Occurrence of Ligula colymbi (Cestoda) in spined loach (Cobitis taenia) and its effects on reproduction and growth of the host

The infection prevalence of Ligula colymbi plerocercoids in the spined loach Cobitis taenia inhabiting brackish waters of the Baltic Sea, and the effects of the parasite on the reproduction and growth of the host were studied. There was no significant trend in the infection rate among the different sampling dates in May-July. In total, 24.6% of females and 15.4% of mates were infected. In most cases (>95%) there was only one plerocercoid per host. In females, there was no correlation between the size group of fish and the infection rate, but larger males had significantly higher infection rate than smaller ones. There was no significant correlation between the parasite-host index and length of females. Condition factor of infected females was significantly higher than that of uninfected females. In the uninfected females, average +/- SD diameter of oocytes was 1.19 +/- 0.10 mm and they were in the trophoplasmatic growth stage. In contrast, only small (<0.4 mm) oocytes in the protoplasmatic growth stage were found in the infected females, which indicates that they were not able to spawn in the study year. GSI of infected females (median GSI = 0.07) was significantly lower as compared with that of uninfected individuals (median GSI = 13.24).