Effects of length and robustness on the ingestibility of chain-forming diatoms in Manila clam feeding

Abstract

Most bivalves show particle-selectivity during feeding, and particle size is one of the most important factors influencing the pre-ingestive selection. Diatoms are a major taxon of phytoplankton of inner bays and are considered to form an important food source for suspension-feeding bivalves including Manila clam. However, it remains unclear whether these diatoms are always suitable food for bivalves, because very long, probably difficult to ingest, chain-forming species tend to dominate especially inner bays. We investigated whether Manila clams are able to ingest chain-forming diatoms, and whether the length, robustness of chains affect feeding of clams. In a series of experiments we: 1) compared the mechanical strength of cell–cell bond in chains of two diatom species, Chaetoceros contortus and Eucampia zodiacus, using aeration at several flow rates, and found that cell–cell bond strength is greater in C. contortus; 2) measured feeding rates in clams on the unicellular Chaetoceros sp., robust chain-forming C. contortus, and more fragile chain-forming E. zodiacus, and found the feeding rate on C. contortus to be significantly lower than that on the others; and 3) compared chain length compositions of C. contortus and E. zodiacus before and after feeding by clams, and did not find significant difference in either diatom species. Considering the fine structure of the gill surface and diameter of the oesophageal sinus which determine the upper limit of the ingestible particle size, these results suggest that although the clams are capable of ingesting long and robust chains, they take longer to ingest them than unicellular and fragile chain-forming diatoms.