TWO LIVING SPECIES OF TIVELA CLAMS

Tivela clams (bivalves) have a fossil record dating back to the Eocene. They have medium-sized to large, heavy shells that are strongly built to resist waves in the surf zone. They live today in moderately cool (i.e., but not polar!) to warm to marine waters where they burrow into nearshore sands. Their depth range is typically the surf zone to about 25 m depth (ranging from low tide to very shallow subtidal. Their two valves are symmetrical, sturdy (can be thick), trigonal (triangular), and smooth (so as to enable burrowing in sand). The three cardinal teeth on the hinge teeth of each valve are also sturdy and help hold the two valves securely together (it is tough living in the turbulent waters of the surf zone!). A thin shell layer (the periostracum) is a thin coating on the surface of their valves and, like a coating of varnish protects the outside of their valves from corrosion. 

The classification of Tivela is:

Class Bivalvia

Order Veneroida

Family Veneridae

Genus Tivela

There are about 30 species of this clam in the world today. Two of these are mentioned in this blog.

1) TIVELA STULTORUM [The Pismo Clam]

Tivela stultoram. Exterior and interior views of a Los Angeles County Museum of Malacology (LACM 186650) specimen that was actually collected from Pismo Beach. 

Image of an exhibit from the old Marine Hall exhibit at the Los Angeles County Museum (LACM) depicting the life position of a Pismo Clam. These images were kindly provided by Lindsey T. Groves, LACM Collections Manager of Malacology at LACM.

Tivela stultoram (Mawe, 1823) is well-known as the Pismo clam—a name  derived from the Indian word Pismu, “meaning tar”]

At Pismo Beach (about half-way between Los Angeles and San Francisco), there used to be a great abundance of “Pismo” clams. Over the years, their number was drastically reduced by *over-collecting; namely, hordes of people, digging into the beach sands in order to collect these clams. Nowadays, California law requires possession of a valid ocean license for most ocean fishing, clamming, lobstering and related activities in order to collect.

*An estimated 150,000 people once sought the clams on Pismo Beach during a single weekend! (see Shaw and Hassler, 1989:p. 1).

The geographic range of the Pismo clam is along the Pacific coast from Monterey, northern California to the Bay to Bahia Magdalena, Baja, California, Mexico.

Most Pismo clams reach a size of five inches within five years. The largest Pismo clam on record was 7 1/8 inches across, and was estimated to about 26 years old.

Shells of the Pismo clam vary in both color and pattern. The principal color is pale buckskin. Some individual shells are maker with lines radiating out to the margin. These stripes disappear as the clam ages (Fitch, 1950, p. 287).

2) TIVELA MACTROIDES (from the Caribbean Sea)

Tivela mactroides: Upper image is the exterior of one of the right valve; lower image shows the interiors of both valves of this  specimen.

Some useful references used in the preparation of this blog post:

Fitch, J.E. 1950. The Pismo clam. California Fish and Game v. 36, no. 3, 285-312.

Fitch, J.E. 1961. The Pismo clam. Marine Resources Leaflet No. 1. State of California. 23 pp.

Shaw, W.N. and T. J. Hassler. 1989. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (Pacific Southwest). U.S. Fish Wildl. Serv. Biol. Rep. 82(11.95). U.S. Army Corps of Engineers, TR EL-72-4, 12 pp.