| Summary: | The annual reproductive cycle in two wild populations of three-spinedstickleback was studied. Sticklebacks from the Dead Vistula River(Martwa Wisła) (brackish water) and the Oliva Stream (Potok Oliwski)(freshwater) were exposed to annual environmental changes in their naturalhabitats. Ovaries and livers (females), and testes and kidneys (males) werecollected during 1-2 years. The gonadosomatic <i>I</i><sub>G</sub>, hepatosomatic<i>I</i><sub>H</sub>, nephrosomatic <i>I</i><sub>N</sub> indices, kidney epithelium height(KEH) and size of oocytes were calculated.The number of mature oocytes andpercentage of ovulating females were determined during the spawning season.Histological changes in the ovaries and testes were described throughouta year. Annual reproductive cycles were similar in both populations ofsticklebacks. This is the first histological and morphological study carriedout throughout a year, simultaneously in two wild populations of three-spinedsticklebacks inhabiting different environments. An improved scale of gonadaldevelopment in conjunction with the determined indices and fecundity givea comprehensive description of the reproductive cycle. These new observations,in combination with previously reported features, provide a universal scalethat can be successfully used to distinguish all phases of gametogenesis insticklebacks in different habitats.
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