Histopathological Effects in Fish Exposed to the Toxins from Karlodinium micrum

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AbstractKarlodinium micrum (family Dinophyceae) produces toxic compounds (KmTx's) that are associated with fish kills. For zebrafish Danio rerio larvae (24 h old) exposed to either KmTx 1 or KmTx 2, mortality (100% in 24 h) was observed at toxin concentrations of 1 μg/mL or more, whereas no mortality occurred after 24 h at concentrations of 0.5 μg/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 μg/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 μg 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 μg/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 μg/mL showed similar gill pathology but no mortality after 6 h. Sheepshead minnow exposed to KmTx 2 at 0.5 μg/mL or more all died in less than 1 h. Transmission electron microscopy of gills of moribund zebrafish exposed to 0.1 μg/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 μg/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.