Reproductive biology of the amphidromous goby Sicyopterus japonicus (Gobiidae: Sicydiinae)
To understand the reproductive biology of the amphidromous goby, Sicyopterus japonicus , juvenile and adult gobies of this species were sampled in the ota River, Wakayama, Japan. Sampling was conducted monthly from June 2003 to october 2004, and once each season from april 2005 to March 2006 at 7 sampling stations from the estuary to the upper reaches of the river. the gonadosomatic index values of females (n = 171) ranged from 0.0 to 20.8 and showed high values in July and august (summer, mean: 6.5) that indicated they have a short spawning season. there were no relation- ships between gonadosomatic index and standard length, age, or distance from the river mouth. this indicated that they do not have a spawning aggregation in a certain area in the river. 30 randomly selected oocytes from each ovary of 48 females were measured to observe the monthly changes of oocyte diameters. “Small oocytes” of 5-100 μm diameters were observed all year round, while “large oocytes” > 200 μm were only observed in July and august. only specimens collected in July and August were classified as being mature by histological examination. Potential batch fecundities calculated by the percentage of large oocytes in the gonads were estimated to be 10,800-52,500 (n = 10), and standard length was cor - related with potential batch fecundity. egg clutches of S. japonicus were observed throughout the sampling sites except in the upper- and lower-most reaches in august 2004 to 2008. Spawning places were found in rapids and egg clutches were attached to the undersides of stones. although S. japonicus in the ota River had similar characteristics with tropical Sicy- diinae species, such as large clutches deposited on the undersurface of the stones, their spawning season was shorter than that of tropical species that have almost year-round spawning.