FISHERY BULLETIN: VOL. 72, NO. 4 



The elements of the opercular complex — pri- 

 marily the opercle and preopercle — began to dif- 

 ferentiate at about 15 mm. Shortly thereafter 

 the subopercle and interopercle began to form 

 and by 21 mm most of the opercular series 

 appeared to be about halfway ossified; the inter- 

 opercle seemed to be the most weakly ossified. 

 By 25 to 26 mm, the interopercle had grown to 

 articulate with the quadrate and angular and 

 the hyals. The other opercular bones had be- 

 come quite well developed, but the postero- 

 ventral edges remained unossified, evidently to 

 permit continuous growth. 



Branchial Development 



The ceratobranchials were the first elements to 

 develop in the branchial system, becoming evi- 

 dent at about 10 mm. By about 12 to 13 mm, 

 the ceratobranchials showed a slight uptake of 

 stain, indicating that they had begun to ossify. 

 At 14 mm, the hypobranchials had begun to 

 differentiate, and a few unossified rakers were 

 visible on the ceratobranchials. 



The ceratobranchials, the first, second, and 

 third epibranchials, and the hypobranchials all 

 developed as more-or-less straight rods (although 

 the epibranchials also developed small dorsal 

 extensions for attachment of suspensory liga- 

 ments). The first rakers appeared on the cerato- 

 branchials at about 12 mm (Table 1) and held 

 almost constant between 10 to 13 rakers to 

 about 17 mm. Two or three rakers appeared 

 on the epibranchials and hypobranchials when 

 larvae reached about 15 mm. The numbers of 

 rakers on all three elements gradually increased 

 with the numbers on the epibranchials and hypo- 

 branchials almost equal but from two to four 

 lower than on the ceratobranchials, up to about 

 30 mm (Table 1). At the 30-mm size, all 

 three bones had an equal number of rakers, but 

 beyond that size the ceratobranchials seemed to 

 lag behind the epibranchials and hj^Dobranchials 

 by five or six rakers. In adults, the hypobranchials 

 and ceratobranchials are almost equal in numbers 

 of rakers (30 to 45), producing lower branch 

 counts of 60 to 90, but the epibranchial rakers 

 almost equal the combined lower branch counts. 



The fourth epibranchial (E4) was the first to 

 develop a slight vertical extension when speci- 

 mens measured about 14 mm; the shape of the 

 fourth epibranchial at this stage is more-or-less 



like an inverted "T" — the crossbar (the basal 

 shaft) is ventral and the vertical segment extends 

 dorsally from it. On the dorsal and posterior 

 edges a developing cartilaginous capsule (see 

 Miller, 1969; Figure 2, for the adult configura- 

 tion of the capsule) is fused to the developing 

 E4 bone; this capsule is also joined to the epi- 

 ceratobranchial "elbow." A vertical slit is present 

 in the posterior side of the capsule, and the two 

 raker series that will eventually grow through- 

 out the length of the lumen of the epibranchial 

 organ begin growing along the sides of the 

 posterior slit. Dual fourth epibranchial raker 

 series are present, with the lateral series grow- 

 ing along the ventrolateral edge of the fourth 

 epibranchial bone, outside the epibranchial 

 organ, and the medial fourth epibranchial series 

 is enclosed in the organ along the anterolateral 

 edge of the slit in the cartilaginous capsule. 

 The single fifth epibranchial series grows along 

 the posteromedial edge of the slit. By 15.5 mm, 

 the vertical extension on E4 had grown much 

 heavier and the characteristic posterior foramen 

 had formed (see Miller, 1969; Figures 2 and 4). 

 Three raker tubercles were present along the 

 edges of the posterior slit (which had just begun 

 to form). By 17 mm, the cartilaginous capsule 

 had begun to increase in size, and by 19 mm 

 there were four or five raker tubercles in the 

 posterior slit. Almost no ossification had yet 

 appeared on the fourth epibranchial, and the pos- 

 terior foramen was quite large. Ossification 

 began at about 20 mm, and by about 25 mm 

 the posterior end of the basal shaft had expanded 

 vertically (it resembled an axe blade), and the 

 vertical shaft had also broadened and ossified 

 quite heavily. At this stage there were about 

 eight rakers along the edges of the posterior 

 slit. When larvae measured 26 mm, the lumen 

 of the organ had begun to form; it extended 

 anterodorsally within the cartilaginous capsule 

 from the bridge of the posterior slit, and 10 

 rakers appeared along each edge. One or two 

 rakers were present in the developing lumen. 

 By 30 mm, the lumen had elongated to about 

 one-third of a full loop [the epibranchial organ 

 in Opisthonema is the continuous-tube type 

 (see Bertmar, Kapoor, and Miller, 1969; Miller, 

 1969) in which the lumen and included rakers 

 extend for a full loop in adults] and there were 

 about 15 to 20 rakers in the lumen. By 35 mm, 

 the lumen was about a half of a full loop and 



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