水槽内で観察されたメイチダイGymnocranius griseusの産卵習性と卵および仔魚

抄録

The sea bream, Gymnocranius griseus (Temminck et Schlegel), a coastal fish belonging to the family Lethrinidae, ranges from Sendai Bay in Japan to the Indo-West Pacific Ocean.The present paper deals with the mode of reproduction and early life history of the fish which were reared in an oceanarium (10×10×6 m, depth) at the Marine Science Museum of Tokai University in 1973.
The seventeen parental fishes, measuring 290-340 mm in fork length, were collected from the coast of Suruga Bay and reared in the oceanarium for two or three years.Reproduction occurred during the months of May and June in the oceanarium when the water temperature ranged 18.0-26.5°C (Fig.1).
Mutual courtship of the sea bream begins when one male attempts to lure a female who re-mains motionless in a small school of the fish close to the bottom.The male approaches the female and blocks her way, and then, he taps her belly with his snout.They ascend slowly together towards the surface of the water with the male under the female.If the female is not ready to spawn, the paired fishes return separately to the bottom.After several repetitions of this mutual courtship behavior, the paired fishes, when they reach the height of one or two meters below the surface of the water, take position themselves side by side.The eggs are spawned and are fertilized simultane-ously (Fig.2). Spawning occurs between 20: 30 and 21: 00 at the water temperature of 20.7-22.8°C.
The characteristics of color of the sea bream can be divided into three distinct types and are interchangeable.One type, in which several wavy silver lines appear on the laterals, is observed only in the male during reproductive activity.The other two types are observed commonly in both sexes under normal conditions (Fig.3).
Fertilized eggs of the sea bream are buoyant, spherical, and colorless, measuring 0.76-0.79 mm in diameter.Twenty-five hours after fertilization, an oil globule, which was spherical until that time, warps elliptically or deforms to a gourd-shape in the lateral view.The hatching takes place 38-40 hrs.after fertilization at the water temperature of 20.0-22.4°C.The newly hatched larvae, measuring 1.48-1.50 mm in total length, have 9+24=33 myotomes.The elliptical oil globule once again becomes spherical.Twenty-two hours after hatching, the larvae, measuring 2.35-2.40 mm in total length, have 5+19-20=24-25 myotomes (Fig.4).
The warping of the oil globule in the yolk has been previously observed in early life stage of two lethrinid fishes, Lethrinus nematacanthus and L.choerorynchus (Mito, 1956;Akazaki et al., 1975;present authors, unpublished).However, in both lethrinids, the oil globule does not return to the spherical shape in the newly hatched larvae.Also the characteristics of the newly hatched larvae of G. griseus are compared with those of the two lethrinids and a few related fishes.