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 gravel to mud in half phi increments. A burrowing rate index (BRI), which normalizes burrowing time for specimen mass, was calculated for each species in each sediment. Skewness and kurtosis of the profile of average BRI's across the range of grain sizes was used to categorize species as substrate generalists, substrate sensitive, or substrate specialists. Substrate generalists (Anadara ovalis, Mercenaria mercenaria, Astarte 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 sediments away from the optimum grain size, producing a platykurtic (genty arching) BRI profile. Substrate specialists (Spisula solida, Nucula sulcata, Cardium edule, Abra alba, Cultellus pellucidus) burrowed rapidly in a very limited range of grain sizes, but failed to burrow into either or both grain size extremes, thereby producing leptokurtic (peaked) BRI profiles. Substrate sensitive species (Donax variabilis, D. vittatus, Tellina (Angulus) tenuis, Ensis directus, Clinocardium nuttalli, Protothaca staminea, Petricola pholadiformis, Macoma nasuta, Scrobicularia plana) penetrated a wider range of grain sizes than specialists, although burrowing rates are slower at the coarse textured extreme 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-influenced molluscan communities, respectively. Substrate specialists (Spisula, Nucula) and some sensitive taxa (e.g., Petricola) are sediment-restricted, 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 generalists (Macoma nasuta), based on experimental data on either conspecific or congeneric individuals, show high mean percent occurrences and low coefficient of variation in the Pliocene Pecten Zone communities of the San Joaquin Formation of the Kettleman Hills, California. Generalists 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 found in more than four biostratigraphic units.