title for learn-about section on whelks & relatives in A SNAIL'S ODYSSEY
 

Predators & defenses

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Escape by burial photograph of mud snails Ilyanassa obsoleta

This section on predators & defenses is divided into topics of escape by burial considered here, and
DURING EARLY DEVELOPMENT

ESCAPE BY SWIMMING, CRAWLING, HIDING, OR BITING
SHELL COLOURS & CAMOUFLAGE
SHELL SCULPTURING
SHELL SIZE & THICKNESS, and
NOXIOUS SECRETIONS
, considered in other sections.

 

 

 

 

Mud snails Ilyanassa obsoleta are preyed upon
by fishes, birds, and crabs.  During low-tide periods
their only defense is to burrow under the mud 1.5X

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Research study 1
 

olive shells Callianax biplicata drilled by moon snails PolinicesPredators of olive shells Callianax biplicata in Oregon include octopuses, which sometimes drill a hole and inject toxins, sea stars, moon snails, crabs, shore birds, and fishes.  Defenses of olive shells include quick burial, protective shell and, to evade sea-star and moon-snail predators, high-speed crawling.  Contact with tube feet of certain sea stars or the foot of a moon snail Polinices spp. induces a backwards flip followed by fast crawling.  Burial is not much defense against a moon snail, and several Callianax may be caught and held in the soft body tissues of the predator for later consumption.  Polinices drills in a standard location on the shell of its prey.  Incomplete bore holes are suggestive of death of the olive shell by suffocation.  Edwards 1969 Veliger 11: 326.

NOTE  drilling by octopuses is considered elsewhere in the ODYSSEY: LEARN ABOUT OCTOPUSES/FEEDING & GROWTH/PREY HANDLING & DRILLING

 

Note that several of these randomly collected
dead shells have incomplete boreholes

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Research study 2
 

graph showing relationship between size and density of olive shells Callianax biplicata and tidal height on an Oregon beachStudies at the Oregon Institute of Marine Biology, Coos Bay, Oregon show that larger-sized olive shells Callianax biplicata live higher on the shore than smaller-sized ones, possibly owing to their greater tolerance to drying and temperature extremes (see graph).  The size difference at extreme tidal levels is not great but, as suggested by the author, the behaviour may provide a degree of refuge from predation for at least a portion of the population.  As a comparable size-separation can be observed in olive shells kept for 72h in a long tray of sand exposed to a gradient in light, it is thought that light may be cueing the behaviour.  Edwards 1969 Am Zool 9: 399.

NOTE note also in the graph, however, that densities are much greater lower on the beach where predation risk may be higher

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Research study 3
 

graph showing daily activity rhythms of olive shells Callianax biplicata in a laboratory settingphotograph of several olive shells Callianax biplicata in different stages of burial




















Since male Callianax biplicata are larger than females, then it is probable that the high-shore dwellers are males.  Laboratory studies at the Bodega Marine Laboratory, California show that olive shells emerge from burial at night to crawl around on the sand surface, and that the light/dark cycles are quite regular.  Field observations show a similar activity cycle (see graph).  Phillips 1977 Veliger 20: 137.

Olive shells Callianax biplicata in
assorted stages of burial 0.1X

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Research study 4
 

photo sequence showing burial of an olive shell Callianax biplicata in response to the chemical scent of a predatory sea star Pycnopoida helianthoidesOlive shells Callianax biplicata in intertidal sandflats are usually buried when the tide is out.  Studies at the Bodega Marine Laboratory, California show that olive shells will also bury themselves in response to water-borne chemicals released from some sea-star predators but not others.  Tests of responses of Callianax to 4 sea-star species show that 3 carnivorous species Pisaster ochraceus, P. brevispinus, and Pycnopodia helianthoides induce fast burial in the snail, while the omnivorous scavenger Patiria miniata elicits no or only weak response. Phillips 1977 J Exp Mar Biol Ecol 28: 77.

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