Observations on the histopathology and probable progression of the disease caused by Trichomaris invadens,an invasive ascomycete,in the Tanner crab,Chionoecetes bairdi

Fifty-three Tanner crabs with varying degrees of surface encrustations and internal invasion by Trichomaris invadens were necropsied, processed, and examined to determine the extent of internal tissue invasion and histopathological effects of the infection. Sufficient data were accumulated to provide some insight on the progression, but not the rate, of tissue invasion. The epidermis and subepidermal layers are invaded and virtually replaced by proliferating hyphae. The invasion then proceeds internally, predominantly via the connective tissue. Major muscles and blood vessels are invaded, then invasion of deeper organs, including the hemopoietic tissue, gastrointestinal tract wall, and wall of the heart follows. The eyestalk is infected, usually early in the disease, and the invasion follows a similar pattern to that in the cephalothorax. Massive invasion and destruction of the retina is the major pathological effect. The gill may also be involved, with destruction of the lamellar tissues and invasion and destruction of podocytes in the stem of the gill.