The parturition of 4 Ditrema temmincki females (153-162mm TL, 1 year old), captured off Tsuyazaki (130°29'E, 33°47'N), northern coast of Kyushu, Japan, was observed in the aquarium using VTR from May 20 to 26, 1985. The 4 female released 6, 8, 8, and 10 embryos, respectively, within 1 or 2 days during the daytime. Of these, 29 were born alive and 3 dead. Just before and at parturition, respiration of the female became frequent and 2 white spots appeared on the female's side. At the initial phase of parturition of living embryos, the caudal fin appeared first from the gonopore. Within a minute to several hours, the caudal peduncle protruded and the embryo (51.4-58.2mm TL) slipped out. It was also observed that several embryos were released successively at intervals of several seconds to about 10 minutes. The three dead embryos, 1 of which was degenerating, were extruded headfirst. The young formed an aggregation and fed immediately after birth. Female parents swam indifferently from their young and did not show parental care. Observations on metamorphosis of embryos taken out from the female revealed that the embryos just before parturition had ruddy body color and large fins with a developed capillary system and spatulate extensions at their margins. These embryonic features disappeared within 1 or 2 days after embryos were transferred into seawater.
The Biwa-sheatfish, Parasilurus biwaensis Tomoda, is endemic to Lake Biwa, and is the largest species of the Japanese silurid fishes. From 23 June to 17 July, 1988, the reproductive behaviour of this fish was investigated during night-time on the rocky shores in the south of the lake. Spawning occurred frequently during midnight following heavy rain. The behavioural patterns observed were as follows: A female first searches for a spawning site, with a male following behind. Immediately the female stops, the male first places his head under that of the female and then, by bending his body towards her anal fin, positions himself such that his tail is towards the end of the female's snout. From this position the male then begins to wrap his tail around the head of the female, gradually winding his body tighter and moving along the body of the female until positioned at the centre of her body. The male then winds his body tightly around the dorsal side of the female's abdomen for 20-30 seconds. The female then shakes her head from side to side several times, and orientates her body downward. This behaviour causes the male to become separated from the female. Immediately after separation, the female releases a large number of eggs, and circles around with the male following on the inside. Although gamete release by the male was not actually observed, it seems likely that the eggs are fertilized during such circling. After circling, the pair turns round twice violently, causing the eggs to become widely scattered. The pair, then swim away with the female in the lead. Reproductive behaviour of the Biwa-sheatfish was compared with that of the Japanese sheatfish, Parasilurus asotus Linnaeus, in temporary waters around paddy fields. Some differences in reproductive behavioural traits were recognized between the two species.