Chronic toxicity and histopathological studies with Benlate^®, a commercial grade of benomyl, in Penaeus vannamei (Crustacea: Decapoda)

Juvenile Pacific white shrimp, Penaeus vannamei (Crustacea: Decapoda) were exposed to 0, 0.01, 0.1, 1.0 mg/l of benomyl (Benlate^® OD; DuPont) in a 30-day static renewal bioassay. The levels tested were below the 96 h LC₅₀ of approximately 10 mg benomyl/l for juvenile penaeid shrimp. Exposure to benomyl at 1.0 mg/l resulted in death of some shrimp after 11 to 12 days, and approximately 80% cumulative mortality by 19 days. No overt toxicity or histopathological changes were noted at 0.01 or 0.1 mg benomyl/l after 30 days of exposure. Moribund shrimp (in the 1.0 mg benomyl/l treatments from day 11 to 19) displayed lethargy, anorexia, opaque musculature, and cuticular hyperchromatophorism. Histological examination of affected shrimp revealed distinctive pathological changes which were confined to the hepatopancreas (HP) and the associated anterior midgut caecum. The most significant lesions included a marked atrophy of the HP, intertubular hemocytic inflammation, often with melanized foci, and necrosis and desquamation of the HP epithelial cells. Moreover, all shrimp examined from the 1.0 mg/l dose level displayed HP epithelial cells that were enormously hypertrophied and presented as conspicuous giant megahepatopancreatocytic (MCy) cells. These benomyl-induced MCy cells are a unique lesion which is distinct from other reported toxic and infectious diseases of penaeid shrimp. Hence, the presence of MCy cells in degenerate HP tubules may provide pathognomonic indicators of benomyl toxicity in penaeid shrimp and perhaps other crustaceans.