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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Predation on adults by sea stars

  The topic of predators & defenses is divided into predation on adults by sea stars, considered here, and PREDATION ON ADULTS BY CRABS, PREDATION ON ADULTS BY FISHES, PREDATION ON LARVAE, and CAMOUFLAGING PROTECTION considered in other sections.
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Research study 1

photograph of snail LacunaMarmorataTrampusGoodmanarmorataResearchers at Bodega Marine Laboratory, California describe escape behaviours in the lacunid snail Lacuna marmorata  in response to predatory sea stars Leptasterias hexactis.  The snails are customarily found on low intertidal rocks and algae or on the blades of surfgrass Phyllospadix scouleri, where they may encounter Leptasterias.  In laboratory observations, within seconds of perception of the sea-star’s scent Lacuna raises its shell and twists it back and forth through an arc of 360 degrees while waving its tentacles.  This is often followed by a running response that may take the animal up and out of the water in its beaker.  While 60 out of 60 individuals react to Leptasterias with twisting and tentacle waving,  only 58/60 respond to the non-predatory sea star Patiria miniata, and 0/60 to being touched by a metal probe.  If touched by a tube foot of Leptasterias, the snail rears up, then releases its attachment and falls.  In the field, if this happens while on a blade of surfgrass the snail may remain attached to its original perch by a mucous thread.  On touching another blade, it quickly re-attaches and crawls away.  In tests with 75 Lacuna on surfgrass blades in the field, 74 respond vigorously to contact with Leptasterias, 68 of which involve falls.  Fishlyn & Phillips 1980 Biol Bull 158 (1): 34. Photograph courtesy Trampus Goodman & LifeDesks Marine Biodiversity of British Columbia.

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Research study 2

Members of genus Littorina are rarely preyed on by sea stars owing to their high positions on the shore.  A related gastropod, however, the slipper limpet Crepidula adunca,  shares its intertidal and subtidal locations with several asteroid species, any one of which would happily make a meal of it.  Crepidula’s situation is made doubly jeopardous by the fact that it is parasitic on the shells of other gastropods, so a hungry sea star could be photograph of a slipper limpet on its host snail Calliostoma ligatumsensing the limpet, its host, or both.  If the host gets caught, then both may be eaten.  Studies at Friday Harbor Laboratories, Washington reveal that the principal host for the parasitic C. adunca on nearby shores is the trochid snail Calliostoma ligatum, and it happens that this snail species has an effective shell-twisting, and running escape response to attack by asteroid predators.  A second line of defense for the limpet is that its shell fits closely to that of its host, making a strong, tube-foot and crab-claw resistant seal.  Costs may be high for the host Calliostoma, for the presence of even a single limpet can add up to 30% to the mass that the host must carry about.  This reduces its escape speed from an attacking sea star by up to 20% and, if dislodged to land aperture-up, can significantly increase righting time (by a factor of 2.5 for Calliostoma and 1.5 for Tegula).  Are there any advantages for the host snails in having attached limpets?  The authors remark that one possible benefit could be that the limpet’s presence may effectively increase the size of the host snail to be subdued by a potential predator.  On the flip side, disadvantages to the limpet could include not being able to suspension-feed when the host is exposed to low-tide conditions, a situation that would be more common with limpets on the intertidal Tegula than on the generally subtidal Calliostoma.  Vermeij et al. 1987 The Nautilus 101 (2): 69. Photograph courtesy Richard Lowell, University of Alberta, Edmonton.

NOTE  also used in the study is another trochid snail Chlorostoma funebralis, collected from open-coast regions of Washington where it hosts the slipper-limpet C. adunca

Slipper limpet Crepidula adunca on
its host snail Calliostoma ligatum 4X
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