Which factors affect sex ratio of spined loach (genus Cobitis) in Lake Muggelsee?

The sex ratio of a diploid-polyploid complex of spined loaches (Cobitis; Co bitidae) from Lake Muggelsee (Germany) was 1:22 in favour of the females. T he percentage of males increased during three years of investigation from 2 .9% to 10.6%. Within the years, male proportion changed significantly betwe en the different dates of sampling. In laboratory, 50 specimens were reared from eggs in slightly warmer and softer water than in the lake. Since sex ratio was 1:24 in this group, the influence of environmental sex determinat ion, hermaphroditism or sex selective mortality in the lake was assumed to be low. Erythrocyte measurements on 59 wild-caught specimens revealed a dip loid-polyploid ratio of 1:7.4. The dominance of the polyploid sperm parasit e was taken as the main reason for the observed unbalanced sex ratio. The a bsolute host-dependence of the parasitic polyploids demands a lower thresho ld value for male percentage, which should have been reached in spined loac hes from Lake Muggelsee.