Abstract
Morphological development of jack mackerel, Trachurus japonicus (Temminck et Schlegel) is described from 196 specimens reared from eggs to juveniles in the laboratory at Tosa City, Japan.The egg is spherical in shape, and of moderate size, 0.87-0.90mm in mode.It is characterized by a light brownish oil globule, 0.17-0.22mm in diameter, and a narrow perivitelline space.During the late stages of embryonic development, the oil globule becomes situated rather close to the head, and on hatching is immediately under the forward part of the head.Hatching takes place about 40 hours after fertilization at water temperatures 20-22°C.
The newly hatched larvae are 2.3-2.5mm in total length, with 18-21 (8-10+10-11) myomeres.The most characteristic feature of the larvae is the extension of the anterior end of the yolk sac beyond the anterior margin of the head.On the 4th day after hatching the body reaches approximately 3mm in length, and the yolk material has been absorbed.
During the early postlarval stage, the caudal rays, the base of 2nd dorsal and anal fins begin to develop, the body attaining a length of about 6mm.The preopercular spines appear for the first time in the 7th day when the larvae reach about 3.4mm in length, and increase in size and number during larval development.During the late postlarval stage, the body is elongated to approximately 13mm in length.The 1st dorsal and pelvic fins are becoming evident at the end of this stage, the 29th day after hatching.
Soon after reaching the early juvenile stage, the preopercular spines become less conspicuous and have almost disappeared at the end of this stage.Small scutes developed along the lateral line can be observed in individuals as small as 19mm in length.Both the scutes and body scales are well developed in individuals longer than 26.5mm in length.At the late juvenile stage, complete number of rays is present in the pectoral and caudal fins.The juveniles attain about 55mm in length at the end of this stage, about 60 days after hatching.
