Phytoplankton Ingestion by the Fingernail Clam, Sphaerium Transversum (Say), in Pool 19, Mississippi River

In Pool 19 of the Mississippi River, the fingernail clam, Sphaerium transversum, appears to feed non—selectively on phytoplankton, although green algae were in higher percentages in guts than in plankton samples. Diatoms were of paramount importance in clam guts and water samples during most sampling periods, especially from December to April when mean monthly water temperatures were less than 4 degrees C. Ingestion by clams was heaviest in spring, early summer, and fall. Clams appeared to stop ingesting when water temperatures fell to about 2—4 degrees C. In February and March, 80—90% of the clams had ceased ingesting. Ingestion did not cease simultaneously in all clams. Throughout the year some contained little or no phytoplankton. Much of the phytoplankton in the gut near the anus appeared completely undigested. Phytoplankton diversity (d) in clam guts and in water was high in July and August and low in February and March. A maximum of 33 phytoplankton genera occurred in clams in August; 38 genera were present in water samples in July and August. Concentrations of over 50,000 plankters/ml (primarily Stephanodiscus hantzschii) occurred in March 1968; minimum concentrations occurred in July.