Reproductive biology of Okinawan amphidromous gobies
Reproductive biology and early development of three amphidromous gobies (Sicyopterus japonicus, Awaous melanocephalus, and Stenogobius sp.) were investigated in streams on Okinawa Island in southern Japan. Gonad examination, observation of spawning habits, and seasonal occurrence of newly hatched drifting larvae suggested the following: S. japonicus and Stenogobius sp. mature at approximately 40 mm and 35 mm in standard length (SL), respectively; A. melanocephalus matures at approximately 40 mm SL, but the smallest parent observed at a spawning nest was 70 mm SL; females of the larger S. japonicus and A. melanocephalus may spawn several hundred thousand eggs in a single clutch, while the batch fecundity of Stenogobius sp. is comparatively smaller (max. 72,000); the spawning seasons of S. japonicus and A. melanocephalus are May-August and June-November, respectively; and Stenogobius sp. can spawn throughout the year, but it is usually inactive from January to April. The eggs of S. japonicus and A. melanocephalus are small (0.4 mm in diameter) and almost spherical, while those of Stenogobius sp. are elliptical (1.0 mm long). The three species hatch as similar small undeveloped larvae, but they can be differentiated by melanophore arrangement. The small eggs, higher fecundity, undeveloped phase at hatching, and spawning during warmer seasons are all considered to be characteristics that are common to the subfamily Sicydiinae and the genera Awaous, Stenogobius, and Eleotris, inhabiting tropical and subtropical insular streams. However, minor variations are observed between species or genera in the timing and duration of the spawning season, construction of spawning nests, egg laying habits, egg morphology, and the size of newly hatched larvae and their yolk sacs.