possible chemical camouflaging from incorporation of sulphated flavenoid pigments from the plant into its shell.  As it seeks out prey, Leptasterias probes widely with its relatively long tubefeet.  When contacted by the tubefeet, Lottia clamps down and allows the sea star to crawl overtop of it (the authors note zero captures in 20 such encounters seen in the field).  The predator appears not to detect the limpet against the chemical background of the host plant.  This clamping behaviour contrasts with the active escape behaviours exhibited by the other snails living sympatrically with Lottia (including twisting, falling off, and running away).  Additionally, the authors note that certain algae settle indiscriminately both on Lottia’s shell and on the surfgrass, suggesting that the algal spores identify the former as surfgrass habitat and possibly add further to the camouflaging.  Fishlyn & Phillips 1980 Biol Bull 158: 34.

NOTE  the other species are the snails Lacuna marmorata and Alia carinata.  All 3 species have defensive adaptations that reduce the frequency of predation by Leptasterias.  Only the limpet is considered here

NOTE  lit. “narrow place” G., referring not to the size of the habitat, but to the fact that the limpet lives only on surfgrass

Does the colour of a limpet’s shell provide camouflage protection against visual predators such as fishes, crabs, and birds?  This is tested with 2 sympatric species Lottia digitalis and L. pelta inhabiting the barnacle/mussel zone on rocky shores in Barkley Sound, British Columbia. The first species is a predominately light-coloured species that occurs on light-coloured substrata such as goose barnacles Pollicipes polymerus and thatched barnacles Semibalanus cariosus.  The second species, L. pelta, is a predominately dark-coloured species that occurs on dark-coloured substrata such as rocks and sea mussels Mytilus californianus. 

First, to test for predation by snail-eating perches Rhacochilus vacca and Embiotoca lateralis, and crabs Hemigrapsus spp., the researchers partition wooden trays into 4 spaces into which are placed, alternately, rocks with Balanus glandula growing on them (representing a light-coloured substratum) and sea mussels (representing a dark-coloured substratum; (see photo lower Right).  Eighty limpets of each species are then added to each tray, divided equally between the substratum types.  Copper¹-based anti-fouling paint is applied to the edges and partitions of the trays to prevent the limpets from crossing over or escaping, and the trays are placed at the +1.5m level of the intertidal zone on a shore where sea birds² are absent.  To control for fish predation but still allow crabs to enter and eat, identical trays with mesh screens installed at a 5-cm height above the substrata are also included. The limpets are counted daily, missing ones replaced, and the experiment is run for 8d.  The expectation is that more limpets of each species will be eaten from the substratum type that is most contrasting with the limpet species’ shell-colour. 

The results, tallied as mean number of limpets missing³ after 12-h exposure to fish and crabs, or to crabs only, show that in the unscreened trays, significantly more L. digitalis are missing from the dark mussel-substratum than from the light barnacle-substratum (29 vs. 18, respectively), and significantly more L. pelta are missing from the light substratum than from the dark (15 vs. 7, respectively), as predicted. In the screened trays (protected from fishes but not crabs), there are fewer missing limpets of both species, but no obvious differences between the numbers eaten on the different-coloured substrata. Thus, both fishes and crabs appear to be preying on both species of limpets. Overall, the results show that L. digitalis are 64% more likely to disappear from dark mussels than from light barnacles, and L. pelta are 130% more likely to disappear from light barnacles than from dark mussels, supporting the hypothesis. The authors conclude⁴ that the 2 species of limpets, otherwise overlapping in distributions in at least part of their vertical ranges and eating the same diatom and algal foods, tend to partition their microhabitats by colour, thus minimising interspecies competition and reducing predation. Mercurio et al. 1985 Ecology 66: 1417.

NOTE¹  copper is toxic to, and avoided by, limpets

NOTE²   a similar experiment to test for bird predation (mainly gulls, black oystercatchers, and turnstones) using natural rock/barnacle/mussel arenas on the shore shows that significantly fewer L. pelta go missing from a dark mussel/rock substratum than from a light barnacle/rock substratum, again, as would be predicted if camouflage were operating as a defensive strategy

NOTE³ while disappearance of some limpets from both unscreened and screened trays may be related to death through trauma associated with prying them from their original rocks, the numbers are not great enough to affect the overall conclusions

NOTE⁴ although there are likely to be other factors influencing the outcome of these experiments, such as physical protection provided from the 3-dimensionality of the 2 types of habitats, and more food or different types of food being available, on one or other of the different habitats, the results support the findings of other studies dealing with substratum and shell colours of limpets. These are considered elsewhere in this section on limpets: HABITATS & ECOLOGY: SHELL GROWTH (SHAPE) & COLOUR