Patterns of oocyte growth, vitellogenin and gonadal steroid concentrations in greenback flounder

Abstract

The pattern of oocyte development in association with changes of plasma concentrations of vitellogenin (Vtg), 17β‐oestradiol (E2) and testosterone (T) was investigated in maturing female greenback flounder Rhombosolea tapirina over the first part of a reproductive season (February to June). Examination of oocyte size‐frequency distributions showed that the oocyte developmental pattern in R. tapirina is multiple group synchrony, and that reproductively mature fish were present at all sampling times. There were no significant temporal variations in the gonado‐somatic index (IG), hepato‐somatic index (IH), or plasma concentrations of Vtg, E2 and T during the sampling period, which indicates that reproductive development is not synchronized within the population. Significant increases in IG, IH and plasma concentrations of Vtg, E2 and T, however, were observed in vitellogenic fish, and in fish undergoing final maturation. A positive relationship was also found between the growth of oocytes and plasma concentrations of Vtg, E2 and T, although the patterns of increase were different for each variable. Plasma concentrations of Vtg and E2 rose steadily across oocyte sizes from 100 to 450 μm, but the rate of increase of plasma E2 was slower than that of Vtg, and both reached a saturated concentration at oocyte sizes of c. 450 μm. In contrast, plasma concentrations of T showed no marked increase until oocytes grew beyond 400 μm.