FISHERY BULLETIN: VOL. 77. NO 4 



leading to the adult condition. The rays of the 

 dorsal fin first develop during Stage 3; the rays of 

 the pelvic fin first appear in Stage 4. 



A series of S. pseudodiaphana taken from the 

 southeastern Pacific (lat. 33°-39° S, long. 80°-120° 

 W) show a similarity in morphology and pattern of 

 development to North Atlantic forms. The data 

 indicate that the sequences of both appearance 

 and completion of the various photophore groups 

 are similar, although the relative timing of com- 

 pletion for certain groups may differ slightly. For 

 example, while the completion of PV in North 

 Atlantic forms apparently occurs prior to the in- 

 itiation of SC, in southeastern Pacific forms it oc- 

 curs afterwards (Table 1). Pigmentation in speci- 

 mens from these two populations is essentially 

 alike, but a small pigment spot located near the 

 posterior end of the dentary in Pacific forms was 

 not noted in the Atlantic material. As in the 

 adults, differences between postlarvae from the 

 two areas, then, does occur. 



Slight differences in larval characteristics be- 

 tween populations of the same species have been 

 shown for certain lanternfishes (Pertseva- 

 Ostroumova 1974) and do occur in the genus Ster- 

 noptyx, differences which we suspect, based on 

 Argyropelecus, may be more extensive than indi- 

 cated here. They can render species differentiation 

 difficult in certain areas. Early-metamorphic in- 

 dividuals of S. diaphana and S. pseudobscura are 

 superficially similar and in tropical Atlantic col- 

 lections only late-metamorphic stages can be 

 separated with certainty. Metamorphic individu- 

 als of S. pseudodiaphana, on the other hand, are 

 highly distinctive. A series of S. diaphana taken 

 off Bermuda, an area where S. pseudobscura is 

 apparently rare, allowed for some comparison be- 

 tween the midmetamorphic forms of this species 

 and S. pseudodiaphana . 



The caudal spot so conspicuous in the young of 

 S. pseudodiaphana at Stage 1 (Table 1) is found 

 neither in S. diaphana nor S. pseudobscura prior 

 to completion of photophore development. Pig- 

 mentation of the pectoral fin rays has been found 

 in S. diaphana, although not consistently, up to 

 Stage 3. At any given stage, S. diaphana appears 

 to be in a more advanced state both morphologi- 

 cally and in terms of pigmentation. Thus the 

 configuration of the anal fin pterygiophores at- 

 tains the juvenile appearance during Stage 3, ap- 

 pearing in Stage 4 in S. pseudodiaphana; the pel- 

 vic fins differentiate earlier (Stage 3 versus 4), as 

 does the pigmentation of S. diaphana in general 



(Figure 9). Even so, the pigmentation of S. 

 pseudodiaphana tends to be denser in the more 

 advanced specimens, which are conspicuous by the 

 dark color of the dorsum. Elbert H. Ahlstrom'* 

 recognizes three forms of postlarval Sternoptyx 

 spp. in his North Pacific collections, none of which 

 bear a caudal melanophore. As populations of S. 

 pseudodiaphana are unknown north of the 

 Equator in the Pacific, then, tentatively, postlar- 

 val S. obscura also lack caudal pigment. Sternop- 

 tyx pseudodiaphana may, therefore, be distin- 

 guished from congeners by this character. 



General Comments 



During metamorphosis postlarval Sternoptyx 

 (ca. 6-14 mm) undergo extensive change from an 

 elongate premetamorphic form to a deep-bodied 

 juvenile. In earlier stages, metamorphic individu- 

 als are somewhat shorter than premetamorphic 

 forms, a pattern of apparent loss in length also 

 observed in the related hatchetfish genus Ar- 

 gyropelecus (e.g., Brauer 1906; Jespersen 1915; 

 and others). While the sequential pattern of 

 photophore addition appears identical among the 

 species examined, timetables for the differentia- 

 tion of other external characters do not necessarily 

 coincide. As indicated, S. diaphana appears in a 

 more advanced state of morphological differentia- 

 tion and development than S. pseudodiaphana at 

 comparable photophore stages. A similar pattern 

 has been observed by Baird (unpubl. data) among 

 species of Argyropelecus. Geographic variation 

 both among and within species is apparent. It ap- 

 pears that there can be appreciable flexibility 

 among species in the timing of photophore addi- 

 tion in relation to the development of other mor- 

 phological characters, though the adaptive sig- 

 nificance of these observations is presently 

 unclear. Growth rate, the functional significance 

 of photophore presence at a given size, and broader 

 ecological considerations such as predation or re- I 

 source availability, are likely complexly related to 

 patterns of photophore development. 



GEOGRAPHIC AND BATHYMETRIC 



DISTRIBUTION OF 



STERNOPTYX SPECIES 



The genus is widespread, occurring in all oceans 



*E. H. Ahlstrom, Southwest Fisheries Center La JoUa 

 Laboratory, National Marine Fisheries Service, NOAA, La 

 Jolla, CA 92038, pers. commun. November 1975. 



814 



