Food utilisation in fingernail and pill clams

The contents of digestive systems in six freshwater bivalve species of the family Sphaeriidae were investigated. Microorganisms in the stomachs and intestines of the bivalves were the same as found suspended in the water, which implies none or very limited preingestive selection, at least of the organic particles. Most of the organisms (coccal algae, diatoms, flagellates, euglenophytes, Zygnematophycaea, monadoid algae, bacteria) probably pass through the digestive system unharmed, as they were found alive even in the alimentary bolus in the hindgut. Probably only a small proportion of them is digested, in particular the bacteria and monadoid algae, the abundance of which decreased especially following starvation. In starved animals, food particles were also present in the digestive diverticula, apparently in order to increase food utilisation.