CORRELATION OF SHAPE AND HABIT WITH SEDIMENT GRAIN-SIZE FOR SELECTED SPECIES OF THE BIVALVE ANADARA

Burrowing ability was experimentally determined for a range (3-150 g) of sizes of the blood ark, Anadara ovalis, in sieved sediments ranging from very coarse sand (-1 phi) to silt (>4 phi). Small individuals (< 5 g) could penetrate the complete range of sieved sediments, but the a bility to reburrow apparently decreases with size (age) so that adults larger than 50 g can only reburrow in fine sands (2.5-3.0 phi). This semi-infaunal species has a length/height ratio of 1.16 and is most co mmonly found in sand. The infaunal A. chemnitzi and A. braziliana, wit h L/H values of 1.10 and 1.18, respectively, are also most common in s ands. The ovate (L/H ratio = 1.07-1.25) forms of the Pliocene A. trili neata from the Kettleman Hills of California are inferred to have been infaunal, based on the absence of muricid boreholes and the infrequen t occurrence of epizoans. Ovate specimens are restricted to fine sands (2-3 phi) in the Etchegoin and San Joaquin Formations, sediment textu res into which the adults could probably reburrow. Elongated shell mor photypes (L/H = 1.5-1.67) have muricid boreholes and epizoans concentr ated posterodorsally on the inferred exposed portion of the shells in life position. Populations of elongated shells were associated with gr avelly, sandy and silty sediments, as is the living epibyssate A. lien osa (L/H = 1.72) and A. transversa (L/H = 1.48).