Incidence, Microscopy, and Etiology of Exoskeleton Lesions in the Tanner Crab, Chionoecetes tanneri

Abstract

As part of a seasonal population survey of the tanner crab, Chionoecetes tanneri Rathbun (Brachyura:Majidae), off the Oregon coast over 1400 adult and juvenile crabs were examined. A significantly high incidence of exoskeleton lesions was observed among adult females (76% infected) and to lesser extent among adult males (29%). No lesions were observed on juvenile specimens. Multiple lesions, some of which extended through the exoskeleton, were frequently observed on the carapace and appendages. Scanning electron miscroscopy of normal and diseased exoskeleton showed dense populations of bacteria associated with lesions and with the thin epicuticle of undiseased crabs. This suggests microbial degradation and/or mechanical damage to the epicuticle as a prerequisite to exoskeleton lesion formation. Chitin-digesting bacteria were isolated from all lesions examined. Sixty isolates were examined as to their phenotypic similarities to known chitin-digesting species that have been implicated as causing exoskeleton lesions in various crustaceans. Twenty-eight of the isolates were luminous and capable of digesting chitin rapidly at low temperatures. These isolates were compared with known luminous bacteria and found to be a Photobacterium sp. that did not precisely compare with known species. The possibility that multiple, luminous exoskeleton lesions might have a significant effect on some aspect of C. tanneri behavior is considered. Key words: exoskeleton lesions, crab, chitin, microbiology, crustacean, Photobacterium