There are approximately 250 species of shells, ranging in size from a quarter inch to several inches, that are relatively common on California beaches. Include species smaller than a quarter inch, and species that live in water species that live in water 100 feet or deeper, and there are at least 10 times more species of marine mollusks in California that the typical beachcomber will find. The deeper water species that wash up after storms are rare surprises for those who walk the beaches in search of shells. The tiny species under a quarter of an inch are for the specialist who has the time and patience to search for and study these microscopic treasures.
There is a major temperature difference between northern and southern California ocean waters. Because some species thrive in warmer water, and others are adapted to cooler water, there is a suite of species unique to each region. Two examples of southern California species not found in northern California are the Chestnut Cowrie, Cypraea spadicea, and the California Cone, Conus californicus. In California we only have one species of each; in the tropical oceans of the world, there are large numbers of species in both families. Two species only found from central California north are the Frilled Dogwinkle, Nucella lamellosa, and the Thorn Purpura, Ceratostoma foliatum.
These cool water species are more abundant in the Pacific Northwest and Alaska. A large number of other species have a wider range of tolerance to the temperature differences between the northern and southern part of the state and occur along the entire coast.
Mollusks are an important part of marine shore communities. On rocky shores, mussels are eaten by starfish and predatory snails, such as the Frilled Dogwinkle. Species of clams that bore into rock open up space for other marine organisms to live, but also can contribute significantly to shoreline erosion. On sandy shores, Pismo Clams and Donax Clams, Donax gouldii (pictured at the top of the page) provide a hard surface for colonies of stalked bryozoans to grow.
The diversity of form observed in shells is often an adaptation of species to the environment in which they live. For example, the tall graceful form of the California Horn Snail, Cerithidea californica, prevents the snail from sinking into the mud upon which it lives. The knobs and spines of many beautiful sea snails, such as the Gem Murex, Maxwellia gemma, Kellet’s Whelk, Kelletia kelletii, and others encourage the growth of encrusting algae, bryozoans, marine worms and other organisms which help camouflage these species against potential predators. And thick, heavy shells of sandy shore clams, such as the Pismo Clam, Tivlela stultorum, and the White Venus, Amiantis callosa, help these species stay anchored down in the stormy currents of the surf.
The world’s largest moon snail, Lewis’s Moon Snail, Polinices lewisii, and the world’s largest chiton, the Gumboot Chiton, Cryptochiton stelleri, occur in California. Eight species of abalones are found along the coast of California. The Pacific Ocean is the region of the world richest in abalone species.
The largest species of abalone in California, the Red Abalone, Haliotis rufescens, was the most important commercially fished abalone from the 1940s to mid ‘70s, after which time the numbers of all species harvested began to decline drastically. Today, a number of abalone hatcheries in the state raise mostly Red Abalones to help meet the great world demand for abalone meat. Because of a slow growth rate, the maximum economical size of these species raised in captivity is around three inches, only about a third of the potential adult size.
In the early ‘80s, a disease caused by a species of bacteria in the digestive gland began to adversely affect abalone populations, particularly the Black Abalone, Haliotis cracherodi, which has been eliminated in some places and reduced to one percent of former numbers in other areas. The Black Abalone is currently being reviewed for listing as an endangered species. The White Abalone, Haliotis sorensoni, the rarest of abalones in the state, was listed as endangered in 2001, because densities of adults in the field have dropped so low, with adults spaced so far apart that they may no longer be reproductively viable. Facts such as these remind us of the importance of preserving the diversity of marine life along our California shores.
To find out more about the kind of habitats that California mollusks live in, see Souvenirs from the Sea on the Museum website.
SAN DIEGO NATURAL HISTORY: FIELD NOTES, JUNE 2005