Littorina saxatilis is a small Atlantic periwinkle that has been found on the Pacific Coast only in San Francisco Bay. Its shell has 4-5 whorls (a whorl is one full spiral turn of the shell) with a conspicuous shoulder where one whorl attaches to the preceding one, and strong spiral ridges on its surface. It grows to a height of 12 mm in San Francisco Bay, and 18 mm in the Atlantic Ocean. In the bay the commonest color is yellow, or yellow on the lowest whorl and gray or brown on the upper ones, sometimes with brown or white spots or spiral brown stripes. The interior of the shell and the operculum—the rigid oval plate that closes the aperture when the snail withdraws into its shell—are typically brown. Occasionally, shells from San Francisco Bay may be white or dark blackish brown.

In San Francisco Bay, the shells of Littorina saxatilis are often infested with a marine fungus that is also found on native barnacles and periwinkles. The fungus makes microscopic pits in the shell, filled with black fruiting bodies, which can turn the shell dark when they are abundant. Some shells have bright green alga growing on the spire.

In the Atlantic, Littorina saxatilis lives in a variety of habitats, including the high intertidal and splash zone on moderately-to-highly exposed coasts, where it may be restricted to protected crevices; in the intertidal and shallow subtidal zones in lagoons and estuaries, on the rockweeds Ascophyllum and Fucus; in salt marshes on cordgrass stems; and on firm mud and on beaches on scattered stones, shells and debris. In favorable habitats, population densities can reach as high as thousands or even hundreds of thousands of snails per square meter.

In San Francisco Bay, Littorina saxatilis is found on rocks in the high intertidal zone along with the native barnacle Balanus glandula, typically nestling in rock crevices or empty barnacle shells when the tide is out. Densities of around 2000 snails per square meter have been observed.

Littorina saxatilis feeds on diatoms, filamentous algae and plant litter, and can survive in salinities down to 5-7 ppt. Unlike many periwinkles, it broods its larvae internally and releases them as tiny snails. It can live for up to 6 years.

Similar Species

There are several native Littorina species on the Pacific Coast that could be confused with Littorina saxatilis. Three of these occur in the San Francisco Bay area, but are narrower and higher-spired than Littorina saxatilis and never have a yellow shell or yellow lower whorl as Littorina saxatilis often does. Two other Pacific Coast species are sometimes very similar to Littorina saxatilis in shape and color, but these are not found south of Humboldt Bay. There are also two other Atlantic species that are frequently transported to California with marine bait worms. The descriptions here are based primarily on Reid (in press).

Two very similar Littorina species found in San Francisco Bay have shells that are high-spired and conical compared to Littorina saxatilis' more rounded shell. Their shells grow to 18-19 mm high, and are mainly black or have a dotted, banded or checkered pattern on a brown, olive or black background (never yellow), and the operculum and interior of the shell are black. Both species range from Alaska to Baja California. Littorina plena occurs on moderately exposed or sheltered shores and in salt marshes. It often has a pale stripe on the inside of its shell within the aperture, and the tentacles on its head are all black or have a longitudinal black stripe. Littorina scutulata occurs on wave exposed to moderately protected shores. It usually does not have a pale stripe on the inside of its shell, and its tentacles have transverse black bands.

The third native species in the San Francisco Bay area is Littorina keenae, which ranges from Coos Bay, Oregon to Baja California. It is found on bare rock in the high intertidal zone on the open coast. It grows up to 23 mm high, its shell is black or brown with irregular white flecks, and it has a conspicuous white stripe on the inside of its shell. The inner edge or lip of the aperture is flat and smooth.

Two northern species are close relatives of Littorina saxatilis (all three are in the subgenus Neritrema), and harder to distinguish from it. Littorina sitkana ranges from Alaska to Cape Arago, Oregon, and is usually found on sheltered rocky shores. It grows up to 21 mm high, and is highly variable in shell shape (from high-spired and thin-shelled to rounder and thicker-shelled), surface texture (from smooth to having strong spiral ridges) and color (usually blackish brown, but sometimes white, yellow, orange or banded). Littorina subrotunda is found from Humboldt Bay north, and is lives in habitats ranging from highly wave-exposed rocky shores to salt marshes. It grows up to 12 mm high, and is similarly variable in shell shape, surface texture and color (including black, white, orange and banded forms).

Littorina littorea is a European species that was introduced to eastern North America, where it is now common. It is often found in the algae used to pack shipments of New England bait worms and lobsters. Single snails or small populations have occasionally been found in San Francisco Bay since the 1960s. It grows up to 30 mm high, and has a thick brown or blackish shell with fine spiral ridges. Its profile is smoothly conical, with no obvious shoulders where one whorl attaches to the preceding one. The inner edge or lip of the aperture is white, and its tentacles have transverse black bands.

Littorina obtusata is another north Atlantic species that is common in the algae used to pack bait worms. It grows up to 12 mm high, and has a smooth shell and a squatter shape than Littorina saxatilis. It has a very flat spire, with the lowest whorl making up about 90% of the height of the shell. It can be yellow, orange or brown, or rarely green, black or banded.

Native Range

From boreal/subartic shores in the North Atlantic, including Hudson Bay, Baffin Island, Greenland and the Barents Sea, south along the America coast to Chesapeake Bay, and along the European coast to the Straits of Gibraltar. Outlying populations in the Azores and Canary Islands, at Venice, at Tunisia, and at three sites in Namibia and South Africa may represent remnants of formerly more widespread populations, or introductions by human activities or migrating birds.

Introduction and Distribution on the Pacific Coast [with dates of first record]

• California: San Francisco Bay [collected in 1993]

Littorina saxatilis was first collected at Emeryville Marina in San Francisco Bay in 1993, and has since been found at several other sites in the bay including Coast Guard Island, the east shore of Alameda Island, southern San Leandro Bay, Brisbane Lagoon and Coyote Point. At each of these sites it has a very restricted distribution adjacent to a public boat ramp, a marina, or a popular fishing spot,

Since Littorina saxatilis does not have planktonic larvae, it is a poor candidate for transport in ballast water. Instead, Littorina saxatilis was probably introduced into San Francisco Bay with marine baitworms (pileworms and bloodworms) imported from Maine. The worms are packed in a seaweed (Ascophyllum nodosum) which anglers frequently discard into the bay. We have found live Littorina saxatilis to be very common in this seaweed in Bay Area bait shops, and estimate that over 10,000 Littorina saxatilis are thereby discarded into the bay each year.

Littorina saxatilis' spread from initial sites of introduction is expected to be slow. In the Atlantic it typically takes 2-10 yr to colonize new habitats within a range of 1 km or less, with dispersal mainly occurring on drifting seaweed. Dispersal within and between Pacific Coast bays may be slower, since these lack the abundant cover of Ascophyllum nodosum, with its numerous float bladders, that characterizes many Atlantic shores. While Littorina saxatilis' production of crawl-away young rather than planktonic larvae may limit its opportunities for transport, it should increase its chances of establishment once it is transported to a distant site, since the released snails are likelier to maintain the population density needed for successful reproduction. Its latitude range in the Atlantic suggests that it could become established on the Pacific Coast from Baja California to Alaska (Carlton & Cohen 1998).

Additional Global Distribution [with dates of first record]

As noted above, isolated populations in southern Africa, Tunisia [reported in 1893], Venice [reported in 1792], the Canary Islands and the Azores may have resulted from introductions by either human activities or migrating birds.

Other names that have been used in the scientific literature

Littorina saxatila, Littorina rudis, Littorina groenlandica, Littorina tenebrosa, Littorina castanea, Littorina davidus, Littorina jugosa, Littorina obligatus, Littorina vestita and others

Literature Sources and Additional Information

Carlton, J.T. and A.N. Cohen. 1998. Periwinkle's progress: the Atlantic snail Littorina saxatilis (Mollusca: Gastropoda) establishes a colony on Pacific shores. Veliger 41(4): 333-338.

Cohen, A.N. and J.T. Carlton. 1995. Nonindigenous Aquatic Species in a United States Estuary: A Case Study of the Biological Invasions of the San Francisco Bay and Delta. U.S. Fish and Wildlife Service, Washington, DC (p. 53).

Reid, D.G. 1996. Systematics and evolution of Littorina. The Ray Society, Andover, Hampshire, England.

Reid, D.G. In press. Genus Littorina. In: The Light & Smith Manual: Intertidal Invertebrates of the California and Oregon Coasts. J.T. Carlton (ed.), University of California Press, Berkeley and Los Angeles.

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