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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  Defensive chemicals
 

The topic of defensive chemicals is considered in this section, and topics of ATTACHMENT-STRENGTH PROTECTION, SHELL PROTECTION, ESCAPE-CRAWLING FROM SEA STARS, PREDATION BY BIRDS, and CAMOUFLAGE, 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.

Also included here is a short section on chemical defenses in limpet-like pulmonates.

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Research study 1
 

There are few examples of defensive chemicals in limpets, even on a world-wide1 basis.  However, a novel triterpene called limatulone has been isolated from the foot of limpets Lottia limatula at the Scripps Institution of Oceanography, La Jolla, California.  Laboratory and field experiments suggest that the chemical acts in defense against some intertidal predators but not others.  Thus, tidepool cottid fishes Girella nigricans and hermit crabs Pagurus samuelis reject pieces of the foot of L. limatula, but readily eat foot tissue from co-occurring limpets L. scabra and L. gigantea.  In contrast, various sea stars2 (e.g., Pisaster giganteus) and sea gulls Larus californicus readily eat foot-portions of L. limatula.  The intertidal fish, Gibbonsia elegans, known to eat limpets in the photograph of Lottia limatula: tidepool cottid fishes Girella nigricans and hermit crabs Pagurus samuelis reject pieces of the foot of L. limatula, but readily eat foot tissue from co-occurring limpets L. scabra and L. gigantea courtesy Pawlik et al. 1986 Mar Ecol Progr Ser 30: 251field, reject food pellets containing approximately 0.5ppt of limatulone.  Limatulone is found only in foot tissue and not in viscera, shell, or foot mucus. 

In one boulder-field area, the risk of partial shell loss3 from impact damage in L. limatula, which results in loss of the outer ring of shell ("skirt") and which would otherwise expose the now-visible limpet’s foot to possible predation, may be mitigated by the presence of the defensive chemical. The authors note the absence of limatulone from gut tissue and conclude that it is most likely synthesised4 from related but inactive dietary precursors, possibly seaweeds. Albizati et al. 1985 J Org Chem 50: 3428; Pawlik et al. 1986 Mar Ecol Progr Ser 30: 251.

NOTE1 other world species of gastropods, most notably the pulmonate “limpet” Siphonaria spp. contain secondary metabolites known as polyproprionates but, at the time of the publication of these papers on L. limatula, the pharmacological significance of these metabolites had not been investigated

NOTE2 other defensive strategies, such as as fast escape crawling, may “kick in” with respect to sea-star predators.  This is considered elsewhere in this limpet section of the ODYSSEY: DEFENSES & PREDATORS: ESCAPE-CRAWLING FROM SEA STARS

NOTE3 occurrence of such limpet “skirts”, especially from L. limatula. in washed-up beach debris has been linked to predation by crabs Pachygrapsus crassipes.  Details of this work can be found elsewhere in this limpet section of the ODYSSEY: DEFENSES & PREDATORS: SHELL PROTECTION

NOTE4 in a later paper by researchers primarily in Japan, but also at the Scripps Institution of Oceanography, L. limatula is shown actually to synthesise 3 stereoisomers of limatulone. Mori et al. 1992 Nat Prod Lett 1 (1): 59.

 
 

LIMPET-LIKE PULMONATES

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Research study 1
 

photograph of a sea star Pisaster ochraceus reacting to the chemical secretion from an intertidal pulmonate Trimusculus reticulatus
A researcher at Long Marine Laboratory, U.C. Santa Cruz describes a mucous secretion produced by the marine pulmonate Trimusculus reticulatus that deters predation by sea stars Pisaster ochraceus and P. giganteus.  Not only does the secretion deter attack, but when spread on shells of limpets Lottia spp. the treated ones are eaten by Pisaster significantly less often than clean ones.  The mucus operates by apparently “stunning” the tube feet of the predator, leaving them temporarily functionless.  In choice tests, P. ochraceus  will eat 5 times more Lottia spp. than Trimusculus reticulatusRice 1985 J Exp Mar Biol Ecol 93: 83.

NOTE  like all pulmonate gastropods, Trimusculus requires air to survive.  Its usual habiat is on the roofs of caves and undersurfaces of overhangs in the low intertidal zone

NOTE  a recent description of these glands and their defensive secretions is provided by South African researchers for the related Trimusculus crispus: Pinchuck & Hodgson 2012 J Moll Stud 78: 44.

 

 

View from below of an ochre star Pisaster
ochraceus
responding to the milky-white secretion
from the mantle tissue of Trimusculus reticulatus 0.6X

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Research study 2
 

Researchers at Scripps Institution of Oceanography, La Jolla, California later report on the potential defensive role of chemicals isolated from the mantle, foot, and mucus of the pulmonate “limpet” Trimusculus reticulatus. The mucousy secretion contains 2 types of diterpenoids, likely the active factors noted in the stunning effect on tube feet of sea stars in the preceding Research Study 1.  On the basis that the metabolites might also play a role in deterring possibly competing sessile invertebrates, the authors test its efficacy in preventing settlement of larvae of the sabellarid tubeworm Phragmatopoma californica, and also test feeding responses of wrasses to food pellets containing small amounts of the pure compounds.  Results for the first suggest that small amounts of chemical (mainly diol 1) mixed with sand in an aquarium tank completely prevents larval settlement; but, for the second, that neither diterpenoid substance appears to deter feeding by the wrasses.  Manker & Faulkner 1996 J Chem Ecol 22 (1): 23.

NOTE  known as diol 1and diol 2

NOTE  for several reasons, one of which being that the researchers mistakenly believe they are working with veliger larvae rather than trochophores or older larvae of these polychaetes, the design and results of these experiments are not convincing, and further work may be needed

 
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