Research Study 1
Shells and shell parts of Lottia limatula found at San Nicolas Island, California
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 field, 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.
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: LIMPETS & RELATIVES > PREDATORS & DEFENSES > ESCAPE-CRAWLING FROM SEASTARS
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: LIMPETS & RELATIVES > PREDATORS & DEFENSES > 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 three stereoisomers of limatulone. Mori et al. 1992 Nat Prod Lett 1 (1): 59.
Albizati et al. 1985 J Org Chem 50: 3428
Pawlik et al. 1986 Mar Ecol Progr Ser 30: 251
Mori et al. 1992 Nat Prod Lett 1 (1): 59
Research Study 1: Defensive chemicals in limpet-like Pulmonates
View from below of an ochre star Pisaster ochraceus responding to the milky-white secretion from the mantle tissue of 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 five times more Lottia spp. than Trimusculus reticulatus.
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.
Rice 1985 J Exp Mar Biol Ecol 93: 83
Pinchuck & Hodgson 2012 J Moll Stud 78: 44
Research Study 2: Defensive chemicals in limpet-like Pulmonates
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 two 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.
NOTE known as diol 1 and 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
Manker & Faulkner 1996 J Chem Ecol 22 (1): 23
Manker & Faulkner 1987 Tetrahedron 43 (16): 3677