Matsui: Description of young platytroctids from northeastern Pacific Ocean 



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the transitional specimens (Table 1). Two pelvic rays 

 are discernible in the most advanced transitional speci- 

 men (SI065-439). Adult counts of dorsal (17-20), anal 

 (14-16), and pelvic (9) fin rays are usually present in 

 30-mm juveniles. Pectoral fin rays form last, with the 

 first rays appearing at approximately 23 mm SL and 

 adult counts (16-20) by 45 mm SL. 



Branchial region Branchiostegal rays appear early 

 and only the larval specimen and the least developed 

 transitional specimen had fewer than the adult count 

 of 8. 



Gill rakers are absent in the larva, but as many as 

 12 are on one side of the 1st gill arch in the transitional 

 specimen. Nearly all of these gill rakers are on the 

 lower arch. Only one transitional specimen (SI065-439) 

 had gill rakers on the 1st epibranchial. The lowest count 

 for juveniles was 1 on the epibranchial and 12 total on 

 the 1st arch, and ranged from 1 to 5 on the upper arch 

 and 12 to 19 total in juveniles <20mm SL. The smallest 

 individual with the adult gill raker count of 25 was 

 27 mm SL and all individuals 42 mm and larger had 

 counts in the adult range of 25-30. 



Medial gill rakers of the 3d and 4th arches are in an 

 uninterrupted row in juveniles as small as 19 mm, but 

 no medial gill rakers form on the 1st and 2d arches until 

 after 20 mm. By 45mm SL, there are about 6 medial 

 rakers on the epibranchial and on the ceratobranchial 

 of the 2d arch, and 5 on these elements of the 1st arch. 



There are 2 pseudobranchiae in the larva and tran- 

 sitional specimens. The smallest individual with a 3d 

 pseudobranchium measured 17 mm and the smallest 

 with a 4th was 20 mm. No specimen <50mm SL had 

 attained the highest adult count of 8. Counts varied as 

 much as 4 between individuals of similar lengths. 



Dentition In the study material, only the larva is 

 toothless. Teeth on dentary, premaxilla, maxilla, 

 vomer, palatines, basihyal, and on the lateral face of 

 the dentary (mid-dentary teeth) appear during tran- 

 sition. Teeth are easily dislodged, contributing sub- 

 stantially to individual variation in counts. The most 

 advanced transitional specimen (SI065-439; Table 1) 

 had a single medial tooth on the basihyal, a tooth on 

 each palatine, and a total of 7 premaxillary, 2 maxil- 

 lary, 24 dentary, and 2 vomerine teeth. Only one tran- 

 sitional specimen had mid-dentary teeth. There are 

 fewer maxillary than dentary teeth in the early stages, 

 but this gradually changes and the numbers are about 

 even in individuals 25 mm and larger (Table 2). Highest 

 count of premaxillary teeth among transitional indi- 

 viduals was 4 on a side. The numbers increased to 4-6 

 in 13-20 mm juveniles, with counts as high as 10 at 

 50mm SL. The inner pair of premaxillary teeth point 

 horizontally beeinninff from about 25mm SL. Fre- 



quently, a second smaller tooth forms adjacent to 

 the 1st. 



Mid-dentary teeth are probably more susceptible to 

 damage than other dentition and most individuals 

 smaller than 30 mm were without them, although 

 counts of 4 on one side occurred as early as 17 mm. In- 

 dividuals 30-50 mm long usually had 4-5 mid-dentary 

 teeth. In the youngest stages, a single tooth was usually 

 present on each palatine. The number variably in- 

 creased to as many as 4 in individuals over 30 mm. Most 

 prejuveniles and early juveniles had a medial tooth on 

 the basihyal, with occasional individuals with 2-4 in a 

 medial row. Basibranchial and mesopterygoid teeth 

 appeared after 40 mm, and ectopterygoid teeth were 

 only found in the adults. 



Photophores Photophores of young platytroctids are 

 oriented horizontally (Matsui and Rosenblatt 1971). The 

 posterior gular organ (G0 2 ; Fig. 1) is the only photo- 

 phore present during most of the yolksac stage. It is 

 an opaque spot outlined by dark pigment in the larva, 

 and forms at the posterior, narrow end of a black, 

 conically-shaped pouch. Near the end of the yolksac 

 stage, the intraventral organ (IVO) develops inside a 

 silver-lined, anteriorly facing pouch. The photophore 

 at the subopercle (SBO) appears later in juveniles. 

 However, it is considered rudimentary as it is sur- 

 rounded by opaque tissue and is without an anterior 

 opening. These photophores are covered over and lost 

 in larger individuals. 



Additional photophores begin appearing in some 

 juveniles of 26 mm but are uncommon until after 

 28mm. Unlike earlier photophores, they face ventral- 

 ly and persist in adults. Earliest to appear are (1) a 

 transversely barred thoracic organ (THO) located on 

 the ventral body margin midway between the pectoral 

 and pelvic fins, (2) two elliptical supraventral organs 

 (SVO) located anterolateral to the bases of the ventral 

 fins, and (3) two elliptical supraanal organs (SAO), 

 lateral to the anal opening. Other adult photophores 

 appear soon after and include a transversely barred 

 midventral organ (MVO) located anteroventral to the 

 SVO, the elliptical branchiostegal organs (BRO), the 

 infracaudal organ (ICO) located on the ventral margin 

 of the caudal peduncle, the pectoral organ (PO) located 

 on the ventralmost ray of pectoral fins, and a longi- 

 tudinally barred jugular organ (JO) located between the 

 bases of the pectoral fins. All adult photophores are 

 usually present by 50 mm. 



Sagamichthys abei (Fig. 2) 



Pigmentation The single larval specimen is nearly 

 unpigmented. Most heavily pigmented areas are the 

 eye, shoulder organ, subopercular photophore (SBO), 



