Genetic distance and the relationships among 15 species of genera Trachurus, Decapterus, Selar, and Selaroides were estimated from 18 electrophoretically detectable isozyme genes. Es-timates of genetic distance (D) between every pair of species within the genus Trachurus ranged from 0.005 to 0.560 with a mean of 0.322, and from 0.484 to 1.868 with a mean of 1.022 within the genus Decapterus. Between species of different genera, estimates of D ranged from 0.786 to 2.863 with a mean of 1.784. From these results, Decapterus species could be considered as having evolved over a long period while Trachurus is a newly arisen genus. A relationship among species suggests that the Decapterus species are expanded to offshore and deep area after being divided into some groups, and that the Trachurus species are divided presumably into at least two groups, one group of which is coastal and the other of which is offshore. Trachurus japonicus and Trachurus novaezelandiae could be considered subspecies on the basis of allelic distribution and genetic distance.
Specimens of Canthigaster rivulata (Temminck et Schlegel) were collected from Kominato and Hayama, central Japan, from May, 1985 to October, 1986. On the basis of the gonadosomatic index, gonadal histology and results of artificial fertilization of these specimens, the spawning season is considered to extend from late June to mid-September. The specimens exhibited the following dimorphic differences associated with sex: 1) The male is larger than the female. 2) Ventral side of the body is brownish orange in the male with vermiculated or reticulated patterns of bright violet, while it is white in the female. 3) The male has a well-developed skin fold along the mid-dorsal and mid-ventral lines, which is greatly elevated during courtship; whereas the female's skin folds are not or slightly developed and conspicuous only during courtship.In an aquarium with the water temperatures of 22 to 26°C, a pair of fish spawned every four days late in the morning for three consecutive months. Courtship and spawning occurred in a pair. The male swam in front of the female, and elevated the skin folds both dorsally and ventrally, fully spreading the unpaired fins, with the ventral side of the body flashing bright blue and the dorsal side turning dark. Both fish swam in a circular fashion, elevating the skin folds. The male followed the female nudging her abdomen with his snout. Both fish turned upward, and released gametes. The eggs are spherical, 0.53-0.73mm in diameter, demersal, adhesive, transparent, and pale yellowish orange in color, and contain a cross-shaped or asteroid cluster of oil globules. The egg membrane was thick and consisted of about 14 concentric layers. The incubation period ranged from 73.5 hours at 28.2-28.5°C to 145.0 hours at 22.1-22.4°C. The newly hatched larvae were 1.38-1.98mm in total length (TL) with 8+11-13=19-21 myomeres. The yolk was absorbed when the larvae attained 1.49-2.22mm TL, three days after hatching. The larvae were fed on oyster larvae, blue mussel larvae, sea-urchin larvae and rotifers, but all of them died in 16 days. During the embryonic and early larval stages, the only pigment cells that appeared on the body were the black chromatophores.
Reproductive behaviour of the catfish, Silurus asotus was studied in temporary waters around paddy fields. Spawning occurred nocturnally during the first week from the initiation of irrigation. In reproductive activities, a male first energetically pursued a female with its head near to the female's belly (chasing) and then began to cling to the female's body from the side, bending its tail or head (clinging). Finally the male enfolded the female's body, with its anus near to the female's (enfolding). In some cases, 2-4 males pursued a single female and two males enfolded a female at the same time. Although no aggressive behaviour was evident between males, it was always the largest male that could most frequently approach and enfold the female. The mating pair moved a long distance in a ditch, paddy field and/or creek, performing reproductive activities. It is thought that the spawning site and period of spawning of the fish enable the larvae to avoid the danger of predation and to efficiently feed, firstly on plankton and later on larvae of other fishes which become abundant during the irrigation period. Although some eggs and larvae may die due to the drying out or high water temperatures of such unstable temporary waters, scattering eggs may reduce the incidence of the annihilation of the young.