As unusual as it seems, the olive shell Callianax biplicata is able to swim away from contact with potential predators.  Usually on contact with a sea star such as Pisaster brevispinus, Callianax is more likely to turn sharply and crawl away, or to burrow into the sand.  Sometimes, however, it rears up, withdraws its propodium, and extends the metapodia sideways and forwards.  This flips the snail onto its back in a half somersault. The metapodium is now extended horizontally on both sides as a flat membrane and is flapped.  The motion lifts the snail from the substratum and carries it a short distance in upside-down swimming.  Edwards 1969 Veliger 11: 326.

NOTE shown here is swimming of the related Callianax zanoeta from the Gulf of California.  Drawings from Farmer 1970 Veliger 13: 73.

Olive shell Callianax biplicata creates a shallow
trench as it crawls along the sand surface 0.5X

Observations of whelks Nucella ostrina at the Bodega Bay Laboratory, California reveal that they respond to scent or touch of sea stars Pisaster ochraceus with defined escape behaviours.  These include rocking of the shell, turning, and 180^o reversal of direction of movement.  Miller 1986 Veliger 28: 394.

Experiments on whelks at the Bamfield Marine Sciences Centre, British Columbia show that Nucella lamellosa can discriminate between the effluents of predatory and non-predatory crabs.  Thus, while Nucella crawls away from seawater that has passed over known predatory crabs Cancer productus, they are indifferent to seawater pasing over non-predatory kelp crabs Pugettia producta and lithode crabs Lopholithodes mandtii.   Interestingly, Nucella actually appears to be attracted to the scent of small shore crabs Hemigrapsus nudus, a species known to feed on small-sized snails and thus to be a potential predator of juvenile Nucella.  The results suggest that Nucella can assess from a distance the relative risks posed by different species of crabs and respond accordingly.  As for the apparent attraction to H. nudus, the authors suggest that it may aid in the snails finding protective under-rock refuges, habitats also occupied by the crabs. It is an interesting suggestion, but one that probably will need further researching. Marko & Palmer 1991 Biol Bull 181: 363.

NOTE  the test device is a plastic platform, perforated to allow water to drain through, that receives seawater from holding containers on either side.  A snail placed on the platform thus simultaneously contacts seawater from 2 sources, either crab vs. no-crab, or other combinations.  A snail crawling off the platform to one side or the other is scored as making a choice

Three whelks Nucella lamellosa, probably feeding
on barnacles Balanus glandula. The snail at
the lower left is Lirabuccinum dirum 1.4X