KUBOKAWA, K.*; MIZUTA, T.; Ocean Res. Inst., Univ. of Tokyo, Japan; Ocean Res. Inst., Univ. of Tokyo, Japan: Spontaneous spawning of amphioxus in aquarium

Several hundreds of mature amphioxus, Branchiostoma belcheri, were collected by dredging from sand bottom of 20 m depth in Enshu-Nada Sea, Japan in July, 2000 and July, 2001. They were kept in aquaria under natural day-light and constant room temperature conditions at Misaki Marine Biological Station, University of Tokyo until the end of August. Spontaneous spawning was repeated in aquaria at intervals for 1 to 11 days. The environmental conditions, temperature, tide level, lunar cycle, atmospheric pressure, and day-light length did not correlate with spawning. Swimming and spawning activities of animals in aquaria were recorded with a black-and-white video camera in 2000 and by an infrared video camera in 2001, and analyzed from the image of the video film. Fertilization was confirmed by the presence of embryos in aquarium water next morning. On spawning days, animals swam up from the sand bottom, reached to the water surface of 90 cm from the bottom of the aquarium and then released their gametes. The swimming speed was approximately 40 cm/sec. A peak of the swimming activity between 20:00 and 00:00, and spawning was concentrated in a short period from around 20:00 to 21:30. Every day when spawning was observed, the first spawning was performed by a male, and the following spawnings showed no regular male-female order, i.e. spermiations and ovipositions were randomly performed with no male-femlae synchronization. The interval between a given spermiation and the following oviposition was longer than one minute. These results show that the spawning pattern might be classified into a type of group spawning, although a male initiated the spawning every spawning day.