Histopathological effects in fish exposed to the toxins from Karlodinium micrum

Karlodinium micrum (family Dinophyceae) produces toxic compounds (KmTx's) that are associated with fish kills. For zebrafish Danio rerio larvae (24 It old) exposed to either KmTx 1 or KmTx 2, mortality (100% in 24 h) was observed at toxin concentrations of 1 mu g/mL or more, whereas no mortality occurred after 24 h at concentrations of 0.5 mu g/mL or less. Zebrafish and sheepshead minnow Cyprinodon variegatus juveniles (60-90 d old) exposed to KmTx 2 were more sensitive to the toxin's effects than larvae were; mortalities in the juveniles began at 0.1-0.5 pg/mL. In whole, sectioned juvenile zebrafish, gills were the primary site showing injury by light microscopy. Histology of gills in both species treated with 0.5 mu g KmTx 2/mL (100% mortality in 1 h) showed epithelial necrosis and shortening or loss of secondary lamellae. Histology of zebrafish gills treated with 0.05 and 0.1 mu g/mL (0-44% mortality in 4 h) showed clubbing and bridging between secondary lamellae within 4 h of exposure. Sheepshead minnow exposed to 0.1 mu g/mL showed similar gill pathology but no mortality after 6 h. Sheepshead minnow exposed to KmTx 2 at 0.5 mu g/mL or more all died in less than 1 h. Transmission electron microscopy of gills of moribund zebrafish exposed to 0.1 mu g/mL revealed extensive cellular hypertrophy and lysis of epithelial and chloride cells. Because concentrations of KmTx 1 and KmTx 2 range from 0.1 to 0.8 mu g/mL in filtered water samples from K. micrum-associated fish kills, these results suggest that the concentrations of KmTx 1 and KmTx 2 found during fish kills are acutely toxic to fish and that gills are a primary target.