Based on original and literature data, this study discusses the close relationship that seems to exist between the pre-ovulatory hydratation (POH) on fish oocytes and the marine environment, contrasting with the absence of this process on freshwater species. The difference is mainly explained by the necessity or not of intraoocitary water on eggs flutuation, in one or another situation, according to the surroundings. Once accepted this hipothesis, it is proposed that POH should be an efficient indicator of the adaptation to freshwater environment in fish species originated from marine groups.