Digestion and nutrition in freshwater bryozoans (Phylactolaemata)

As suspension feeders, freshwater bryozoans (Phylactolaemata) ingest a wide variety of particles. After this material is processed in the gut, the remaining wastes are ejected in a pellet. Curiously, fecal pellet contents appear perfectly intact, often still alive, and apparently unharmed. This raises questions about the true sources of bryozoan nutrition and the entire digestive process itself. This article summarizes the results of controlled feeding experiments as well as findings from gut examination by scanning electron microscopy. Extensive feeding tests with bryozoan species in three families show that colonies ingesting protozoans and small rotifers grow much faster than those on any other diet. There is no evidence for digestion of green algae or bacteria, including cyanobacteria. Close observations reveal muscular contractions of the stomach crushing and abrading fragile prey organisms, with possible assistance from indigestible algal particles. A dense carpet of microvilli lines the walls of the stomach and intestine. In most instances, these are organized in a three-dimensional matrix that offers an extensive surface area. Because all food is handled in sequential batches, there is limited time for many particles to be digested.