ウメノハナガイ Loripes pisidium (Dunker) の発生とその卵について

抄録

Loripes pisidium (Dunker) is a small, globular, whitish bivalve and the valves are scored faintly by crossing grooves. In the vicinity of the Fishery Laboratory of Tokyo University, Ikawazu, Aichi Pref., this species lives abundantly with the little-neck clam, Venerupis semidecussata, in the sand-gravel bottom somewhat above the low tide mark. In Hukue Bay, the spawning season begins in the middle of June, and continues on through summer. The author could secure spawning individuals every spring tide throughout the period from June 17 to August 31. The spawning took place always in early morning between six and eight o'clock in my laboratory aquarium. There are some evidences that this is also true under natural conditions. In either sex, the reproductive element is issued slowly through the single siphon which is stretched one to two times as long as the shell length. The discharge may continue for 20 to 30 minutes with little or no interruption. The ova flow out in a continuous stream together with gelatinous substance and, in quiet water, settle to the bottom as a soft, milky, pear-shaped gelatinous mass. That is to say, ova are entangled in the gelatinous mass. The size of the gelatinous mass varies 2.5 to 5.0 cm in longer axis, 1.5 to 3.5 cm in shorter axis. Ova are of alike sizes, averaging 0.08 mm in diameter. The number of ova is not so many in each gelatinous mass. In the male, on the other hand, a continuous stream of milt scatters immediately into water. The ova undergo their early development within the gelatinous mass. The first and the second polar bodies appear from 30 to 35 minutes after fertilization, lacking a polar lobe. At water temperature 27.0∿30.5℃, embryoes in the gelatinous mass begin a rotatory movement by means of the cilia, when they are 11 hours old. At the age of 15 hours, they develop to trochophore larvae. In this stage, the larvae come out of the gelatinous mass, and commence the free swimming life. At the age of 20 hours the prodissoconch is complete, and the larvae become very active swimmers. The velum has well-developed cilia, but without a flagellum. Five-day-old larvae measure 0.14 mm in length. When larvae become about 0.18 mm long or about 20 days old, the otocyst appears. About 30 days old larvae begin a crawl-life with a slender foot, when the gill appears. The specimens 0.21 mm long have one or two gill papillae. The age at which various organs appear during the later metamorphosis can be stated only approximately, since artificially cultured larvae have rarely undergone normal development. The author could culture the larvae for two months. The maximum size attained by the cultured larvae was 0.22 mm in length and 0.19 mm in height.