larval pectoral (without rays), (2) caudal, (3) 

 pectoral, (4) second dorsal and anal. (5) first 

 dorsal, and (6) pelvic. 



Larval pectoral fins. — The larval pectoral fins 

 are formed on the smallest larva, 2.8 mm. long. 

 The fin membrane is large and fanshaped, and 

 fin rays do not develop until the larvae are weW 

 past 6 mm. (table 8). 



Caudal fin. — The caudal fin of wahoo devel- 

 ops first as a thickening ventral to the posterior 

 portion of the notochord in larvae about 4.4 

 mm. long (fig. 2C) . As the size of this thicken- 

 ing increases, the posterior portion of the noto- 

 chord turns upward ; at this stage the larvae 

 are shorter than 6.8 mm. (fig. 3A). This thick- 

 ening eventually develops into the hypural 

 bones. 



The principal caudal rays, those that are 

 ultimately supported by the hypural bones, 

 develop before any of the secondary caudal 

 rays are formed. When the larvae are about 

 5.8 mm. long, the first two to four rays near 

 the middle of the posterior edge of the hypural 

 thickening develop almost simultaneously. The 

 midline separating the ultimate dorsal and 

 ventral lobes of the tail is clearly discernible, 

 owing to the wide spacing between the two 

 medial rays. The initial group of rays lies at 

 an oblique angle to the notochord, but as the 

 posterior part of the notochord turns upward, 

 it pulls the rays up to a horizontal position. 

 Subsequent ray developnient proceeds dorsally 

 and ventrally from the medial rays. As in most 

 percomorph fishes, there are 17 principal cau- 

 dal rays in the wahoo : 9 are dorsal and 8 ven- 

 tral to the midline of the fin. All the principal 

 rays are completely developed in larvae over 

 8.4 mm. long (fig. SB). 



The secondary caudal rays first appear on 

 larvae about 8.4 mm. long with the develop- 

 ment of two dorsal rays anterior to the tip of 

 the notochord. The sequence of ossification is 

 from posterior to anterior for both the dorsal 

 and ventral secondary rays. At a length of 

 23.7 mm., the caudal fin has 15 dorsal and 

 15 ventral secondary rays, which is within the 

 range for adults. 



The shape of the caudal fin changes as the 

 rays are being developed. At first the larval 

 caudal fin, represented by the median fin fold, 



is roundly lobed. As the principal rays develop, 

 the initial medial rays are longer than the rest 

 of the fin fold. With the flexion of the notochord 

 and the subsequent shift in position of the rays 

 to a horizontal plane, the fin outline becomes 

 angular (fig. 3A). After all the principal rays 

 have developed, the dorsal and ventral rays be- 

 gin to outgrow the medial rays, so that the pos- 

 terior margin of the fin gradually assumes a 

 square and finally a forked shape. The fork 

 in the caudal fin is evident in the 10.7-mm. 

 larva (fig. 3C) and is pronounced in larvae 

 above 13.2 mm. long (fig. 3D). 



Pectoral fins. — The larval pectorals already 

 are present in the smallest wahoo (fig. 2A). 

 The initial pectoral rays begin to develop near 

 the dorsal part of the fin when the larvae are 

 about 6.6 mm. long, and are much shorter than 

 the pectoral membrane. At a length of 8.4 mm., 

 these rays extend to the margin of the larval 

 pectoral fin. As is usual in pectoral-fin develop- 

 ment, the dorsal rays appear first. Twenty- 

 three pectoral rays are developed in the 23.7- 

 mm. juvenile. This total is within the range 

 for adults. 



Second dorsal fin and finlets. — The base of the 

 second dorsal fin first appears as a thickening 

 on larvae as small as 5.8 mm., and the first rays 

 are developed in larvae 6.6 mm. long. The first 

 two or three rays develop simultaneously near 

 the middle of the fin; subsequent rays develop 

 anteriorly and posteriorly to these. The first 

 few finlets develop before the formation of the 

 anteriormost ray of the second dorsal fin. The 

 last finlet, however, develops only after the 

 formation of all the second dorsal fin rays. 



In the early stages of development, it is ex- 

 tremely difiicult to differentiate rays from fin- 

 lets. Consequently, the number of ray and fin- 

 let elements for the larvae were combined. A 

 similar grouping for the adults shows that the 

 full complement of rays and finlets for the sec- 

 ond dorsal fin ranges from 20 to 22 (table 2). 

 As discussed earlier, two elements, which are 

 separated in the larvae, form the last adult fin- 

 let. Consequently, the total number of larval 

 fin ray and finlet elements comparable with 

 those of the adult must be more than 21. The 

 full complement of rays and finlets in the sec- 

 ond dorsal is already present in the 13.2-mm. 



MORPHOLOGY AND DISTRIBUTION OF LARVAL WAHOO 



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