Chang Sook Kim, Sam Geun Lee and Hak Gyoon Kim
Harmful Algal Blooms Research Division, National Fisheries Research and Development Institute, Busan 619-902, Rep. of Korea

Abstract

Cochlodinium polykrikoides is one of the most frequently appearing harmful dinoflagellate responsible for fish kill and the clear toxic mechanisms has not been identified. To elucidate the ichthyotoxic mechanisms of the C. polykrikoides, physiological responses of fish exposed to this algal blooms and whether or not the existence of toxins in this algal cells were investigated, respectively. Particularly, based on our previous results (J. Plankton Res. 21, 2105-2115; J. Exp. Mar. Biol. Ecol. 254, 131-141; J. Kor. Fish. Sci. Tech. 3, 111-117), the loss of structural integrity of gill membranes was examined. The gills of red sea bream exposed to dense blooms > 3000 cells/ml for 24h showed severe epithelial separation: a sever edema formation was found in the secondary epithelium and interlamella regions of primary lamella. Moreover, the membrane swelling and/or lysis were found to be out of all proportion to the level of algal cells and exposure time. Meanwhile, the aqueous and lipid-soluble fractions of Cochlodinium extracts did not showed toxicity against mice and fish. In addition, no effects of algal extracts on the growth of lymphoid cells as well as the mammalian mitochondrial respiration were also observed. The algal extracts did not showed any peaks corresponding to PSP on the fluorometric HPLC chromatograms. These results suggest that the mechanism of fish kill by a live C. polykrikoides is distinct from the special ichthyotoxic compounds-mediated toxic effects. What we can confirm this finding is that the gill of fish subjected to C. polykrikoides could be damaged, eg. the loss of functional and structural integrity of cell membranes, by some factors containing reactive oxygen species(ROS). Consequently, both physical clogging of gill by the algal cells debris and abnormal mucus excretion as well as gill damages by ROS may be involved in fish kill by C. polykrikoides.