FISHERY BULLETIN: VOL. 72. NO. 2 



faint traces of the fin fold and oil globule 

 pigment could be distinguished. 



Morphological Development of Yolk 

 Sac Larvae 



Omaka larvae hatch in a relatively undif- 

 ferentiated state, the only conspicuous structures 

 being the large yolk sac, the unpigmented eyes, 

 otic vesicles, and heart. The oil globule, positioned 

 forward of the head at the extreme anterior 

 margin of the yolk sac, is characteristic of 

 carangids (Ahlstrom and Ball, 1954). Ten hours 

 after hatching (1.80 mm SL), the larvae had 

 developed a narrow, straight gut tube (it became 

 convoluted on the fifth day) terminating at the 

 anal papilla and urinary bladder (Figure 2B). 

 The gradual yolk resorption resulted in the oil 

 globule's shifting its position posteriad while 

 remaining at the anterior margin of the sac. The 

 oil globule lay just ventral to the head at 10 h. 



Pectoral buds appeared in the larvae by the 

 end of the second day and the jaw buds by the 

 third day. After three days most of the yolk 

 had been absorbed, and the oil globule had 

 diminished in size to a small, barely noticeable 

 spherical body. The end of the third day was thus 

 selected as the termination of the yolk sac stage 

 of the larvae. 



By the fourth day, the eyes were completely 

 pigmented, the mouth was open and the broad, 

 membranous pectoral fins were functional. The 

 small collapsed yolk sac containing the now 

 minute oil globule was still evident ventral to the 

 anterior portion of the abdominal cavity. 



LARVAE 



Pigmentation 



Head Pigmentation 



Following yolk absorption (in three-to four- 

 day-old larvae), head pigmentation was present 

 in the following areas: (1) the median dorsal 

 surface of the midbrain (optic lobes) consisting 

 of one or two small melanophores; (2) the floor 

 of the otic vesicle with two or three expanded 

 melanophores which remained visible until 

 obscured by the overgrowth of tissue in older 

 larvae at about day 10 (Figure 2C); (3) along 



the dorsal margin of the opercle which exhibited 

 a few faintly pigmented melanophores; (4) on the I 

 lower jaw with a melanophore situated at the tip , 

 of the lower jaw and another at the angular I 

 bone, with most of the larvae having a melano- ' 

 phore midway between these two. 



As the larvae grew, the density of head pig- j 

 mentation increased — particularly over the mid- 

 and forebrain region and on the jaws. The 

 number of melanophores increased on the postero- 

 lateral half of the midbrain lobe while a ring 

 of melanophores concurrently outlined the mar- 

 gin of the midbrain capsule. Larvae of 

 approximately 3.5 mm (day 8-not illustrated) 

 exhibited a cluster of expanded melanophores 

 over the midbrain which gradually extended 

 antero-ventrally to the forebrain and snout 

 region. Pigmentation on the surface of the head 

 had intensified in the older larvae, with the cap 

 over the midbrain being especially conspicuous. 



By the tenth day (4.0 mm SL), most larvae 

 possessed a melanophore at the tip of the upper 

 jaw in addition to those on the lower jaw; 

 pigmentation subsequently increased over the 

 premaxillary, maxillary, and dentary region 

 as the larvae advanced. Melanophores located on 

 the jaws were smaller and more punctate than 

 those on the top of the head and along the 

 operculum. 



Pigmentation on the membrane overlying the 

 branchiostegal rays and along the gular region 

 developed in eight- to ten-day-old larvae (3.5- 

 4.0 mm SL) (Figure 2D). The most anterior 

 branchiostegal rays were initially pigmented with 

 pigmentation proceeding distad until the full 

 complement of seven rays was pigmented. 

 Pigmentation of each ray also proceeded distad 

 resulting in larvae of 20 to 22 days (8.3-9.5 

 mm SL) possessing as many as two or three 

 melanophores over the basal end of each branchi- 

 ostegal ray (Figure 3A). This pigmentation was 

 barely discernible in larvae of 26 days (10.9 

 mm SL) and eventually lost altogether in larvae 

 of 28 days (11.4 mm SL). Melanophores along 

 the median gular region similarly increased in 

 density, forming an almost continuous dotted 

 line of contracted melanophores posterior to the 

 isthmus in larvae of 14 days (5.2 mm SL). 

 Additional melanophores formed along this line 

 but pigmentation in this region gradually dis- 

 appeared, like the branchiostegal pigmentation, 

 in the advanced larvae. 



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