Early Development and Reproductive Behavior of the Gobiid Fish, Mugilogobius abei

Reproductive behavior,spawning season and development of eggs and larvae of Mugilogobius abei were studied.Materials used in this study were taken periodically f rom T akahama Channel,Tokyo Bay ,during the period from April to October,1979 .The spawning season seems to continue from April to August ,with a peak in May.Spawning was accomplished at least more than one time during the spawning season .The oval eggs were 0.98 mm(mean)in length ,0.45 mm (mean)in breadth.Eggs hatched 97 hours after their discovery at 24.0•‹•`24.6•Ž.The larvae reached the juvenile phase and wholly aban doned their planktonic mode of life 44 days after hatching at 22 .5•‹•`29.1•Ž.It was assumed that larvae in the field return ,not immediately,but rather gradually to the native habitat , after completing the planktonic mode of life . Mugilogobius abei(Jordan et Snyder)is a very common goby,reaching 5•`6 cm in total length,which not only inhabits sandy and muddy bottoms in estuarine waters,but also occasionally enters freshwaters .This species is known to occur from the Kanto district (middle part of Honshil)southward to the Ryukyu Islands in Japan and the southern part of China including Hainan Island(19.00N, 109.30E)(Okada et al.,1965;Nakamura ,1975; Aoyagi,1957).Its northern range seems to extend to Miyagi Pref.,northern Honshil , Japan (Nakamura,1976) . There is little information concerning the life history of this species except for observations of the egg shape (Dotsu,1979)and estimation of the spawning season(Iwata et al ., 1979). In this paper the spawning season ,reproductive behavior and the development of eggs and larvae are described and discussed based on specimens collected from Takahama Channel near the Tokyo University of Fisheries . Materials and methods The sampling area is located near the mouth of Takahama Channel,Tokyo Bay.Sea water enters the channel at high tide(13 .49•ñin salinity at low spring tide in April).The bottom of this area consists mainly of sandy mud and small gravel.Discarded tires,broken concrete blocks,pieces of glass and cans lie scattered about the bottom. A total of 150 males,157 females and 71 individuals of unknown sex were caught(Table 1)using dip nets once a month at low spring tides,during the period from April to October, 1979.The spawning season was estimated from the gonad index of female specimens and the appearance of egg masses in the surveyed area.The gonad index was obtained by the following formula:gonad weight(g)/body weight (g)•~102. Five pairs of mature males and females of 58 individuals captured on 26 April were maintained to 1 November,1979 in an aquarium , 114cm•~30cm•~43cm,120l in capacity,containing 50% sea water(15.19•ñin salinity) with a closed water circulation system .Water temperature during the period ranged from 20.8•‹to 29.8•Ž.Natural and fluorescent light was used during day time.The fish were fed chopped bivalves and midge larvae,Chironomus prasinus. During six months of rearing,curved slate plates were placed on the bottom as spawning beds.The reproductive behavior was observed and fertilized eggs were obtained . Some of the fertilized eggs from one spawning were transferred to a 13 cm diameter glass basin,containing the same water as their parents were kept in,where observations of the development of eggs were carried out.The water temperature for eggs was kept at 24 .0•‹•`