The reproductive cycle of the zebra mussel Dreissena polymorpha (Pallas, 1771) in the hydroaccumulation Čakovec

Terenska istraživanja provedena su od svibnja do listopada 2006. godine. Primjenom klasičnih histoloških tehnika proučeni su razvojni stadiji gonada, a računalnom analizom slike LUCIA G 4,81 izmjeren je gonadni volumen, volumen probavne žlijezde i volumen ostalih tkiva utrobne vreće. Rezultati rada su pokazali da je u istraživanoj hidroakumulaciji omjer spolova 1:1. Iako je reproduktivni ciklus školjkaša uglavnom ujednačen, svi razvojni stadiji gonada najprije su uočeni kod ženki. U svibnju su gonade većine jedinki bile u stadiju pred mriještenje. Do mriještenja je došlo u lipnju i početkom srpnja što je potvrđeno porastom broja jedinki koje su bile u stadiju poslije mriještenja. Stadij mirovanja gonada uočen je kod manjeg broja ženki već u kolovozu. Proces nove gametogeneze počeo je u rujnu i nastavio se tijekom listopada. Najviše vrijednosti gonadnog volumena i gonadnog indeksa utvrđene su u lipnju, a najniže u rujnu kad je najveći dio jedinki bio u stadiju mirovanja. Za razliku od gonada, vrijednosti volumena i indeksa probavne žlijezde bile su najniže u lipnju, a najviše u rujnu. Tijekom istraživanja utvrđena je i prisutnost endoparazita metilja.Field research was conducted from May to October 2006. Using conventional histological examination developmental stages of gonads were studied. Gonad volume, volume of the digestive gland and volume of other tissues of visceral mass were measured with computerized image analyses LUCIA G 4.81. The results showed that in the studied hydroaccumulation sex ratio was 1:1. Although the reproductive cycle of mussels generally synchronized, all the developmental stages of gonads were first observed in females. In May, the gonads of most individuals were at the prespawning stage. The spawning occurred in June and early July, which was confirmed by the increase in number of individuals which were at the postspawning stage. The stage of inactive gonads was observed in a small number of females in August. The process of new gametogenesis started in September and continued during October. The highest volume of gonads and gonad indices were determined in June, and the lowest in September when the most of the individuals were at the inactive stage of gonads. In contrast to the gonads, the values of volume and digestive gland indices were lowest in June and highest in September. During the research the presence of endoparasites trematoda was also identified