Reproductive dependence of a branchiobdellidan annelid on its crayfish host: confirmation of a mutualism

The ectosymbiotic branchiobdellidan Cambarincola ingens Hoffman, 1963 can increase the growth and reduce mortality of the crayfish Cambarus chasmodactylus James, 1966. These results suggest that these two species are engaged in a cleaning symbiosis mutualism. However, the potential benefits C. ingens derives from the association, specifically its reproductive dependence on the crayfish, have not been examined experimentally. We conducted two experiments to determine if C. ingens was reproductively dependent on C. chasmodactylus. In the experiments, worms only laid cocoons on live crayfish. Also, significantly more cocoons were laid on larger crayfish which was consistent with a survey of worm and cocoon abundance on crayfish collected from a local river. These results suggest that C. ingens is dependent on its crayfish host for reproduction. The association between C. chasmodactylus and C. ingens is a mutualism, at least under environmental conditions in which crayfish survival and growth are enhanced by the worms.