The myxosporean fauna of the Black Sea is predominantly marine with a few brackish and fresh-water species. Seasonal changes in water temperature influence the life-cycle of Black Sea myxosporeans primarily through the biology of the host. This is because the mass formation of spores in Myxosporea takes place when hosts form the largest shoals. The high saturation of the water in Tendrovsky Bay with hydrogen sulphide accounts for the mass mortality of fish, but and at the same time for the fact that many fish species were found to be free of parasites. Myxosporeans occur practically at any depth where their hosts can live.