INFLUENCE OF SEDIMENT GRAIN-SIZE ON THE BURROWING OF BIVALVES - CORRELATION WITH DISTRIBUTION AND STRATIGRAPHIC PERSISTENCE OF SELECTED NEOGENE CLAMS

Burrowing ability of 21 species of clams from Oregon, New Jersey, and Scotland was determined in each sieved sediment ranging from fine grav el to mud in half phi increments. A burrowing rate index (BRI), which normalizes burrowing time for specimen mass, was calculated for each s pecies in each sediment. Skewness and kurtosis of the profile of avera ge BRI's across the range of grain sizes was used to categorize specie s as substrate generalists, substrate sensitive, or substrate speciali sts. Substrate generalists (Anadara ovalis, Mercenaria mercenaria, Ast arte sulcata, Venerupis decussata, Venus striatula, Venus casina, Mya arenaria) burrowed slowly into a wide range of grain sizes. Burrowing rate gradually diminished in progressively coarser and finer grained s ediments away from the optimum grain size, producing a platykurtic (ge nty arching) BRI profile. Substrate specialists (Spisula solida, Nucul a sulcata, Cardium edule, Abra alba, Cultellus pellucidus) burrowed ra pidly in a very limited range of grain sizes, but failed to burrow int o either or both grain size extremes, thereby producing leptokurtic (p eaked) BRI profiles. Substrate sensitive species (Donax variabilis, D. vittatus, Tellina (Angulus) tenuis, Ensis directus, Clinocardium nutt alli, Protothaca staminea, Petricola pholadiformis, Macoma nasuta, Scr obicularia plana) penetrated a wider range of grain sizes than special ists, although burrowing rates are slower at the coarse textured extre me in comparison to generalists. In San Francisco Bay and Mississippi Delta habitats, generalists (Anadara, Mercenaria, Mya) occur commonly in most sediment categories and show high mean percent occurrence and low coefficient of variation from the seven and eight sediment-influen ced molluscan communities, respectively. Substrate specialists (Spisul a, Nucula) and some sensitive taxa (e.g., Petricola) are sediment-rest ricted, show fidelity to one or two communities, have low mean percent occurrence, and high coefficient of variation. Substrate generalists (Anadara trilineata, Mya arenaria), and species transitional with gene ralists (Macoma nasuta), based on experimental data on either conspeci fic or congeneric individuals, show high mean percent occurrences and low coefficient of variation in the Pliocene Pecten Zone communities o f the San Joaquin Formation of the Kettleman Hills, California. Genera lists show stratigraphic persistence, i.e., they are found in 12-14 of 20 successive biostratigraphic units in the Etchegoin and San Joaquin Formations, whereas specialists (Spisula, Acila (Nucula)) are never f ound in more than four biostratigraphic units.