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 spec­ imens. Average 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 after hatching. Growth was especially accelerated after 20 days from hatching. 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 characteristic melanophore pat­ tern appeared and it was partially maintained until day 3 after hatching. At approximately 4 mm BL, larvae had developed melanophore patterns simi­ lar 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 mid­ lines of trunk and tail, and on the lower jaw. Erythrophores appeared at 4.63 mm BL in the caudal area. Jaw teeth appeared at 5–6 mm BL. The preoper­ cular angle spine, anterior preoper­ cular spine, and posttemporal spine, developed at approximately 7 mm BL, when erythrophores appeared on the trunk and tail. The notochord flexion occurred between 6 and 8 mm BL. At approximately 8 mm BL, erythro­ phores disappeared and juvenile color­ ation appeared on the trunk and tail, consisting of dense patches of melano­ phores at the dorsal and anal-fin bases, embedded melanophores, and melano­ phores at the periphery of the eye. The adult complement of fin-ray counts was attained at 10 mm BL, when the juve­ nile melanophore pattern was attained, although the pattern was not fully developed. Specimens larger than 20 mm BL did not have erythrophores. Squamation began at 27 mm BL and head spines disappeared by 38 mm BL. Manuscript accepted 10 April 2001. Fish. Bull. 99:601–616 (2001). Morphological development and growth of laboratory-reared larval and juvenile Thunnus thynnus (Pisces: Scombridae) Shigeru Miyashita Yoshifumi Sawada Tokihiko Okada Osamu Murata Hidemi Kumai Fisheries Laboratory of Kinki University, 3153, Shirahama, Wakayama 649-2211, Japan E-mail address (for S. Miyashita): miyasita@cypress.ne. jp The Pacific bluefin tuna is a large scomMorphological, physiological, and be­ brid that migrates transoceanically havioral information has recently been between the western and eastern Pacific collected from laboratory-reared Pacific Ocean (Orange and Fink, 1963; Fujita, bluefin tuna specimens (Harada et al., 1998). The Pacific bluefin tuna, Thun1971a; Kaji et al., 1996; Takii et al., nus thynnus orientalis (Temminck and 1997; Kumai, 1998; Miyashita et al., Schlegel), is considered a separate sub1998; Miyashita et al., 2000, in press; species from the Atlantic bluefin tuna, Sawada et al., 2000). The ability to proT. thynnus thynnus Linnaeus (Gibbs duce a complete series of early-stage and Collette, 1967; Collette and Nauen, specimens provides an opportunity to 1983). Recently, Cho and Inoue (1993) enhance our understanding of the ear­ showed reproductive isolation between ly development of bluefin tuna. In this the two subspecies. Most recently, Colstudy, we describe the morphological lette (1999) considered these subspedevelopment and pigment patterns of cies to be separate species on the basis reared larval and juvenile bluefin tuna. of morphology and molecular data. Our knowledge of the early life his­ tory of oceanic species, such as tunas, Materials and methods is incomplete. Morphological character­ istics common to scombrid larvae are Eggs were obtained on 10 July 1994 large head, gape, and eyes; development from bluefin tuna that had been caught of head spination; and posterior migraoff Ohshima when 20–40 cm in total tion of the anus (Collette et al., 1984). length [0-year class] and that had been In studies of early-stage bluefin tuna raised for seven years naturally in a net (e.g. Yabe et al., 1966; Matsumoto et cage at the Ohshima Experiment Sta­ al., 1972; Richards and Potthoff, 1974; tion of the Kinki University Fisheries Kohno et al., 1982), melanophore patLaboratory. The net cage was equipped terns were used to identify tuna larvae. with a polyvinyl chloride sheet surErythrophore patterns were proposed rounding the top 2 m of the net side as another characteristic by Ueyanagi wall to prevent eggs from flowing out of (1966) and Matsumoto et al. (1972). the net. Fertilized eggs were collected However, ontogenetic changes of melaat the surface with a 330-μm mesh net. nophore and erythrophore patterns and Eggs were transferred to a tank in a morphological characteristics have not land-based nursery center and kept in sufficiently been investigated because a fine mesh net at 24.5°C for approxi­ of the difficulty in obtaining a complete mately 12 hours. The following morn­ series of wild specimens in their early ing, eggs were transferred to a 20-m3 developmental stages. rearing tank and larvae hatched at 602 Fishery Bulletin 99(4) approximately 2200 h on 11 July. The first 24 hours after hatching were counted as day 0 (day-0) after hatching. Water temperature ranged from 24.5 to 27.7°C during the rearing period ( x =25.5°C, Fig. 1). The feeding scheme for larvae and juveniles was as follows: rotifers, Brachionus rotundiformis, from day 2 to day 22 after hatching, Arte­ mia nauplii from day 10 to day 25, and other live fish larvae (Oplegnathus fasciatus) from day 12 to day 30. At 1000 h daily from day 0 (12 July) to day 30 (11 Au­ gust), 20 tuna were sampled. Before preservation in 5% formalin, individual computer-captured video images were made with a video camera attached to a binocular micro­ scope while all specimens were under MS-222 anesthesia. Ten body parts were measured from these images: total length (TL), body length (BL), preanal length, body height at the base of the first dorsal spine, head length, head height, snout length, eye diameter, caudal peduncle depth, and upper jaw length. Video image measurements to 1/102 mm with an accuracy of ±1% were obtained from NIH Im­ age (version 1.61) image analysis software (NIH, 1997). Before and during notochord flexion, BL was measured from the tip of the upper jaw to the end of the notochord. After notochord flexion, BL was measured from the tip of the upper jaw to the posterior margin of the hypurals. All specimens (n=620, 3.49–37.78 mm BL) were used to de­ termine growth estimates and to record pigmentation pat­ terns and spine and fin development. Sixty-nine specimens (5.26–33.68 mm BL) were stained with alizarin red-S to describe spine development, and squamation. Pigment patterns, especially the position of body melanophores in relation to myomeres, were deter­ mined for specimens from 3 to 16 days after hatching (n=180, 3.63–9.96 mm BL). At irregular intervals from day 0 to day 22, one to five fish (a total of 53 specimens, 3.38–26.76 mm BL) were examined while anesthetized with MS-222 to determine erythrophore distribution. Ad­ ditional samples were examined at irregular intervals from hatching to day 2. A representative series of specimens from this study was deposited in the Aquatic Natural History Museum of the Fisheries Research Station, Kyoto University (FAKU 129041–129075).