Busby and Ambrose. Development of larval and juvenile Odontopyxis trispinosa and Xemeretmus latifrons 



409 



tively large unpigmented area anterior of this patch is 

 an important diagnostic feature (Fig. 7, A and B). 



Both the dorsal and ventral finfold melanophore 

 patches are constricted nearly to the body margins at 

 a point about 80%-90% of the distance between the 

 anus and notochord tip (Fig. 7B). As notochord flexion 

 progresses, the ventral patch of pigmentation poste- 

 rior to the constriction on the anal finfold is seen as 

 the post-hypural pigment present on the caudal fin of 

 postflexion specimens (8.7-22.0 mmKFig. 7C). This cau- 

 dal-fin pigmentation extends posteriorly to about 15- 

 20% of the caudal-fin length (Fig. 7, C and D). 



The remaining melanophores on the dorsal and anal 

 finfolds migrate toward the body margins as the finfold 

 recedes. Discrete melanophore patches remain on the 

 anterior and posterior margins of the second dorsal 

 and anal fins in postflexion larvae (Fig. 7D). 



Osteology Although precursors of some bony struc- 

 tures such as dermal plates and fin rays are discernable 

 as early as 8.0 mm, ossification in X. latifrons does not 

 begin until approximately 13.0 mm. The sequence of 

 ossification of bony structures in X. latifrons is nearly 

 identical to that described previously for O. trispinosa . 

 The most important difference to note is that ossifica- 

 tion in X. latifrons begins later (13.0 mm), progresses 

 more rapidly for lateral body plates, and is slower for 

 most skeletal elements than O. trispinosa. The intent 

 here is to highlight important differences between the 

 two taxa. 



Cranium All parts of the cranium, with the excep- 

 tion of the sphenotic, prootic, epiotic, tabular, and 

 pterosphenoid are ossified at 13.8 mm. These remain- 

 ing cranial elements ossify at about 30.5 mm. 



Mandibular region Ossification of all mandibular 

 structures is complete at 13.8 mm. 



Spines Head spination is generally reduced in X. 

 latifrons larvae. Xeneretmus latifrons larvae have no 

 inferior infraorbital, postocular, or posttemporal plates 

 and have only four superior infraorbital spines. 

 The rostral spine, which is weaker than that of O. 

 trispinosa, and sclerotic plates do not develop until 

 the late postflexion stage (about 25-30 mm). The tym- 

 panic and frontal spines are more pronounced in X. 

 latifrons larvae than in O. trispinosa. 



Palatine region The palatine, quadrate, meta- 

 pterygoid, mesopterygoid, ectopterygoid, and symplectic 

 are ossified at 13.8 mm. 



Opercular region The preopercle, opercle, sub- 

 opercle, and interopercle are ossified by 13.8 mm. 



Hyoid region The basihyal, hypohyal, urohyal, 

 ceratohyal, epihyal, interhyal, glossohyal, and bran- 

 chiostegal rays are ossified by 13.8 mm. The hyo- 

 mandibula is ossified at 30.5 mm. 



Branchial region The pharyngobranchial teeth, 

 pharyngobranchials (n=A, fused as in O. trispinosa), 



ceratobranchials (n=5), and epibranchials (rc=4) ossify 

 at about 21.0 mm. The remainder of the branchial ap- 

 paratus including the basibranchials (n=3) and 

 hypobranchials (n=3), are ossified by 30.5 mm. 



Appendicular region The cleithrum, postcleithrum, 

 supracleithrum, and coracoid are ossified by 13.8 mm. 

 Pelvic-fin spines and rays and all pectoral-fin rays are 

 complete at 13.8mm (Table 7). The basipterygium and 

 posttemporal ossify by 21.0 mm. The scapula and three 

 radials supporting the pectoral fin are ossified by 

 30.5 mm. 



Median fins All dorsal-, anal-, and caudal-fin spines 

 and soft rays are ossified by 13.8mm (Table 7). The 

 caudal fin has 6 superior principal, 2 superior pro- 

 current, 6 inferior principal, and 1 inferior procurrent 

 rays (2+6+6+1=15 total). Ossification of the hypural 

 plate and the pterygiophores supporting the dorsal- 

 and anal-fin rays was not complete in the largest speci- 

 men examined (39.2 mm). 



Vertebral column Notochord flexion begins at ap- 

 proximately 8.5 mm and is completed by 11.0 mm. All 

 vertebral centra and the urostyle are ossified by 

 13.8 mm. All except the two posteriormost neural and 

 three haemal spines are also ossified at 13.8 mm. Ossi- 

 fication of all neural and haemal spines is complete by 

 14.5 mm (Table 7). 



Dermal plates All dermal plates are ossified by 

 13.8 mm. Sequence and direction of formation and os- 

 sification is the same as previously described for 

 O. trispinosa. Xeneretmus latifrons, however, has higher 

 DLP+MDP, ILP, and VLP+MVP lateral body plate se- 

 ries counts than O. trispinosa (Tables 4 and 7). A maxi- 

 mum of five pectoral-fin plates was counted in 

 X. latifrons. 



Discussion 



Summary comparison of O. trispinosa * M/ 1 

 X. latifrons larvae 



Larvae of O. trispinosa and X. latifrons can be distin- 

 guished by pigmentation, morphological, and meristic 

 characters. 



Larvae of O. trispinosa possess a semicircular patch 

 of melanophores that nearly covers the entire caudal 

 finfold. This character is diagnostic and present 

 throughout development. The caudal finfold of pre- 

 flexion X. latifrons lacks pigmentation except for a 

 patch located near the ventral margin of the noto- 

 chord tip. This patch becomes elongate as notochord 

 flexion progresses and becomes a band at the hypural 

 margin. This band extends onto the caudal fin and 

 may cover as much as 20% of its anterior surface. 



Preflexion larvae of O. trispinosa possess a small 

 patch of melanophores on the anal finfold immediately 

 posterior to the origin, X. latifrons larvae have a large 



