REMARKS ON SYSTEMATICS, DEVELOPMENT, AND DISTRIBUTION OF 

 THE HATCHETFISH GENUS STERNOPTYX (PISCES, STOMIATOIDEI) 



Julian Badcock' and Ronald C. Baird^ 



ABSTRACT 



Sternoptyx pseudodiaphana Borodulina is reported from the eastern North Atlantic in sympatry with 

 S. diaphana, providing conclusive evidence that the former represents a species distinct from S. 

 diaphana. Patterns of geographic variation among various characters are apparent in species of 

 Sternoptyx as is allometric growth. These patterns render species identification difficult in certain 

 allopatric populations, particularly those from the Atlantic and Pacific Oceans. Each species has 

 distinct patterns of horizontal and vertical distribution and where species occur in sympatry, their 

 centers of abundance do not coincide. Members of the genus Sternoptyx inhabit the "lower mesopelagic 

 depth zone" (sensu Baird) from 500 to 1,500 m. Geographic variation in depth of maximum abun- 

 dance for various species can be demonstrated. These appear correlated with variations in tempera- 

 ture and light although competitive mteractions may also contribute to observed depth ranges. 

 Photophore development is similar in the three species described and postlarval individuals of S. 

 diaphana and S. pseudodiaphana are readily distinguishable. Characters useful in distinguishing 

 the various species are presented in relation to patterns of geographic variation. 



A single ancestral species which gave rise to the four presently recognized species, each exhibiting 

 slight morphological divergence, is advanced as a parsimonious mitial hypothesis of evolutionary 

 relationship. 



The genus Sternoptyx has, until recently, been 

 thought to contain but a single polymorphic 

 species (Schultz 1961, 1964). However, Baird 

 (1971), and more recently Haruta and Kawaguchi 

 (1976), have demonstrated the validity of three 

 morphologically similar species, S. diaphana 

 Hermann, S. obscura Garman, and S. pseudob- 

 scura Baird, each with broad but distinct geo- 

 graphic ranges. Baird (1971) also noted a mor- 

 phologically distinct population of S. diaphana 

 from the subtropical convergence region of the 

 South Pacific. In view of the degree of character 

 similarity and lack of sympatry with other popula- 

 tions of S. diaphana, he considered his data in- 

 sufficient to substantiate the Southern Ocean 

 form as a distinct species. Borodulina (1977) sub- 

 sequently described the Southern Ocean form as 

 S. pseudodiaphana £md has recently published a 

 synopsis of the hatchetfish genera Argyropelecus 

 and Sternoptyx based on Russian collections 

 (Borodulina 1978). 



Sternoptyx pseudodiaphana from the eastern 

 tropical Atlantic occurs in sympatry with S. 



'Institute of Oceanographic Sciences. Wormley, Godalming, 

 Surrey, U.K. Authorship alphabetical. 



^Department of Marine Science, University of South Florida, 

 St. Petersburg, Fla. 



diaphana. Our new data provide conclusive evi- 

 dence that S. pseudodiaphana represents a species 

 distinct from S. diaphana. Patterns of geographic 

 variation are not well known in deep-sea fishes 

 and patterns occur in the genus Sternoptyx which 

 tend to obscure species distinctions among certain 

 allopatric populations. Characters found useful in 

 distinguishing among various species and popula- 

 tions are presented which complement and expand 

 the treatments of Baird (1971) and Borodulina 

 (1978). We describe metamorphic and postlarval 

 development for various species and include com- 

 parisons among species. Additional data on the 

 geographic and vertical distribution of the genus, 

 including information from discrete-depth trawl- 

 ing studies, are presented which add considerably 

 to our knowledge of the distribution of this wide- 

 spread group of mesopelagic fishes. 



METHODS 



All four species of Sternoptyx were examined. 

 The material and its sources are listed in Appen- 

 dix Table 1. The specimens were fixed in Forma- 

 lin^ and preserved either in alcohol (70% ethyl or 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Manuscript accepted June 1979. 



nSHERY BULLETIN VOL 77. NO 4. 1980 



803 



