Bohlen, J. & D. Ritterbusch

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

Environmental Biology of Fishes 59: 347-352

Environmental Biology of Fishes 59: 347-352 (2000)

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

BOHLEN J. & D. RITTERBUSCH

Abstract

A biased sex ratio in fishes generally indicates ecological or genetical disturbances in the concerned population and should focus scientific attention on this population. The sex ratio of spined loach (genus Cobitis; Cobitidae) from Lake Müggelsee was found to be strongly biased. During three years of study from 1996 – 1998 the average sex ratio was 1 : 22 in favour of the females. The percentage of males increased during this period of investigation from 2.9 % to 10.6 %. Within the years, male proportion changed significantly between the different dates of sampling. In order to identify the responsible reasons for the biased sex ratio, 50 specimens were reared from egg stage in laboratory under different regimes of temperature and water chemistry than in the lake. The sex ratio was 1 : 24 in this group. Since this ratio was similar to that in the field, the dominance of females was fixed genetically. No evidence for environmental sex determination, hermaphroditism or sex selective mortality was found. Blood cell measurements on 59 specimens from Lake Müggelsee revealed the existence of diploid and polyploid specimens in a ratio of 1 : 7.4. The dominance of the polyploid, gynogenetical reproducing form was taken as the main reason for the observed unbalanced sex ratio. The ecological relationship between the two level of ploidy is a parasite-host interaction (sperm parasitism). The absolute host-dependence of the parasitic (gynogenetic) polyploids demands a lower threshold value for male percentage. Below this value the total number of fertilised eggs from this population is limited by male availability. It can be assumed, that the threshold value for male availability is reached in spined loach from Lake Müggelsee.

Key words: sex determination, sperm parasitism, polyploidy, gynogenesis, reproductive limitation.