BADCOCK and BAIRD: SYSTEMATICS OF STERNOPTYX 



Figure 5. — Sternoptyx pseudobscura . 54 mm SL. 



(Table 3). Sternoptyx obscura is further distin- 

 guished from S. pseudobscura by its lower place- 

 ment of the photophore SAN and by uniformly 

 dark pigment of body and trunk, as well as the 

 presence of a dark corona along the caudal fin rays, 

 radiating from the fin base. 



Geographic Variation " 



The degree of genetic differentiation and nature 

 of geographic variation in populations of midwa- 

 ter fishes have not been thoroughly explored, 

 though evidence is now accumulating that such 

 variation does exist and may be widespread in 

 species with broad geographic ranges (e.g., Naf- 

 paktitis 1968; Baird 1971; Pertseva-Ostroumova 

 1974; Karnella and Gibbs 1977). Baird (1971) was 

 able to distinguish separate populations in several 

 species of the related hatchetfish genus Ar- 

 gyropelecus . Populations tended to remain distinct 



over time and differences among populations were 

 generally associated with zoogeographic bound- 

 aries. The present evidence indicates that similar 

 patterns of geographic variation occur in species of 

 Sternoptyx, the extent of which awaits more ex- 

 tensive investigation. 



Geographic variation is apparent in both S. 

 pseudobscura and S. pseudodiaphana. The sys- 

 tematic problems arising from such variation are 

 illustrated in Figures 6 and 7. In addition to the 

 indicated allometry, the suitability of the two 

 character complexes (trunk depth/SAN photo- 

 phore height and SAN photophore depth) for dis- 

 tinguishing species differs, depending on the 

 populations being compared. Both characters are 

 distinctive among the three species illustrated (S. 

 diaphana, S. pseudodiaphana, and S. pseudob- 

 scura) for sympatric populations in the North 

 Atlantic. However, where southeast Pacific popu- 

 lations of S. pseudodiaphana are compared with 



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