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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  Escape-crawling from sea stars
  Escape-crawling from sea stars is considered in this section, and topics of ATTACHMENT-STRENGTH PROTECTION, SHELL PROTECTION, 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
 

table showiing running responses of several species of west-coast limpets to four species of predatory sea starsschematic showing distribution of limpet species on the shore in relation to distribution of predatory sea-star speciesWhen the tide is in sea stars are likely to be the main predators of limpets, and the primary defense of the prey is to run.  Accompany-ing a fast gliding-type escape may be rocking or side-to-side swiveling of the shell, and turnng movements away from the predator.  Intuitively, we would expect strongest escape responses from sea-star species that live in the same habitat as the limpets being tested.  The results of studies on west-coast limpets at Friday Harbor Laboratories, Washington and sites in California support this notion.  Note in the schematic above that some limpet species, notably L. persona and L. digitalis, live high on the shore mostly out of contact with any sea stars. Other species, such as Lottia scutum live low on the shore or subtidally, and are in contact with many types of sea stars.

When tested by contact with several common sea stars, Lottia persona and L. digitalis, which live above the usual range of sea stars, tend not to respond to their touch, while L. pelta and L. scutum, which live at a lower level on the shore, tend to run away from the contact (see schematic on photograph: when confronted with a sea-star predator such as  Pycnopodia helianthoides, the normal response of the limpet  Acmaea mitra is to sit tightRight).  The inconsistent response of L. fenestrata may be explained by its preference for smooth, sand-abraded rocks, a habitat not favoured by sea stars.  The lack of response by the dunce-cap limpet Acmaea mitra to several sea stars is notable, and this species may rely on strong attachment to the substratum and possibly chemical camouflage conferred by growth of coralline algae on its shell for defense (see photo on Left).  Bullock 1953 Behaviour 5: 130; Margolin 1964 Ecology 45: 191.

NOTE  L. pelta also shows a strong escape response to the touch of Evasterias troschelii and this sea-star species, along with the other 3 indicated in the schematic, are known to prey on L. pelta in the field



When confronted with a sea-star predator such as
Pycnopodia helianthoides, the normal response of the limpet
Acmaea mitra
is to sit tight, as this one seems to be doing 0.7X

 
Research study 2
 

One of the above studies also shows that the limpets are able to discriminate between predatory and non-predatory sea stars.  For example, Lottia pelta and Lottia scutum, which run fairly consistently from the carnivorous forcipulate sea stars Pisaster ochraceus, Leptasterias hexactis, Evasterias troschelii, Pycnopodia helianthoides, and others, will virtually ignore non-predatory species such as Henricia leviuscula, Pteraster tesselatus, and Dermasterias imbricata, or specialist feeders such as Solaster stimpsoni, Luidia foliata, and Crossaster papposus.  Margolin 1964 Ecology 45: 191.

 
Research study 3
 

photographs showing relative sizes of sea stars Pisaster giganteus, Leptasterias hexactis, and Pycnopodia helianthoidesAs shown in the 2 previous Research Studies, flight responses of limpets to sea stars are strongest to ones naturally encountered as predators in their habitats, and these responses can include crawling upwards.  For example, offshore from Hopkins Marine Station at Pacific Grove, California limpets Lottia scutum and L. limatula respond to the scent of sea stars Pycnopodia helianthoides, Pisaster giganteus, P. ochraceus, and Leptasterias aequalis by crawling up vertical surfaces (about 80% of individuals tested do this), but do not respond, or do so only weakly, to the scent of Pisaster brevispinus or Asterina miniata.  The first 4 sea-star species co-inhabit the same parts of the intertidal region with the limpets, while the last 2 species live mostly subtidally.  Moreover, while the first 4 sea stars are active predators of other motile invertebrates, P. brevispinus mainly digs for clams in sandy substrata and A. miniata is an omnivorous scavenger. The upward escape response is thought to be adaptive because the limpets, after venturing lower in the intertidal region to access richer food resources, can readily escape by crawling a short distance upwards to areas not inhabited by the sea stars. Phillips 1976 Oecologia 23: 83; Feder 1963 Ecology 44: 505.

NOTE how can the comparatively tiny Leptasterias chase down and eat fast-crawling limpets?  One suggestion is that with their small size and thus small-sized “chemical signature”, they may be able to approach closer and their prey is given less time to run away.  Phillips 1976 Oecologia 23: 83.

 
Research study 4
 

photograph of several limpets Lottia scutum in a small tidepool
Further studies at Hopkins Marine Station, Pacific Grove, California on responses of limpets Lottia scutum and L. limatula to Pisaster ochraceus show that normal rheotactic responses are reversed if the limpets sense the upstream presence of the predator.  Thus, limpets that are normally positively rheotactic (i.e., crawl towards a current) will reverse this response and become negatively rheotactic (i.e., crawl in the same direction as the current).  Phillips 1975 J Exp Zool 191: 199. For a comprehensive review of chemosensory assessment of predation risk in animals in general see Kats & Dill 1998 Ecoscience 5: 361.

 
Research study 5
 

photograph of a limpet Lottia limatulaWhat chemoreceptors in limpets are involved in mediating avoidance behaviour to predators, such as sea stars?  The most likely candidates would seem to be the cephalic tentacles, mantle tentacles, and osphradim, as they have known chemotactile and water-tasting functions.  In fact, studies at Hopkins Marine Station, Pacific Grove, California show that in the limpets Lottia scutum and L. limatula the chemoreceptors are located in the mantle edge, possibly in the tentacles.  Thus, heat cauterisation of the mantle margin eliminates the response to Pisaster ochraceus scent, while similar cauterisation of the osphradium/ctenidium does not diminish the avoidance behaviour. Limpets without functional mantle margins respond with normal, vigorous escape movement when touched on the cephalic tentacle with a Pisaster tube foot after the experiment.  The experimental treatment does not, however, eliminate the mantle tentacles as the chemosensory site, as they are destroyed along with the mantle edge, nor does the study exclude the cephalic tentacles as possible distance chemoreceptive sites.  Phillips 1975 J Exp Biol 63: 403. 

NOTE  this “water-tasting” organ is positioned on the left upper wall of the mantle-cavity opening in such a location as to intercept water flow into the mantle cavity

NOTE  these treatment groups died within a week of the cauterisations, a circumstance that does not, however, detract from the conclusions presented in the study

 
Research study 6
 

photograph of a limpet Lottia insessa close to its feeding scar on the kelp Egregia menziesii
Most west-coast species of limpets live on rocks, but at least 3 species, Lottia paleacea, L. insessa, and L. instabilis, live on surfgrasses or kelp plants.  Lottia insessa (see photo on Right) actually creates a grazing scar on feather-boa kelps Egregia menziesii, and its response to contact with predatory sea stars varies depending on whether it is on its scar or not.  Studies at Bodega Marine Laboratory, California show that if on its scar, L. insessa usually responds to contact by elevating its shell (“mushrooming”) and rocking from side-to-side, but rarely moving away from the scar.  If contact is made with a predatory sea star when off its scar, photograph of a limpet Lottia instabilis courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, WAthe limpet crawls rapidly away.  Significant positive responses are usually obtained from contact with the sea stars Leptasterias hexactis, Pycnopodia helianthoides, and Pisaster ochraceus (all known to be predatory), while non-significant responses are usually obtained from contact with Henricia leviuscula and Asterina miniata (both non-predatory). 


photograph of a limpet Lottia paleacea on a blade of surfgrass Phyllospadis scouleriReaction of L. instabilis (photo abpve Left) to a predator almost always involves rapid and vigorous crawling away from the point of contact.  In contrast, touches with a control probe invariably elicit short-duration (30sec) clamping-down responses. Lottia instabilis responds to sea stars in a pattern similar to that of L. insessa, but generally more vigorously. 

The third plant-inhabiting limpet species, L. paleacea, shows no response to any species of sea star, possibly relying instead on chemical crypsis and a shell shape that exactly fits the width of its seagrass-blade (photo on right).  Phillips & Castori 1982 J Exp Mar Biol Ecol 59: 23. Photo of Lottia instabilis courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC

NOTE tests involve touching the mantle margin or cephalic tentacle of a test limpet with either the tube foot of a sea star or with a clean glass probe, no more than once in a 24-h period

NOTE more on this can be found in the CAMOUFLAGE part of LEARN ABOUT LIMPETS & RELATIVES

 

 
Research study 7
 

number of limpets Lottia scabra eaten by various invertebrate and fish predators when the limpet is inside or outside of its home scarphotograph of a limpet Lottia scabra in its home scarThe protection conferred to Lottia scabra by occupation of scars is assessed in a series of laboratory experiments in Sonoma County, California.  Groups of limpets on rocks are exposed to single predators in 1-d experiments.  Half the limpets are presented to the predators still in their original scars, and half are pried out and allowed to re-attach to bare rocks.  Each predator has access to 20 limpets (10 of each type) over a 3-d period.  As limpets are eaten they are replaced.

Results show that occupation of scars provides significant protection against most of the predators tested (see table on Left, "n.s."=differences in data are non-significant).  Exceptions are the sea star Pisaster ochraceus and the crab Cancer antennarius.  Other behavioural observations by the authors indicate that sea-star predators cover the prey, whether they are in scars or not, and then ingest the limpets after 5-10min.  Since scars should not provide any protection against this type of predator, it is not clear why the data for the sunflower star Pycnopodia helianthoides is not also non-significant. Cancer crabs, including C. antennarias, have finely serrated chelae and have less difficulty in removing a limpet from its scar than the more bluntly chelaed grapsoid crab Pachygrapsus crassipes. All the other predators tested, including the flatworm Freemania litoricola, the cottid fish, and the octopus, eat significantly more limpets that are outside their scars.  Kunz & Connor 1986 Veliger 29: 25.

NOTE  actually, both sets of limpets are pried out of their scars, and then half of each set is replaced in their scars.  This is a good design in that it controls for the effect of possible trauma incurred during removal on a limpet’s later susceptibility to a predator

 
Research study 8
 

graph of crawling speeds of limpets Lottia scutum and Lottia peltaAt Bodega Bay and Dillon Beach, California distributions of limpets Lottia pelta and L. scutum are separated vertically (with the former living higher than the latter), but both overlap that of a predator, the sea star Pisaster ochraceus.  When presented with Lottia pelta and L. scutum simultaneously in laboratory dishes, Pisaster preferentially eats pelta over scutum.  The difference in susceptibility to the predator relates to different abilities of the limpets to avoid capture.  First, L. scutum graph of cephalic-tentacle length in limpets Lottia scutum and Lottia peltacrawls faster than L. pelta (0.9 vs. 0.3cm . sec-1) and can actually outrun the predator(which crawls at about 0.4cm . sec-1; see graph on Left)

Second, when scutum runs from a sea star postioned upstream from it, it tends to move away from the predator, while pelta in the same circumstance tends to move in whatever direction it is facing at the time.  Third, scutum’s cephalic tentacles are relatively longer than pelta’s, thus enabling earlier touch perception of a Pisaster closing in from a downstream orientation.

Finally, the 2 species show no significant difference in tenacity. Although certainly aware that many additionaly factors are involved in regulating distributions of the 2 limpet species, the author suggests that scutum, with its better defensive abilities, is able to live lower in the intertidal region than pelta, and this may contribute to their vertical separation.  Bros 1986 Bull Mar Sci 39: 92.

NOTE  escape speed is significantly related to shell size in scutum, but not in pelta

NOTE  measured in field animals by glueing aluminum soft-drink tabs onto their shells, then pulling them sideways from their attachments using spring-loaded weighing scales hooked to the tabs

 
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