Are shell-crushing fishes like pile perch Rhacochilus vacca that forage in the upper intertidal area at high tide, important predators of winkles?  These fishes are daytime visual predators that commonly eat mussels, most notably Mytilus trossulus, but are also known to eat winkles.  The prey is gathered in by means of small conical teeth on the jaws, then crushed between large grinding plates situated at the back of the pharynx.  There are 5 plates: a large lower one, 2 side ones, and 2 top ones. All 5 plates are involved in crushing, as evidenced by their wear patterns (see photos on Right).  No shell fragments are spat out; rather, the crushed prey is encapsulated by mucus into slippery bundles and these are defecated 24-72h after consumption.  A pile-perch’s gut is stomach-less, a feature possibly testifying to the effectiveness of this processing system.  Perch mouth and other drawings modified from DeMartini 1969 Wasmann J Biol 27: 177 and Brett 1979 Can J Zool 57: 658.

NOTE  the only other snail-eating perch on the west coast is the striped perch Embiotoca lateralis.  Boulding et al. 2001 J Shellf Res 20: 403

Several striped perches Embiotoca lateralis
watch attentively as a winkle passes by

Opinions of researchers are actually divided as to whether pile perches Rhacochilus vacca are important predators of winkles.  In captivity a perch will eat one Littorina sitkana in about 20sec and up to 33 in a day.  However, total nutrient and energy return of all these winkles are unlikely to come close to that of a single mussel prey.  Even the largest L. sitkana, 11mm shell width, is able to be eaten by perches in the laboratory, suggesting that there is no refuge in size for the winkles in the field.  Authors of 2 studies done in neighbouring sites in Barkley Sound, British Columbia, both using tethered L. sitkana in pile perch habitats (see photograph on Left), agree that perches eat the snails, but disagree on the intensity and importance of this predation source.  In the later study listed below, most of the winkle mortality is actually atributable to “peeling” by crab predators and not to fishes.  McCormack 1982 Oecologia 54: 177; Boulding et al. 2001 J Shellf Res 20: 403. Photograph of pile perch courtesy James Watanabe, Stanford University, California and SeaNet; photograph of tethered L. sitkana courtesy Elizabeth Boulding, U Guelph, Ontario.

Pile perch Rhacochilus vacca 0.2X

In areas of Puget-Sound, Washington spiny-nose sculpins Radulinus (Asemichthys) taylori and other cottids Artedius spp. prey on small snails including Lacuna spp., Margarites spp., and Alvania spp., as well as on benthic amphipods and other small invertebrates, including limpets, bivalves, and hermit crabs.  Snails comprise about 40% of the diet of Radulinus.  Non-gastropod prey is simply swallowed whole, but about 75% of gastropods are “punched” as they are being swallowed.  The punching is done by a robust toothed device, known as a vomer, located in the buccal cavity of Radulinus and other cottids (see photographs upper Right). Note the replacement teeth on both sides of the vomer. With wear, these migrate anteriorly and eventually become the main, functional, row of the vomer.

The vomer in Radulinus is small, no more than about 5mm wide in a 50-mm long fish. However, it is powerful, and leaves characteristic punch-holes in the shells of its prey (see photographs lower Right). The extent of punching of a species does not relate to degree of shell sculpturing.  Gastropods lacking an operculum, such as limpets Lottia spp. and slipper limpets Crepidula spp. are never punched, and do not survive passage through the gut tract.  Hermit-crab shells are rarely punched, but the crab occupants are digested anyway. The fish is able to distinguish vulnerable from protected prey, probably on first encounter by taste and texture, and then later possibly by sight based on experience.  Some shells are unpunched, possibly a result of a fish suction-capturing snails too rapidly, with the prey passing the vomer too quickly. Interestingly, of the 25% unpunched snails swallowed, almost half will pass through the fishes’ digestive tracts unharmed and alive. The author credits this to the protective nature of the shell and operculum.  The author suggests that an ability to resist digestion in this way may provide a dispersal mechanism for otherwise sedentary species.  Norton 1988 Science 241: 92. E-micrographs courtesy the author; colour photographs of snails courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE  Lacuna has a smooth shell, Margarites has spiral sculpturing, while Alvania has both spiral and axial sculpturing (see photos lower Right). Note the scratch marks on the shell of Lacuna made by individual teeth of the vomer