Ecological Studies of the Branchiobdellidae (Oligochaeta)

Crayfish collected from a central Texas farm pond are hosts for two Branchiobdellidae. Evidence indicates the branchiodbellids are commensals, since they survive for long periods in the absence of crayfish, but are dependent upon them as a site for reproduction. Branchiobdellid cocoons, which have incubation periods of 10 to 12 days, are attaches to the exoskeleton of the host mostly on the ventral abdominal surface. Cocoons are not deposited on very small crayfish, apparently because of selection brought about by the loss of eggs through the frequent molts of young crayfish, since eggs left on exuviae perish. Transfer between hosts is accomplished mostly through body contact of the crayfish. The logarithm of the branchiobdellid populations yields a straight line when plotted against crayfish body lengths. This may be accounted for by the increased surface area and the decreased frequency of molts of larger crayfish. Population densities are minimum during the winter, increase distinctly in early spring, and reach a maximum in June. Adverse environmental conditions during the summer and winter likely cause decreased reproductivity activity or increased mortality. A greater proportion of the total brachiobdellid population inhabits small crayfish during the early summer than at other seasons. This may result from population pressures created by overcrowding on larger crayfish during this period of maximum branchiobdellid population density. The structure of the bronchiobdellid populations varies seasonally, with adults composing about half of the winter population and a smaller proportion in other seasons. This can be accounted for by decreased reproductive rates or selective mortality depleting the numbers of young worms during the winter.