Morphological development and growth of laboratory-reared larval and juvenile Thunnus thynnus (Pisces : Scombridae)

Morphological development and growth of larval and juvenile Pacific bluefin tuna, Thunnus thynnus, were studied from laboratory-reared specimens. Aver age body length (BL) of newly hatched larvae was 2.83 mm and larvae grew on average 5.80 mm by 10 days, 10.62 mm by 20 days, and 35.74 mm by 30 days a fter hatching. Growth was especially accelerated after 20 days from hatchin g. Newly hatched larvae had small melanophores scattered over their bodies except for the finfold. On day 1 after hatching (3.35-3.74 mm BL), a charac teristic melanophore pattern appeared and it was partially maintained until day 3 after hatching. At approximately 4 mm BL, larvae had developed melan ophore patterns similar to those of preflexion Thunnus spp. larvae, such as melanophores on the dorsum of the gut, midlateral trunk, and tail, at the dorsal and ventral midlines of trunk and tail, and on the lower jaw Erythro phores appeared at 4.63 mm BL in the caudal area. Jaw teeth appeared at 5-6 mm BL. The preopercular angle spine, anterior preopercular spine, and post temporal spine, developed at approximately 7 mm BL, when erythrophores appe ared on the trunk and tail. The notochord flexion occurred between 6 and 8 mm BL. At approximately 8 min BL, erythrophores disappeared and juvenile co loration appeared on the trunk and tail, consisting of dense patches of mel anophores at the dorsal and anal-fin bases, embedded melanophores, and mela nophores at the periphery of the eye. The adult complement of fin-ray count s was attained at 10 mm BL, when the juvenile melanophore pattern was attai ned, although the pattern was not fully developed. Specimens larger than 20 min BL did not have erythrophores. Squamation began at 27 mm BL and head s pines disappeared by 38 mm BL.