ハナジロガジ (新称) の生活史

抄録

Opisthocentrus tenuis Bean and Bean, 1897, is known from Hokkaido, Mutsu Bay, Aomori Pref., Iwate Pref.and Sado Island in Niigata Pref.This species dwells in small schools among Zostera in Mutsu Bay.It feeds mainly on benthic gammarids and caprellids, and usually swims slowly above the bottom in an L-shaped posture by fanning its large pectoral fins.
In Mutsu Bay, the spawning period seems to extend from early December to late January.The water temperature during this period falls to 5-10°C. From experiences with spawning experiments in the aquarium and observations on natural egg masses, the egg mass is always guarded by the spawned female.For studies in a small aquarium, a vinyl pipe was used as the spawning nest.In the natural site at Moura, Mutsu Bay, 5 to 6 m deep, each egg mass was found with the female parent coiled about it in a nest on the stony bottom.A female spawns only one time in one spawning season.
Eggs are attached to each other by their adhesive points, though they have no adhesive processes, and form a single spherical egg mass about 3 cm in diameter.The egg is translucent.The yolk is colorless, containing a large light yellow oil globule and many small ones with white cloudy materials surrounding the large oil globule.The egg membrane is about 2_3 mm in diameter.
Eggs spawned in the aquarium developed in water temperature varing from 4.6 to 8.0°C and larvae hatched about 58 to 64 days after the spawning.Newly hatched prolarvae, provided with large remnants of yolk, are slender and 11.0 to 11.8 mm in total length.The newly hatched prolarvae reach the postlarval stage about 20 days after hatching.Hatched larvae were reared in a small vessel for about two months.They were fed nauplii of brine shrimp Artemia sp., but the largest one reached only 19.0mm.
In May, 1979, at Moura, young fish, some more than 3 cm TL., were observed swimming close to the bottom in schools.From examinations of over four hundred specimens collected from Mutsu Bay, one year old fish seemed to grow to 6 to 10 cm TL., two year olds to 11 to 15 cm and three year olds to 16 to 19cm.Most fish attain sexual maturity in two years.The life span is two years in most fish.