the length of the snout (more than twice the 

 diameter of the orbit) . 



The development of the larval and early 

 juvenile stages is discussed in the following 

 sequence : (a) changes in pigmentation, (b) 

 changes in body form, and (c) sequence of 

 ossification. The term "larva" includes all 

 specimens from the smallest to the juvenile 

 stage, which starts at the time the full com- 

 plement of spines and rays in all fins has ossi- 

 fied. In wahoo this point is reached before 23.7 

 mm. SL. The term "postlarva" is not used here. 

 Standard length is the measure of body length, 

 except when specified otherwise. 



CHANGES IN PIGMENTATION 



As is true for all fish larvae preserved in 

 Formalin, the only pigment spots visible in 

 wahoo larvae are the melanophores. Unlike 

 larvae of Trachurus ( Ahlstrom and Ball, 1954) , 

 Exocoetidae (personal observation), Istiopho- 

 ridae (Ueyanagi, 1963), Coryphaena (Mito, 

 1960) , and others, the wahoo larvae have com- 

 paratively few melanophores. Large changes in 

 pigmentation as the larvae increase in size are 

 seen only in four areas: (1) snout, (2) base of 

 second dorsal fin, (3) base of anal fin, and (4) 

 digestive tract (figs. 2 and 3). The following 

 descriptions of pigmentation are based on the 

 left side of the body as seen in lateral view. 



Head Pigmentation 



The midbrain area remains unpigmented in 

 larvae smaller than 4.4 mm. (fig. 2A and B). 

 The only exception was a 3.1-mm. lai-va which 

 possessed a melanophore in this location. A 

 single melanophore is present on the anterior 

 portion of the midbrain in about half the larvae 

 between 4.4 and 5.5 mm. (fig. 2A). Pigmenta- 

 tion in the anterior portion of the midbrain 

 increases gradually with body length ; 6 melano- 

 phores are present in the 10.7-mm. larva (fig. 

 3C), and about 27 in larvae up to 13.2 mm. 

 long (fig. 3D). 



Pigmentation appears on the forebrain much 

 later than on the midbrain and the number of 

 melanophores is small. A single melanophore 

 appears on the forebrain in larvae about 7.4 

 mm. long, and the number of melanophores 

 increases to only four or five in larvae up to 

 17.8 mm. long (specimen similar in appearance 



to that of figure 3D, except for heavier pigmen- 

 tation). 



Lateral pigmentation on the posterior portion 

 of the head is completely lacking in larvae up to 

 13.2 mm. long, but about 50 small melanophores 

 are present in the area posterior to the orbit 

 and on the surface of the preopercle in larvae 

 about 17.8 mm. long. In larvae of yellovdin 

 tuna (Thunnus albacares) and skipjack tuna, 

 some melanophores usually are present in the 

 postorbital region in specimens shorter than 

 6.0 mm. 



Pigmentation on the snout develops in two 

 major areas : near the primordial nasal cavity 

 and on the tip of the upper jaw. It is present 

 on the snout in the smallest larva as a single 

 small melanophore within the primordial nasal 

 cavity and two or three melanophores on the tip 

 of the upper jaw (fig. 2A). The pigmentation 

 at the primordial nasal cavity develops slowly; 

 additional melanophores are added within the 

 cavity and on the surface of the snout anterior 

 to it. As the nostrils form (when the larvae are 

 between 10.7 and 13.2 mm. long) , a few more 

 melanophores appear on the surface of the 

 snout between the anterior and posterior nos- 

 trils. The number of melanophores in this area 

 is small, however, for no more than 13 to 15 

 are present in the 13.2-mm. larva (fig. 3D). 



The number of melanophores increases 

 slowly on the anterior portion of the upper jaw 

 of larvae up to a length of about 4.3 mm. ; how- 

 ever, above this size, as the upper and lower 

 jaws begin to grow more rapidly in relation to 

 body length, pigmentation on the anterior por- 

 tion of the upper jaw increases noticeably. At 

 a length of 6.8 mm. (fig. 3A), two or three 

 rows of 18 to 25 melanophores are on the anter- 

 ior part of the upper jaw. The number of 

 melanophores increases with further growth of 

 the fish, and the entire surface of the upper jaw 

 anterior to the mesethmoid is covered in larvae 

 larger than 13.2 mm. (fig. 3B-D) . Additional 

 melanophores develop posteriorly on the snout, 

 and in the 17.8-mm. larva the pigmentation on 

 the anterior portion of the upper jaw and 

 around the nostrils has merged into a single, 

 large pigmented area. 



The tip of the lower jaw in larvae 2.8 mm. 

 long has few melanophores, but the number 



MORPHOLOGY AND DISTRIBUTION OF LARVAL WAHOO 



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