subtitle for learnabout section of A SNAIL'S ODYSSEY
  Predators & defenses

Intertidal life, especially in the upper shore regions, may be especially risky because of exposure to a greater variety of terrestrial, aerial, and aquatic predators than would be experienced in subtidal areas.  Other than the shell and operculum, which offer physical protection, and the checker-board patterns of Littorina scutulata and L. plena, which may provide camouflaging protection, winkles have no defenses against predators.  The main predators of littorinids are crabs, fishes, and birds, and of lacunids and slipper limpets, sea stars. 

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Camouflaging protection

  The topic of predators & defenses is divided into camouflaging protection, considered here, and PREDATION ON ADULTS BY CRABS, PREDATION ON ADULTS BY FISHES, PREDATION ON ADULTS BY SEA STARS, and PREDATION ON LARVAE considered in other sections.
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Research study 1

drawing of hairy snail Trochotropis cancellata bearing parasitic epifaunaA researcher at Friday Harbor Laboratories comments that no gastropod shell is so richly covered with attached organisms than is the shell of the hairy snail Trichotropis cancellata.  The author’s list of these symbionts reads like the index of a marine-invertebrate text, and includes such things as hydroids, polychaetes, snails, sponges, tubeworms, barnacles, tunicates, and the like (see drawing).  With such a load to carry about, the author suggests that the life style of the host may be quite sedentary.  Trichotropis feeds by suspension-feeding, so sitting in one spot would suit both it and many of its symbionts who also suspension-feed.  The relationship seems a clear case of parasitism, but not if the host snail derives benefit, perhaps in the form of camouflaging protection, from its passengers.  The outer periostracum covering of the shell is drawn up into spines, and may confer protection in one form or other. Trichotropis might present an interesting research project for someone. Yonge 1962 Biol Bull 122 (1): 160.

Snail Trichotropis cancellata hosting 2 large tunicates
Pyura haustor
and a juvenile tubeworm Eudistylila vancouveri

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Research study 2

photograph of hairy snail Trichotropis cancellataLater research on the function of the periostracal hairs of Trichotropis cancellata at Friday Harbor Laboratories involves an ambitious series of experiments with potential predators, water-flow dynamics, and epibiont deterrence, some results of which are summarised here.  1)  some predators, most notably crabs Cancer oregonensis and sea stars Pycnopodia helianthoides, prefer snails with the periostracum removed over control snails, but the results are not overly convincing.  Another potential crab predator C. productus shows no significant preference, 2) collections of dead shells from field experiements are not significantly biased towards smooth-shelled specimens vs. normal hairy specimens (note, however, that sample sizes used in the experiment are small), 3) in laboratory flume-experiments, the hairy periostracum does not alter water flow in any consistent way, 4) when allowed to parasitise tubeworms for their food, hairy- and smooth-shelled snails grow at similar rates, whereas when constrained to suspension-feed, hairy individuals grow more slowly, 5) hairy and smooth individuals falling through the water column land more or less randomly, and righting times after falls are similar, 6) a hairy periostracum significantly deters field settlement by barnacles over a 1yr-exposure period, thus reducing mass and drag of epibiont load to be carried about.  A problem arises in that of 18 snails recovered after the 1yr field deployment, 17 are smooth and 1 hairy.  Fortunately, post-deployment growth has produced a new, hairy body whorl on the 17 smooth specimens, so the authors have used settlement on this to bulk up the “hairy” portion of the data-base.  The authors conclude that the periostracum acts as a slight deterrent to some predators, and a stronger deterrent to barnacle settlement.  Is there anything more that could have been explored?  The authors’ main conclusion that deterrence of larval settlement by barnacles would potentially reduce the mass to be carried about is a good one, but should be considered in light of earlier comments about the abundance of epibiont parasites on Trichotropis made by the author of Research Study 1 above, done also in the San Juan Islands.  Iyengar et al. 2008 Invert Biol 127 (3): 299.

NOTE  the researchers use 2 methods to remove the periostracum: rasping with a file and dipping the live shell in bleach to partially digest the protein (periostracum) covering, which can then be more easily scraped off.  This latter may not be advisable owing to risk of diffusion of bleach into the shell and possibly into the mantle tissue of the body

NOTE  these experiments involve tethering (for easy recovery) hairy and smooth specimens in areas where potential predators are common, either to plastic sheets or to rocks bearing tubeworms (so that the snails can facultatively behave as food-thiefs, or kleptoparasites, from these host worms): see LEARN ABOUT TUBEWORMS: FOODS & FEEDING: FOOD THEFT BY SNAILS

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