FISHERY BULLETIN: VOL. 78, NO. 2 



' In the axial skeleton the first centrum to ossify 

 (as early as 11 mm) supports the first haemal 

 spine. Ossification proceeds both anteriorly and 

 posteriorly, with the urostyle ossifying before the 

 antepenultimate and penultimate vertebrae. All 

 centra are ossified by 17 mm. 



The first caudal supports to ossify are hypurals^'* 

 2 and 3 in larvae as small as 12 mm. Hypural 1 is 

 ossified by 13 mm and hypural 4 at about 15 mm. 

 The epurals ossify last at about 16 mm. We inter- 

 pret the caudal complex of/, isolepis to consist of 

 four hypurals (two below and two above the medial 

 axis) and two epurals. One is a normal-sized 

 epural that supports the uppermost caudal ray, 

 and the other epurallike bone is reduced in size 

 and supports no rays (Figure 6H). No uroneurals 

 are present. 



Fin rays begin to ossify during notochord flexion 

 (10-14 mm) in the dorsal, anal, caudal, and pelvic 

 fins. Ossification is completed in the following or- 

 der: caudal, dorsal, anal, pelvic, and pectoral fins 

 (Table 4; Figure 6). Pectoral fin rays were not fully 

 ossified in the largest stained larva, 22.8 mm. 



The anlage of the base of the caudal fin begins to 

 form by 5.5 mm ( Figure 7 A). Incipient caudal rays 

 are evident from 5.5 to 8.8 mm. By 10.5 mm (Fig- 

 ure 7 B) the notochord begins to flex and hypurals 

 1-3 develop, supporting about eight incipient rays. 

 At about 12.7 mm the notochord is usually fully 

 flexed and three hypurals are evident, supporting 

 10 differentiated but unossified caudal rays ( Figure 

 7C). By 14.3 mm (Figure 7D), hypurals 1-3 are 

 ossified and some caudal rays have started ossify- 

 ing, beginning at the center of the fin and proceed- 

 ing dorsally and ventrally . The full complement of 

 18 rays is consistently ossified by 16.4 mm (Figure 

 7E). By 18.0 mm (Figure 7F) all four hypurals and 

 both epurals are ossified. By 22.8 mm (Figure 7G) 

 the caudal fin essentially resembles that of a 

 juvenile ( Figure 7H) and nearly all elements of the 

 caudal complex are ossified. The 18 rays, consist- 

 ing of (from ventral to dorsal) 3 unbranched rays, 

 12 branched rays, and 3 unbranched rays, are car- 

 ried on the hypurals as follows: epural 1, 1 ray; 

 hypural 4, 2 rays; hypural 3, 6 rays; hypural 2, 5 

 rays; hypural 1, 4 rays. 



Incipient dorsal fin rays may be observed in the 

 proximal portion of the finfold at midbody by 10.5 

 mm. Ossification of dorsal rays begins at midbody 



'"We follow Moser and Ahlstrom's (1970) definition of hypur- 

 als ". . . all bones of hypaxial origin associated with ural centra 

 [are defined] as hypurals, including the lowermost bone." 



by 11 mm and proceeds anteriorly and posteriorly. 

 By 17.5 mm the dorsal rays reach their full com- 

 plement and are completely ossified. 



The anal fin develops in a manner analagous to 

 the dorsal fin and nearly simultaneously. Rays 

 begin to differentiate at midbody with formation 

 progressing both anteriorly and posteriorly. Os- 

 sified rays begin at midbody at about 11 mm and 

 the full complement may be ossified by 17.5 mm. 



Pelvic fin buds may be observed on larvae as 

 small as 10 mm, although not consistently until 

 notochord flexion is complete at about 14 mm, and 

 individual rays may begin ossifying by 11 mm. 

 The full complement of six rays is ossified by 19 

 mm. 



Pectoral fin buds are visible above the yolk sac 

 in newly hatched reared larvae. Larval pectoral 

 fins are present in the smallest stained larvae 

 examined (3.2 mm). The rays begin to differen- 

 tiate by 13.5 mm and individual rays begin ossify- 

 ing as transformation occurs by about 17 mm. The 

 full complement of pectoral rays was not attained 

 in the largest larva examined, 22.8 mm, but is 

 fully developed in a 44 mm juvenile. 



Branchiostegal rays begin to accept alizarin 

 stain at about 9.5 mm. The adult complement of 

 seven rays may be differentiated, but not ossified, 

 by 13.6 mm. All rays are ossified by 16 mm. 



Gill rakers on the first ceratobranchial begin 

 forming at about 7 mm. A maximum of six rakers 

 was formed in the largest stained larva exammed 

 (22.8 mm). No rakers were formed on the epi- 

 branchials of this specimen. The adult comple- 

 ment of four plus seven gill rakers is present on a 

 44 mm juvenile. 



Of the median fin supports, pterygiophores sup- 

 porting anal and dorsal fin rays begin ossifying at 

 about 19 mm. These pterygiophores are com- 

 pletely ossified by 22.8 mm (Figure 8). 



Scales form sometime between 22.8 mm (largest 

 stained larva) and 44 mm (smallest stained 

 juvenile). 



OCCURRENCE 



Off Oregon, larvae of/, isolepis are distributed 

 mainly in the near coastal zone within 18 km of 

 shore, with abundance peaks at 6-9 km 

 (Richardson 1973, see footnote 4; Richardson and 

 Pearcy 1977). Smaller numbers of larvae have 

 been taken as far as 56 km offshore (Richardson 

 and Pearcy 1977; Laroche and Richardson 1979), 

 inside the mouth of Yaquina Bay (Pearcy and 



412 



