Study on histological and morphometric characters of gonad development of Cynoglossus semilaevis Günther

The temporal dynamics of oocyte growth and ovary development were examined during a 12-month ovarian maturation cycle in captive Cynoglossus semilaevis Gunther using light microscopy method. Results showed that the oocyte growth could be divided into 6 phases and ovary development consisted of 6 periods accordingly. Two or three distinct batches of vitellogenic oocytes were found in Cynoglossus semilaevis Gunther ovaries at one development stage,indicating that this species are a non-synchronized multiple batch spawners. The ovaries are in stage I when the fish are less than 6-month old,which only appears once in their whole life.As fish grew,the ovaries enter into stage Ⅱ when they are 9 to 12 months old. The stage Ⅲ which were characterized by cortical alveoli accumulation was attained when the fish were 12-month old and maintained until 24-month old. Deposition of lipids and vitellogenesis began to happen in ovaries of stage Ⅳ when the fish were over 3 years old. The ovaries could be matured (stage Ⅴ) under artificial photothermal control from May to August and fish spawned between September and November. Hydration observed in oocytes indicates that the fish were completely matured for ovulation. After spawning period,the ovaries retrogressed to stage Ⅲ via stage Ⅵ,then continued regressing to stage Ⅱ and was maintained until the next reproductive season. Gonadosomatic index (GSI),hepatosomatic index (HSI) and condition factor (CF) were all found to increase prior to,or during the peak phase of vitellogenic growth corresponding to the gonad development,which are important indices for evaluating the reproductive condition of the parental fish. Cynoglossus semilaevis Gunther appear to strategically utilize episodes of high feeding activity to accrue energy reserves early in the reproductive cycle prior to its deployment during periods of rapid ovarian growth. In the developing ovaries (mainly after stage Ⅳ),some post-vitellogenic follicles failed to undergo final maturation,resulting in widespread preovulatory atresia,which may depress the fecundity or be an indicator of capacity of multiple spawns.