Pathological changes caused by cold-water stress in Japanese eel Anguilla japonica

In the present study, histopathological changes in Japanese eel Anguilla japonica subjected to 'cold-water stress' were examined. Eels were exposed to cold water (13 to 15 degrees C) and warmer water (25 degrees C) as controls. Fish held in warm water did not show any apparent changes. Although none of the eels exposed to cold water died, they displayed nephrotic changes such as cloudy swelling and hyaline droplet degeneration of the renal tubular epithelia. Fish with nephroses had low levels of serum chloride (12 to 23 mg l-1 in fish with hyaline droplet degeneration, 71 to 81 mg l-1 in fish with cloudy swelling) compared with the control fish (87 to 109 mg l-1). In electron microscopy, affected tubular cells had variously damaged mitochondria and formations of secondary lysosomes of variable sizes. Electron microscopy also revealed mitochondrial degeneration in hepatocytes and degenerated granules of neutrophils in the hematopoietic tissue. 'Cold-water stress' was effective in damaging Japanese eels below 15 degrees C.