to the ossification in Pacific mackerel ( Kramer, 

 1960). All the bones in the hypural complex 

 are completely ossified in the 17.8-mm. larva. 



The neural spines are initially based on the 

 anterior end of their centra. As each centrum 

 grows progressively longer, the base of the cor- 

 responding neural spine seems to shift poste- 

 riorward, until it is near the middle of the cen- 

 trum. This change in position of the base of 

 the neural spine is evident in the last six verte- 

 brae (excluding the terniinal vertebra) of the 

 23.7-mm. juvenile. In the adult the bases of 

 the 50th to 59th neural spines have shifted 

 further caudad, to the posterior edge of their 

 corresponding centrum. 



The haemal spines, which first appear near 

 the anterior part of the centra, generally re- 

 main in their original position throughout the 

 larval stage. In the juvenile stage the bases of 

 the last six or seven haemal spines have shifted 

 posteriorward to the middle of the centrum. 

 This condition is evident in the 23.7-mm. juve- 

 nile and is similar to that found in the adult, 

 in which the bases of the 53d to 58th haemal 

 spines have shifted further caudad to the pos- 

 terior edge of their corresponding centra. This 

 posterior shifting of the neural and haemal 

 spines and the simultaneous decrease in the 

 angle formed by the spines and longitudinal 

 axis of the vertebral column produce the nar- 

 row caudal peduncle so characteristic of the 

 scombrids. 



Unlike the Pacific mackerel, which develops 

 zygapophyses at about 8.7-mm. body length 

 (Kramer, 1960), the wahoo develops zygapo- 

 physes when the larvae are 17.8 to 23.7 mm. 

 long. No zygapophysis is ossified in the 17.8- 

 mm. larva, but in the 23.7-mm. juvenile, the 

 neural prezygapophyses of the 2d to 6lst verte- 

 brae and the neural postzygapophyses of the 

 1st to the 60th vertebrae are ossified. Some dif- 

 ferences exist between juvenile and adult neural 

 postzygapophyses of vertebrae near the caudal 

 end. Neural postzygapophyses are widely sep- 

 arated from the bases of the neural spines in 

 juveniles, but in adults those of the 54th to 62d 

 vertebrae have become partially or entirely 

 fused to the neural spines, as each succeeding 

 spine gradually inclines toward the horizontal 

 axis of the vertebral column. 



The haemal zygapophyses also ossify when 

 the larvae are between 17.8 and 23.7 mm. long. 

 No haemal zygapophysis is developed in the 

 17.8-mm. larva, but most of the adult comple- 

 ment are ossified in the 23.7-mm. juvenile. In 

 the latter specimen, the first haemal prezyga- 

 pophysis is seen on the 27th vertebra; the first 

 two or three appear as small projections near 

 the anterior edge of the centrum and anterior 

 to the base of the haemal arch. All succeeding 

 vertebrae possess haemal prezygapophyses. 

 These structures are presumed to develop an- 

 teriorly from the 27th vertebra as the j uveniles 

 increase in length, since in the adults the an- 

 teriormost haemal prezygapophysis is on the 

 19th to 21st vertebra. 



The haemal postzygapophyses follow a se- 

 quence of ossification slightly different from 

 that of the prezygapophyses. In the 23.7-mm. 

 juvenile, haemal postzygapophyses are ossified 

 on the 11th to the 60th vertebra. In the adults, 

 haemal postzygapophyses are present on about 

 the 6th to 57th or 58th vertebra. Consequent- 

 ly, I presume that, as the fish matures, ossifica- 

 tion of haemal postzygapophyses continues an- 

 teriorly, and the postzygapophyses of the last 

 five or six caudal vertebrae fuse with the hae- 

 mal spines. 



Clothier (1950) called the processes of the 

 haemal arches the haemapophyses and named 

 as parapophyses "the bony projections on each 

 side of the anterior ends of the centra in the 

 abdominal region to which the ribs are at- 

 tached." I shall use these two terms as defined 

 by Clothier. Perhaps some of the parapophyses 

 and haemapophyses are already ossified in the 

 10.7-mm. larva, but this cannot be determined 

 because the vertebral column of this specimen 

 did not stain adequately. The anteriormost 

 parapophysis on the side of the centrum is de- 

 veloped on the 13th vertebra in the 13.2-mm. 

 larva, and the anteriormost haemapophysis is 

 developed on the 18th. In the 17.8-mm. larva 

 and the 23.7-mm. juvenile, the anteriormost 

 parapophysis is seen clearly on the third verte- 

 bra in the dorsal third of the centrum, which 

 is the adult condition. The parapophyses on 

 succeeding vertebrae are situated progressively 

 lower on the side of the centrum, and the first 

 pair of haemapophyses at the ventral surface 



MORPHOLOGY AND DISTRIBUTION OF LARVAL WAHOO 



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