enase (alpha glycerol phosphate). This locus has been 

 used to identify adults of the genus Thunnus in the 

 absence of complete morphological data (Dotson and 

 Graves 1984). This paper describes an application 

 of the above techniques, modified for work with 

 small tissue samples, to the identification of an op- 

 portunistic collection of early juvenile tuna, in ex- 

 cellent morphological condition, which were frozen 

 shortly after capture. The early juveniles in this col- 

 lection encompassed the pigmentation patterns 

 reported for both T. albacares and T. obesus. 



Specimens were collected aboard the MV Royal 

 Polaris, a San Diego-based sportfishing boat, about 

 1 km off Clipperton Island in the eastern Pacific (lat. 

 10°23'N, long. 109°15'W), 8 May 1986, from hours 

 of 2100 to 2400. The early juveniles were caught 

 underneath floodlights at a depth of about 1 m with 

 a fine (1 mm) mesh, long-handled dip net. They were 

 not present at the surface. After each pass, the dip 

 net contents were sorted for scombrid larvae. Ap- 

 proximately 100 specimens were collected, most of 

 which were quickly frozen in seawater. 



Adult specimens of T. albacares and T. obesus 

 were collected by hook and line off the Pacific coast 

 of southern and northern Baja California, Mexico, 

 respectively. White muscle tissue samples were 

 removed from freshly caught specimens and quick- 

 ly frozen. 



The early juveniles collected off Clipperton Island 

 were thawed in the laboratory and examined under 

 a dissecting microscope. Those juveniles positively 

 identified to the morphologically indistinguishable 

 T. albacares/T. obesus complex were measured for 

 total length (TL) to the nearest millimeter and ex- 

 amined for postanal ventral pigmentation pattern. 

 Heads were removed and placed in 95% ethanol for 

 otolith studies. The remaining trunk and tail mus- 

 culature was placed in a small (1.0 mL) microfuge 

 tube, and 60 /iL of cold grinding buffer (0.1 M Tris, 

 pH 7.5) was quickly added. Tissues were homog- 

 enized with a cold ground-glass rod contoured to fit 

 snugly within the microfuge tube. Approximately 

 10 seconds of rod rotation were required to com- 

 pletely disrupt the tissues. The homogenate was cen- 

 trifuged for 2 minutes in a microfuge and stored on 

 ice until electrophoresis. 



Two grams of adult tissue were disrupted for ap- 

 proximately 20 seconds in 4 volumes of cold grind- 

 ing buffer in a motor-driven, ground-glass tissue 

 homogenizer. The homogenate was centrifuged at 

 5,000 g, 4°C, for 10 minutes. The supernatant was 

 removed, diluted 10:1 with cold grinding buffer, and 

 stored on ice until electrophoresis. 



Horizontal starch gel electrophoresis was per- 

 formed on 12% (W/V) gels run in the Tris/Citrate 

 II system of Selander et al. (1971). Gels were run 

 at 45 to 50 mA for 3.5 hours. Glycerol-3-phosphate 

 dehydrogenase was stained using the protocol of 

 Shaw and Prasad (1970). Three sets of standards 

 composed of the supernatants of muscle tissue 

 homogenates of adult yellowfin and bigeye tuna 

 were placed in each gel to score the early juveniles. 



A total of 77 early juveniles, ranging in length 

 from 10 to 21 mm TL, were processed. Glycerol-3- 

 phosphate dehydrogenase activity was scored for 68 

 individuals. All early juveniles displayed a muscle- 

 tjrpe glycerol-3-phosphate dehydrogenase band of 

 low anodal mobility, identical to that of the yellow- 

 fin tuna adults (Fig. 1). No individuals with the 

 faster migrating T. obesus glycerol-3-phosphate 

 dehydrogenase band were detected. 



The lack of bigeye tuna juveniles in this study 

 could be the result of two possibilities: either the 

 early juveniles were all yellowfin tuna or both 

 yellowfin and bigeye tuna early juveniles share a 

 muscle-type glycerol-3-phosphate dehydrogenase 

 isozyme of similar electrophoretic mobility. How- 

 ever, differential ontogenetic expression of electro- 

 phoretically distinct isozymes has not been reported 

 for fishes in studies that have used adult allozymes 

 to identify larvae or early juveniles (Morgan 1975; 

 Smith and Crossland 1977; Sidell et al. 1978; Smith 

 and Benson 1980) or in investigations of ontogene- 

 tic expression of electrophoretic loci (Shaklee et 

 al. 1974; Siebenaller 1984). Thus, the electropho- 

 retic similarity of the glyceraldehyde-3-phosphate 

 alleles of the early juveniles investigated in this 

 study most likely indicates that they were all yellow- 

 fin tuna. 



On the same trip during which the early juveniles 

 were collected at Clipperton Island, about 300 adult 

 yellowfin tuna were caught on hook and line but no 

 bigeye tuna were taken. Histological examination 

 of ovarian tissue from several of the adult yellow- 

 fin tuna revealed postovulatory follicles, indicating 

 that spawning was taking place (Anonymous 1987). 

 On the basis of this information, it is not unexpected 

 that all the early juveniles identified electrophoret- 

 ically in this study proved to be T. albacares. 



A wide range of postanal ventral pigmentation 

 patterns (red and black) was displayed by the early 

 juveniles. Since these specimens were collected at 

 night, the pigment cells, when present, were distinct 

 and brightly colored as reported by Matsumoto et 

 al. (1972). While some individuals had no melano- 

 phores in this region, others had from one to eight. 



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