the angle of the preopercle. Additional spines 

 develop slowly; at 4.3 mm., only two .spines are 

 ossified (fig. 2C). The third -spine is first seen 

 on the 4.7-mm. larva, and the fourth and fifth 

 spines appear in larvae longer than 6.8 mm. 

 The number of spines remains constant until 

 the larvae attain a length of 13.2 mm., when 

 two spines are added on the horizontal edge. 

 The 23.7-mm. juvenile possesses the greatest 

 number of spines: five on the horizontal and 

 three on the vertical edge of the preopercle. 

 The preopercular spines of the wahoo are rela- 

 tively shorter than those in larvae of yellowfin 

 or skipjack tuna, and the preopercle has al- 

 ready overgrown most of the spines in the 13.2- 

 mm. juvenile. 



As in larvae of yellowfin and skipjack tuna, 

 but to a lesser degree, additional spine develop- 

 ment occurs on a ridge on the preopercular sur- 

 face anterior to the base of the preopercular 

 spines. In larvae between 4.5 and 5.8 mm., a 

 single short spine is present on this ridge, and 

 two spines are developed in larvae between 

 6.8 and 10.7 mm. In yellowfin tuna, skipjack 

 tuna, and black skipjack (Eidhynnus spp.), 

 three or more spines are already present in 

 larvae less than 5.5 mm. total length (cf. Matsu- 

 moto, 1958, 1959). 



The preopercular surface begins to ossify 

 when the larvae are about 4.3 mm., soon after 

 the first preopercular spine has developed. Os- 

 sification starts at the base of the preopercular 

 spine and, as more spines are added, increases 

 correspondingly. When the larvae are 7.4 mm. 

 long, a small portion of the dorsal and ventral 

 terminals of the preopercle also becomes os- 

 sified. The surface of the preopercle is almost 

 completely ossified in the 23.7-mm. juvenile. 



In larvae of yellowfin tuna, skipjack tuna, 

 and black skipjack, between 5.0 and 6.0 mm. 

 total length, a conspicuous spine develops in 

 the posttemporal region of the head. This spine 

 is absent in wahoo ; instead it is represented by 

 a bony ridge which is present throughout the 

 larval and juvenile stages. This ridge is first 

 apparent in the 8.4-mm. larva. A second ridge 

 is seen dorsal to the first, and a groove appears 

 between the two ridges in the 23.7-mm. juvenile. 

 I presume that with continued growth of the 

 juvenile, these two ridges unite to form a canal 



that becomes part of the lateral line system. 



The opercle begins to ossify when the larvae 

 are about 13.2 mm. long. Ossification progresses 

 slowly, and the opercle is still not completely 

 developed at 23.7 mm. 



Branchiostegal Rays 



The first branchiostegal ray is seen in the 5.2- 

 mm. larva. The number of rays increases to 

 four in the 5.7-mm. larva and to six in larvae 

 6.6 to 6.8 mm. long. The full complement of 

 seven rays is generally ossified in larvae about 

 8.4 mm. (fig. 3B), although a 7.6-mm. larva 

 already had seven rays. The posteriormost ray 

 ossifies first, and subsequent ossification pro- 

 ceeds anteriorly. 



Other Head Bones 



The parasphenoid first begins to ossify in 

 larvae 2.8 mm. or smaller, and its entire length 

 is ossified in about 3.1 mm. 



It is difficult to determine the initial develop- 

 ment of the frontal and parietal bones because 

 these bones did not stain sufficiently in some 

 of the specimens. The first sign of ossification 

 appears in the 5.7-mm. larva. The parietal 

 bone develops faster than the frontal bone and 

 is completely ossified in larvae 10.7 mm. long. 

 The frontal bone, on the other hand, is com- 

 pletely ossified only in the 17.8-mm. larva and 

 the 23.7-mm. juvenile. 



In contrast to these three bones, ossification 

 of the supraoccipital crest, which develops only 

 after nearly all the fin rays and vertebrae have 

 completely o.ssified, starts in larvae about 13.2 

 mm. long. Ossification is completed only after 

 the individuals exceed 23.7 mm. 



Fins 



The sequence of fin formation in wahoo 

 lai'vae is similar to that of jack mackerel and 

 Pacific mackerel (Ahlstrom and Ball, 1954; 

 Kramer. 1960) ; the only difference is in the 

 length of body at which fin development com- 

 mences. In all fins, the actinotrichia first de- 

 velop in the larval fin fold. The.se are replaced 

 by definitive rays, lepidotrichia, which are 

 branched, jointed rays composed of bone. Fin 

 spines arise by alteration and fusion of the 

 joints of the lepidotrichia. The fins will be dis- 

 cussed in the order of their formation: (1) 



314 



U.S. FISH AND WILDLIFE SERVICE 



