engleski

Morphometric characteristics of Dreissena polymorpha oocytes in the Drava River and in the Dubrava Reservoir

Freshwater mussel Dreissena polymorpha (Pallas, 1771) is one of the most dominant invasive species in the European freshwaters, which began to spread in Croatia during the 1980s. Their rapid invasion is enabled by the ability of adult mussels to adapt to a wide range of habitats and by their reproductive cycle flexibility. The aim of this study was to investigate the annual development of gonads in two populations living in the Drava River (site 1) and in the accumulation lake of Dubrava Hydroelectric Power Plant (site 2), differing in environmental parameters. The analyses included measuring the size and diameter of oocytes and their nuclei in the period from March to August. The histological sections were made by the classical histological technique. Measurements were done with the image analysis system (Lucia G 4.8), and the digital video camera Nikon DXM 1200, linked to a Nikon's Eclipse E600 light microscope, was used for taking images. A total of 13 731 oocytes were analysed, of which 6 680 from site 1 and 7 051 from site 2. Based on this analysis, but also the known gonadal volume, the following values were calculated: number of oocytes per one mm2, percentage of oocytes in total ovarian volume, oocyte volume in ovary, and finally the total number of oocytes per female. The difference between the mean values of oocyte and nucleus diameters between sites, by months, was statistically significant (p <0.0001 ; Wilcoxon rank sum test). In addition, differences in diameters between months at the same site were also statistically significant (p <0.0001 ; ANOVA). All other calculated variables were higher at site 1, but only the percentage of oocytes in the total ovarian volume differed significantly (p = 0.0063 ; Wilcoxon sum test). The differences in all calculated variables at the same station, by months, were statistically significant (Kruskal-Wallis test). The highest number of oocytes, 801 977, was recorded in a female sampled in April at site 1. In general, the values of all measured parameters were highest in spring, before spawning, and then decrease. The observed differences between sites are possibly a consequence of the fact that mussels at site 2 have faster development of gonads, due to more uniform living conditions, and have a smaller shell width (body volume), due to high population density. Finally, it is important to emphasize that it has been proven that the oocytes became mature in successive cohorts within one gonad which mean that female may spawn several times during the reproductive season.

reproductive cycle ; gonads ; invasive species ; image analysis

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