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An Introduction to Branchiobdellidans Back to Dr. Gelder's homepage | |||
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Stuart R. Gelder Professor of Biology School of Science and Mathematics University of Maine at Presque Isle
Branchiobdellidans, or crayfish worms, are leech-like, obligate ectosymbionts living primarily on astacoidean crayfish (BRINKHURST & GELDER, 2001). The ectosymbionts form a monophyletic taxon, order Branchiobdellida Holt, 1965, with one family, Branchiobdellidae Grube, 1851 and 21 genera containing about 150 species, within the annelid subphylum Clitellata. The Branchiobdellida forms a sister-group to the Hirudinea and together form a sister-group to the Lumbriculida. Their leech-like locomotion is achieved by alternate attachment using duo-adhesive secretions on the posterior attachment disc (segment 11) and the ventral surface of the ventral lip. The branchiobdellidan mouth is not used as a locomotory sucker as reported in some textbooks and in leeches.
The ectosymbionts
have a Holarctic distribution, with 15 genera endemic to North America and
six to Eurasia. No introduced species have been reported in North America.
Species of Branchiobdella are found in eastern Asia and Europe with
each region having its own unique set of species. The actual distribution
of the endemic species in Europe has probably
Branchiobdellidans are obligate ectosymbionts because they need a live host for their cocoon-containing embryos to develop to maturity. Most branchiobdellidan species live on the exposed exoskeleton of the host and have a diet of diatoms, detritus and small invertebrates. In addition, these species leave the host for periods of time and can be maintained in vitro for months. Therefore, these branchiobdellidans are nutritionally best described as ectocommensals with an opportunistic omnivorous diet. The gill chamber habitat appears to be obligate for a small number of species. Other studies have used the damaged gills of hosts as evidence for branchiobdellidan parasitism; however, many crayfish have diseased gills from bacterial and fungal infections. These situations are complex, but it is obvious that not all branchiobdellidans have the same nutritional association with their hosts.
Ectosymbiont populations range in complexity and number from a single crayfish carrying over 1,800 individuals of the same species to one carrying about 100 branchiobdellidans from eight species representing five genera. Ecological studies on various branchiobdellidan-host associations have provided basic information which show variations based on climate, season, habitat and other parameters.
**************************************** Edited from: Diéguez-Uribeondo, J., Cerenius, L., J. Gelder, S., Henntonen, P., Lom, J., Dyková, I., Jiravanichpaisal, P. and Söderhäll, K. 2006. Pathogens, parasites and ectocommensals. In: Atlas of European Crayfish Distribution and Diseases, Noel, P., Holdich, D., Souty-Grosset, C., Haffner, P., and Reynolds, J. (eds). Muséum National d'Histoire Naturelle, Paris, Patrimoines Naturels. 148-149.
Reference BRINKHURST R. O. & GELDER S. R. 2001. – Annelida: Oligochaeta and Branchiobdellida, in THORPE J. H. & COVITCH F. (eds), Ecology and Classification of North American Freshwater Invertebrates. 2nd edition, Academic Press, New York: 431-463.
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