Abstract

The annelid worms that compose the clitellate order Branchiobdellida comprise 17 genera and 120 nominal species. In their anatomy, they are intermediate between oligochaetes and leeches, but the lack of setae, the fixed number of segments (15), the presence of suckers and unpaired gonopores separate them from oligochaetes; the anterior position of the testes and the different number of segments distinguish them from leeches. They are epizoites on all Holarctic astacoideans. A few occur on freshwater shrimp, crabs and isopods. Some few species are parasitic in the gill or brood chambers of the hosts; others 'graze' on the surface of the host’s body. No genera are inter-continental in distribution, except the Eurasian Branchiobdella, and the zoogeography of the worms does not correspond to that of their hosts. Attempts to explain this discrepancy are based upon assumptions of an early (Cretaceous) attachment to their hosts, an initially more rapid radiation of the worms and their subsequent slower evolution. Little is known of their ecology and physiology and nothing of the mode of transmission from host to host. It is known that different assemblages of branchiobdellids occupy different portions of relatively short streams; that some are adapted to microniches on their hosts; that often the same host animal harbors several species; that they are extremely intolerant of some inorganic pollutants; and that some seem to be facultative anaerobes. This paper is addressed to the problems of the systematics, evolution and zoogeography of the branchiobdellid associates of the Holarctic astacacoideans and, secondarily, to some questions of the ecology and physiology of the worms.

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