Predators & defenses
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  Chemical defenses
  Topics relating to predators & defenses include chemical defenses, considered here, and LARVAL, PHYSICAL, and BEHAVIORAL defenses, considered in other sections.
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photograph of abalone Haliotis kamtschatkana Although the production of defensive chemicals by west-coast gastropods is more commonly found in opisthobranchs, there is evidence that abalones and certain trochid snails release secretions, apparently in defense. Defenses of opisthobranchs are considered elsewhere in the ODYSSEY: LEARN ABOUT NUDIBRANCHS & RELATIVES: PREDATORS & DEFENSES
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Research study 1
  Coincidental with shell-twisting and fast locomotion in abalones, mucus is released onto the shell from the up-curled mantle edge and also from exhalent ventilatory holes.  The first is thought to interfere with attachment of a sea star’s tubefeet, but whether a defensive chemical is involved in this or in the vented mucus is not known.   Studies on the Californian species Haliotis assimilis and H. rufescens show that the mucus may cause flight or irritation responses in conspecifics.  Mongomery 1967 Veliger 9: 359.
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Research study 2
 

Calliostoma species are notable for the pristine condition of their shells and lack of fouling organisms.  The observation that dead shells are often fouled suggests that some property of the living animal is responsible for the external cleanliness of the shell.  Shell wiping, that is, extension of the posterior photograph of trochid snail Calliostoma canaliculatum courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washingtonpart of the foot to deposit a layer of mucus over all parts of the shell surface, is described for C. canaliculatum collected at Pacific Grove, California, and is known for other world species of Calliostoma.  The author comments on the “slippery” feel of the shell of C. canaliculatum, but not of the shell of C. annulatum.  Keen 1975 Veliger 17: 413. Photograph of C. canaliculatum courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC; photograph of C. annulatum courtesy Ron Long, Simon Fraser University, Burnaby, British Columbia.

NOTE  a more complete study is reported for C. zizyphinum in Devon, England. If the mucus is wiped off experimentally, the snail senses its absence and repeats the wiping photograph of Calliostoma annulatum courtesy Ron Long, Simon Fraser University, Burnaby, British Columbiaprocedure.  The author speculates on whether the protection is active, where the act of wiping may remove loose organisms, or passive, where a layer of mucus may act as a physical or chemical barrier to settlement. Jones 1984 J Moll Stud 50: 245.

Calliostoma annulatum
= not slippery 1.2X


Calliostoma canaliculatum

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Research study 3
 

histogram showing level of response of sunflower stars Pycnopodia helianthoides to different concentrations of defensive exudate from trochid snails Calliostoma canliculatumWhen attacked by predatory sea stars (Pisaster, Pycnopodia) photograph of trochid snail Calliostoma canaliculatumthe trochid snail Calliostoma canaliculatum may move away, clamp down, or (occasionally) pull into its shell, depending upon the attacking species.  If escape is prevented, it releases a viscid yellow secretion from its hypobranchial1 gland out of the shell aperture.  Subsequently, the snail may spread the exudate over its shell with its foot.  Studies on sea stars Pycnopodia helianthoides and Pisaster giganteus collected around Monterey, California show that response to the exudate is in graded proportion to its concentration, with 10-3 dilution representing the lower limit of sensitivity2Pycnopodia helianthoides3, for example, responds with arm coiling at low concentrations (3.2 x 10-3 mg dried exudate in seawater) then progresses through graded responses4 of tube-foot withdrawal, tightening of arm tip, and arm coiling with increasing concentrations up to 3.2 mg dried exudate in seawater.  There is only limited response to control treatments of seawater.  Interestingly, Calliostoma does not respond to water-borne odours of either sea star species.  The authors describe the release of a known defensive substance from the hypobranchial gland as being rare among gastropods.  Bryan et al. 1997 J Chem Ecol 23: 645.

NOTE1 a secretory gland located, as the name implies, below the gill

NOTE2 applied in 75µl aliquots via syringe 1cm distance from arm tip

NOTE3  similar, but slightly more sensitive, responses are obtained in trial with Pisaster giganteus (data not shown here)

NOTE4  level of response is scored as:

0 = no response
1 = tube-foot withdrawal (TFW)
2 = TFW and tightening of arm tip around ambulacral groove
3 = TFW, tightening, and arm coiling
4 = TFW, tightening, arm coiling, and movement away.  The values are summed and averaged over 30 replicates

 

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Research study 4

 

photograph of Calliostoma canaliculatum from belowLater isolation and analysis of the biologically active component of secretions from specimens of Calliostoma canaliculatum collected in the Pacific Grove region of California show K-channel-blocking activity.  The authors note that the compound (BrMT) represents a novel class of K-channel inhibitors that exert their effects on channel-gating, rather than actually blocking the pore as found for peptide toxins in other animals (e.g., snakes, cone shells, scorpions, and sea anemones).  The authors remark that BrMT has structural similarity to tyrindoxyl sulphate, a precursor of Tyrian purple dye, also produced in the hypobranchial glands of certain muricid gastropods (Murex trunculus and other species).  Kelley et al. 2003 J Biol Chem 278: 34934.

NOTE  the active compound is not a peptide, but a disulphide-linked dimer: 6-bromo-2-mercaptotryptamine


Calliostoma canaliculatum crawling and feeding on aquarium glass 1.5X

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Research study 5
 

graph showing relationship between sea-star densities and snail (Calliostoma brunnea)densitiesTurban snails Chlorostoma brunnea mostly have shells that are clean of fouling growths such as seaweeds.  However, in areas of northern California such as around the Bodega Marine Laboratory, they are often 75-100% covered in growths of red algae Peyssonnelia meridionalis or crustose corallines Lithothamnion and Pseudolithothamnion spp.  Predators of C. brunnea in this area include ochre stars Pisaster ochraceus and sunflower stars Pycnopodia helianthoides, and the question arises as to whether the algae protect their hosts from predation.  Laboratory feeding tests show, indeed, that the sea stars consume 3-4 times more bare (unfouled) snails than they do ones covered with algae.  That a predator-prey relationship does exist is suggested by an inverse correlation between numbers of snails and sea stars (see graph).  The epibiont algae therefore appear to benefit their hosts via an associational resistance to sea-star predators.  The mechanism of protection is not known, but may be physical or chemical, or perhaps a combination of the two.  The author notes that in addition to this protection, the seaweed fouling may act as camouflage to visual predators such as fishes or birds.  All of these ideas will require further research.  Thomber 2007 Mar Ecol 28: 480.

NOTE  mortality risk for one species is reduced by living near or in close contact with another species

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