boundary of the SEC and may be related to the 

 upwelling and northward transport of surface 

 waters near the equator. In Figure 1, the latter are 

 indicated by the shallow 27°C isotherm between 

 about lat. 1°S and 0°30'N-somewhat south of the 

 indicated faunal change. However, King and Hida 

 (1957) and Murphy and Shomura (1972) point out 

 that, while strength of upwelling and extent of 

 northward surface transport vary irregularly over 

 short periods depending on strength and direction 

 of winds, the area between about lat. 1°S and 

 about lat. 4°N can be regarded as a zone of high 

 primary production and high zooplankton stand- 

 ing crop over most of the year. 



The central equatorial Pacific is thus somewhat 

 unique in that it is the only major offshore and 

 truly oceanic area in the world where upwelling 

 and high primary production are not strongly 

 seasonal. Other areas of upwelling are either rela- 

 tively close to land or, e.g., the Antarctic, are 

 light-limited over part of the year. Thus it is not 

 surprising that the fauna of the zone is different 

 from both that of the oceanic, but relative sterile 

 central water mass and that of the highly produc- 

 tive, but less oceanic eastern equatorial Pacific. 

 Nor is it surprising that five of the species which 

 occur in the zone are either endemic or restricted 

 to it at least in the eastern part of the Pacific. 



The faunal change observed in the data here 

 could in part be a consequence of the change in 

 primary production and availability of food. 

 Ebeling (1962) has suggested that distributions of 

 even deep-living, nonmigrating fishes are as- 

 sociated with differences in surface primary 

 production, and Backus et al. (1969) have suggest- 

 ed that faunal changes associated with thermal 

 fronts are more directly related to differences in 

 primary production which result from differences 

 in thermal structure on either side of the front. It 

 is not unreasonable to suggest that the five species 

 which were found only near the equator are unable 

 to survive in the less productive waters north of 

 about lat. 4°N. 



This is, of course, insufficient to explain other 

 features of the faunal change. It is not clear why 

 some species, many which occur also in sterile 

 central waters, not only occur in the equatorial 

 zone of high production but are markedly more 

 abundant there, while other species, which occur in 

 the highly productive eastern equatorial Pacific, 

 are apparently excluded from the offshore zone 

 near the equator. More knowledge, e.g., the 

 reproductive potential and food requirements, of 



the species concerned and of environmental 

 characteristics of the areas, e.g., types and rates of 

 predation, is needed before even speculation is 

 warranted. 



There are several examples where dominant 

 species are "replaced" by congeners or 

 morphologically similar species. Lampanyctus 

 omostigma and L. hubbsi are very similar con- 

 geners which both occur in the equatorial water 

 mass but with separate distributions. Diaphus 

 schmidti, a central water-mass species, is replaced 

 by D. garmani and D. problematicus in the 

 northern section of the equatorial water mass, and 

 these in turn are replaced by D. malayanus, D. 

 signatus, and D. regani in the zone near the equa- 

 tor. All these Diaphus species are similar 

 morphologically. Diaphus schmidti, D. garmani, 

 and D. malayanus are of similar size at maturity 

 while the others are somewhat larger. In addition 

 to differences in placement of body photophores 

 and development of sexually dimorphic head or- 

 gans, these species are all distinguished by slightly 

 different gill raker counts, suggesting differences 

 in feeding habits. The Diaphus mollis-like forms 

 also show a replacement series of sorts. The cen- 

 tral water-mass species {Diaphus sp. A of Clarke 

 1973) is very similar in size and morphology to the 

 Diaphus sp. taken near the equator; however, no 

 similar form was present in the northern section 

 of the equatorial water mass. 



It is not implausible to suggest that Benthosema 

 suborbitale and the Diogenichthys species form a 

 replacement series. Benthosema suborbitale is a 

 diminutive species of Benthosema and very similar 

 to the Diogenichthys; the two genera are closely 

 related (Moser and Ahlstrom 1970; Paxton 1972). It 

 is apparently replaced by D. laternatus in the 

 northern section of the equatorial water mass. 

 Diogenichthys atlanticus occurs in all three zones, 

 but is an abundant and dominant species only near 

 the equator-where neither B. suborbitale nor D. 

 laternatus occur. 



Clarke (1973) has shown that, at a single loca- 

 tion, closely related species have different night- 

 time depth ranges. The data here suggest that 

 within a given depth range, closely related species 

 are frequently separated geographically. Inves- 

 tigations of the biology of such closely related, but 

 separated species, particularly near their 

 geographic boundaries, would be a promising 

 approach toward understanding the factors de- 

 termining zoogeographic distribution in the open 

 ocean. 



639 



