RICHARDS, MILLER, and HOUDE: DEVELOPMENT OF THREAD HERRING 



had curved anteriorly beyond the width of the 

 vertical shaft of E4. About 30 rakers were 

 present in each series at this stage and the 

 epibranchial organ and E4 bone were assuming 

 adult configuration. Estimates made on adult 

 specimens indicate that in a full-loop, continuous- 

 tube epibranchial organ, the definitive number 

 of rakers in the included medial E4 and single 

 E5 series each at least equal the total of rakers 

 on the whole first gill arch. There are about 

 120 rakers in adults 100 mm or longer (Berry 

 and Barrett, 1963). We may therefore infer that 

 the number of rakers in the epibranchial organ 

 increases gradually with increasing size of the 

 fish, to about 120 or moi-e in each series. 



PIGMENTATION 



Melanophores were present along the ventral 

 midline in the smallest specimens studied (4 mm) 

 — one or two melanophores beneath the heart 

 just anterior to the pectoral symphysis, and a 

 paired row along the base of the hindgut (that 

 portion of the gut posterior to the site of the 

 pelvic fins) continuing to the anus, and a row on 

 the ventral midline posterior to the anus. A 

 dorsolateral row of melanophores occurred on 

 each side of the foregut (that portion of the gut 

 anterior to the pelvic fin region). The eye was 

 pigmented but no other melanophores were pres- 

 ent (Figure 2). By 10 mm, the posterior ventral 

 row was distributed along the posterior edge of 

 the hypural bones, and a few internally placed 

 melanophores appeared dorsolaterally on each 

 side of the hindgut. The remaining melanophore 

 pattern was basically unchanged (Figure 3). 

 By 15 mm, the melanophores had developed into 

 streaks of pigment along the base of the isthmus, 

 dorsolaterally along the anterior gut, along the 

 base of the posterior gut, and on each side of 

 the base of the anal fin. Internal melanophores 

 above the posterior gut had taken on a well- 

 defined, broken-lined pattern and had begun to 

 advance anteriorly, particularly forming a slight 

 arch in the area of the swim bladder (Figure 4). 

 Melanophores appeared on the cleithrum near 

 the hindbrain, and the hypural melanophores at 

 first clustered on the bases of the lower lobe 

 of the caudal fin but, by 17 mm, melanophores 

 appeared on both lobes of the caudal fin. Between 

 15 and 24 mm (transformation period), melano- 

 phores varied somewhat in time of appearance. 



In some specimens a melanophore appeared 

 medial to the left nostril (Figure 5, dorsal view). 

 Melanophores appeared both dorsally and ven- 

 trally on the swim bladder as it developed 

 (Figure 6). During this period melanophores 

 also began to appear on the dorsum, first pos- 

 terior to the dorsal fin, then anteriorly. Melano- 

 phores also appeared along the lateral midline 

 and over the hypural bones internally. The 

 internal pigment associated with the vertebral 

 column of these larvae is quite pronounced, par- 

 ticularly during transformation. A few internal 

 melanophores over the posterior centra were 

 first noted in a cleared and stained 12.1-mm 

 specimen. By 15 mm, one or two melanophores 

 or groups of melanophores were noted above 

 each vertebra. The melanophores over the 

 anterior vertebrae are lost or reduced by 19 mm. 

 Some individual variation, however, is evident 

 in both the internal and external pigmentation. 

 The dorsal external pigmentation is noticeably 

 reduced in the 19.7-mm specimen (Figure 6) as 

 compared to the 17.1-mm specimen (Figure 5). 

 The expanded state of most melanophores (a 

 probable result of rearing under continuous illu- 

 mination) makes detailed counts of melanophore 

 patches quite difficult. Following transforma- 

 tion, melanophores were visible over the brain 

 and on the jaws as well as in increasing quan- 

 tities on the dorsum, lateral midline, over the 

 gut, and in the caudal fin rays (Figure 7). By 

 30 mm, the dorsal pigment had increased, the 

 foregut pigment was lost, and melanophores 

 were seen in the dorsal fin — essentially the adult 

 pattern (Figure 8). 



In life, the larvae are transparent and the 

 only noticeable features are the heavily pig- 

 mented eyes. The gut is usually noticeable be- 

 cause of the food contained in it. In larger 

 larvae, the swim bladder is decidedly noticeable 

 as a bubble above the gut (Figure 6). Melano- 

 phores are invisible until the specimen is exam- 

 ined under magnification. 



ACKNOWLEDGMENT 



Elbert H. Ahlstrom (Southeast Fisheries Cen- 

 ter, National Marine Fisheries Service, NOAA, 

 La Jolla, Calif) and Peter J. Whitehead [British 

 Museum (Natural History)] reviewed the manu- 

 script and their helpful comments and criticism 

 are greatly appreciated. 



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