Whelks living in intertidal or shallow subtidal regions feed on various hard-shelled prey, mostly mussels and barnacles.  Olive shells in sand habitats have more diverse feeding modes and diets. 

Categorisation of prey species eaten by 287 whelks Nucella ostrina in Mukkaw Bay, Washington shows their principal prey (95%) to be 3 species of acorn barnacles (Balanus glandula, Chthamalus dalli, and Semibalanus cariosus).  The remaining 5% of the whelks in the sample are eating mussels Mytilus spp. Paine 1966 The Am Nat 100: 65.

A whelk Nucella ostrina positions itself
on a barnacle Semibalanus cariosus,
perhaps in preparation for drilling 2X

In shallow sands, the subterranean movements of olive shells Callianax (Olivella) biplicata are readily visible from above from its burrowing tracks.  The species apparently has versatile feeding modes, from deposit-feeding, to suspension-feeding, to eating algae and animal prey.  Edwards 1969 Am Zool 9: 399.

Olive shell Callianax biplicata burrowing on a
sand beach near Tofino, British Columbia 1X

The eastern mud snail Ilyanassa obsoleta was introduced to the west coast in shipments of oysters Crassostrea virginica in San Francisco, California in 1907, in Willapa Bay, Washington in 1945, in Boundary Bay, British Columbia in 1952, and possibly in other locations.  The snails feed on detritus, diatoms, algae, dead plant and animal flesh and, based on often high population densities, are highly successful.  They apparently out-compete other mud snails for food, eat their eggs, and even exclude various detrital-eating spionid worms by taking their food.  Their life span is about 5yr. Ilyanassa is unusual in having a crystalline style, a feature usually associated with bivalves.  The authors suggest that it may be an adaptation for processing plant starches in the diet.  Curtis & Hurd 1981 Veliger 24: 91.

NOTE  this species is a vector for several trematode flatworms, at least one type of which causes swimmer’s itch in marine habitats

Mud snails Ilyanassa obsoleta feeding
on a dead crab Cancer magister 0.3X

Field diets of leafy-hornmouth shells Ceratostoma foliatum vary geographically.  For example, in the San Juan Islands, Washington, their diet is mainly barnacles and pholadid bivalves such as Penitella, while in areas of Oregon they eat mainly bivalves, with littleneck clams Protothaca staminea representing 49% of the diet, the California date mussel Adula californiensis 49%, and Hiatella arctica the remaining 2%.

Most littleneck clams (82%) are drilled at the juncture of the shell edges where the shells are thinnest, and only 18% in the main part of the shell valves.  The drawing shows a Ceratostoma attacking a Protothaca within an old pholid burrow in a rock.  The snail has inserted its foot into the burrow. The 2 lobes of the foot form a guide down which the proboscis is extended for drilling.  Spight et al. 1974 Mar Biol 24: 229; drawing modified from Kent 1981 The Nautilus 95: 38.

NOTE the author has drawn the foot and tentacles symmetrically on either side of the labial spine...could that be the function of the spine?

A study of prey species eaten by Nucella emarginata in Pacific Grove, California over a 4-mo period during Jan-May shows an apparent preference for barnacles Balanus glandula out of a total of 7 prey species.  Twenty-four other potential prey invertebrate species, not listed, are present in the habitat but not eaten.  The snails feed only when immersed by the tide, but remain in feeding position during air exposure.  West 1986 Ecology 67: 798.

NOTE  the 7 species include 4 species of barnacles, 2 of limpets, and one of mussels. Further details on diet selection in these whelks can be found in: FOODS, FEEDING, & GROWTH: FACTORS IN DIET SELECTION

A whelk Nucella ostrina feeds on barnacles
Balanus glandula and Chthamalus dalli 2.5X

In Pacific Grove, California whelks Acanthinucella punctulata eat mostly barnacles Chthamalus spp., but many other prey items are also consumed.  With the exception of limpets, the pattern of prey selection is generally in accordance with the relative abundances of prey (see schematic).  Limpets tend to be avoided, perhaps because they can readily escape.  When the whelk’s tentacles contact a suitable prey item, it climbs onto the prey and begins to drill, generally into the thickest part of the shell.  Once a borehole is created it inserts its proboscis and begins to feed, detaching meat in “relatively large chunks” and passing them into the esophagus.  Sleder 1981 Veliger 24: 172. Photograph courtesy Gary McDonald, Santa Cruz, California and CALPHOTOS.