One of Kachemak Bay’s most numerous snails is the pea-sized Sitka periwinkle (Littorina sitkana) found in the upper reaches of our shoreline’s intertidal zone. While a variety of periwinkle snails are found worldwide, the Sitka periwinkle ranges from western Alaska to Puget Sound, Washington. The common and species names indicate where the snail was first described — Sitka, Alaska, while its genus name (Littorina) means “shore dwellers” in Latin.
Periwinkles are small herbivorous snails with drab-colored, fat and roundish shells about three-fourths inch in height. Usually blackish-gray or dark brown, these shells exhibit four or five whorls of varying sculpture and color. Alternating dark and yellowish or white-gray bands are common. The shell’s wide circular opening, or aperture, is almost half as tall as the shell, and the horny trap door, or operculum, is brownish. The animal itself is black with a head that bears long, delicate antennae.
On shorelines with moderate surf this snail may live within the splash zone (even in aquaria Sitka periwinkles remain near the water’s surface). One of the remarkable strategies an exposed Sitka periwinkle uses to survive is to trap moisture within its shell. Mucus produced by the foot hardens at the margins of the aperture as the snail clamps down, forming a seal around the shell. Meanwhile the snail withdraws into its shell and closes the operculum, further tightening the seal and preventing the mollusc from drying out. Researchers studying related European periwinkles have kept these snails dry for 42 days and immersed in freshwater for 11 days without damage. Periwinkles are survivors — they need only wet their gills occasionally, can tolerate relatively low salinities and can last for long periods of time without food.
The coiled ribbon-like tongue, or radula, of a Sitka periwinkle sports 200-300 rows of minute rasping teeth, as well as an entangling coating of mucus, to scrape tiny particles of algae from rocks. These snails are commonly seen among acorn barnacles of the high and mid-intertidal zones. In fact, Sitka periwinkles and pulmonate snails (Siphonaria thersites) are the most abundant grazing invertebrates in the upper intertidal zone of Kachemak Bay’s rocky beaches. Deeper habitats are generally not preferred by these snails because of predation by sea stars, especially little six-rayed stars (Leptasterias hexactis) and true stars (Evasterias troschelii), anemones and other invertebrates. The rubbery masses of small round eggs laid by female Sitka periwinkles become food for intertidal fish such as blennies and sculpins. At lower reaches of a beach, what may look like a mass of periwinkles under a rock is more likely to be an aggregation of hermit crabs in second-hand periwinkle shells.
To observe and learn more about these fascinating molluscs, join the Research Reserve staff at a special Discovery Lab featuring the ecology of snails, clams, and chitons on Jan. 4. To learn more about the Kachemak Bay Research Reserve, please visit www.kbayrr.org.
Carmen Field is a biologist, naturalist and marine science educator at the Kachemak Bay Research Reserve.
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