Abstract
Active protection measures towards the population of lake minnow, Eupallasella percnurus (Pall.), require comprehensive studies on the reproduction biology of this endangered representative of the native ichthyofauna. One of the key issues is the analysis of the reproduction cycle of sexually mature individuals, which enables the explicit determination of the time and the course of natural spawning. The first attempt at investigating this issue was undertaken in May 2009. Gonads of six E. percnurus specimens (3 males and 3 females) were histologically evaluated. The fishes with the mean body length of 4.9 cm (the range 3.3-6.1 cm) and the mean body weight of 3.1 g (the range 0.8-6.2 g) were caught in the reservoir in Zielonka (Mazowieckie Voivodeship). Values of the gonadosomatic index for females ranged from 6.2 to 18.7%, and for males - from 0.7 to 3.1%. The histological analysis of E. percnurus ovaries during the pre-spawning season revealed apparent asynchronicity in the maturation of oocytes. A few (3-4) ova fractions were found in gonads, they were heterogeneous in terms of maturity stages. Single oogonia, previtellogenic and vitellogenic oocytes (both the initial and final stage of the trophoplasmatic growth) were found in female gonads. The cross-sections of E. percnurus testes revealed the dominance of germ cells characteristic of the final cytological stages of spermatogenesis in the central part of seminal ampullae, i.e. spermatids and spermatozoids. Along their edges - free spaces reflecting the loosened structure of gonads occurred. At the borderline between the adjacent ampullae, single spermatogonia were visible, as well as seminal ampullae filled up with primary and secondary spermatocytes, which potentially ensure the spawning readiness of male E. percnurus for a longer period.