FISHERY BULLETIN: VOL. 72. NO. I 



(Judkins, 1972). Among the Atlantic Foramin- 

 ifera listed by Be and Tolderlund (1971) as 

 tropical species only Candeina )iitida shows a 

 geographical distribution similar to that of P. 

 platijchcla. 



In general species characterizing the eastern 

 tropical Pacific, unlike sobriiia, tend to follow 

 the coastline of the Americas from about lat. 

 30° N to 20° S and extend westward to long. 

 160° to 180°W: e.g., Euphausia distinguenda, 

 (Johnson and Brinton, 1963), Eucalanus inermis, 

 (Lang, 1967), Melamphaes spiuifer, (Ebeling, 

 1962), Stomiafi colubrhins, (Gibbs, 1969). 



These distributions are meridionally and 

 zonally more extensive than the limited disper- 

 sion observed in P. sobriiia and others like 

 Pontella danae, P. agasnizi, and Pontellopsis 

 lubbockii (Heinrich, 1964; Fleminger, 1967b; 

 and unpubli.shed records). The dissimilarities 

 probably relate to differences in depth range, 

 the more widespread forms having access to 

 subsurface currents flowing northward (Woos- 

 ter and Jones, 1970) and southward (Wooster 

 and Gilmartin, 1961) under the eastern boun- 

 dary currents as well as westward in the tongue 

 of low oxygen water accompanying the North 

 Equatorial Current which is traceable to the 

 Philippines (Reid, 1965; Wyrtki, 1966; Tsuchi- 

 ya, 1968). 



Distributions of epipelagic species in the 

 equatorial Indian and Pacific Oceans resem- 

 bling that of moiii include a number of other 

 copepods, e.g., several species of Eiicctkunts, 

 (Fleminger and Hulsemann, 1973; Fleminger, 

 1973); Claunucakuni^ ))iiii(>)\ (Frost and 

 Fleminger, 1968); several euphausiids such as 

 Euphausia diomediae, E. paragibba, and Sty- 

 locheiron microphthalina, (Brinton. 1962); and 

 fishes such as Scopeloyadiis iinispiinis, (Ebeling 

 and Weed, 1963) and Stomias o//7»/,s, (Gibbs, 

 1969) though the lattermost is also considered 

 to inhabit the tropical Atlantic. 



Although the ubiquitous plumata overlaps 

 geographically with each of the tropical species, 

 plumata s overall range lies mostly in the enor- 

 mous basin of oligotrophic waters spreading 

 across the tropics and subtropics of each ocean, 

 waters markedly different in vertical thermal 

 structure from those of its tropical congeners. 

 The almost mutually exclusive distributions of 

 plumata and its more localized congeners, 

 platychela in the equatorial Atlantic and aobriua 



in the eastern tropical Pacific, are evidence of 

 relatively intensive environmental gradients 

 and the adaptive response to appreciably differ- 

 ent environmental optima, which separate the 

 distributions of these pairs of species. 



For example, morii has been found at the 

 edge of the south Atlantic as well as the edge 

 of the eastern tropical Pacific; concomitantly 

 sobriiia occurs in the North Equatorial Current, 

 but successfully extends only a few degrees of 

 longitude to the west of its habitat; platychela 

 is adjacent to but fails to establish itself in the 

 Sargasso Sea; finally plumata, despite apparent 

 circumglobal distribution, does not appear in 

 large numbers where its equatorial congeners 

 abound. Thus, the optimum habitats appear to 

 be regionally distributed and those that are 

 contiguous are sufficiently different to prevent 

 colonization by expatriated congeners trans- 

 ported to the margin of the habitat. The possi- 

 bility of interference among the species is open 

 but in the light of available knowledge of 

 calanoids it seems intuitively to be most unlikely. 



Thus, the two classes of epipelagic warm- 

 water distributions found in Po)itelliHa suggest 

 a fundamental dichotomy in the circumglobal 

 warm-water belt. The three tropical species 

 correlate with geographically separated shallow 

 lenses of eutrophic water. Each lens is known 

 to overlie regions of intense temperature and 

 oxygen gradients and to be partially bounded 

 by the similarly intense tropical convergences 

 (Neumann and Pierson, 1966). 



P. plumata, however, correlates with the 

 circumglobal warm-water pool that is largely 

 oligotrophic. The oligotrophic pool tends to be 

 deep, the permanent thermocline often exceed- 

 ing 200 m in depth. Temperature gradients in 

 the thermocline and along its margins at the 

 subtropical convergence are relatively weak, 

 and oxygen is generally at or near saturation 

 (Neumann and Pierson, 1966). Evidence that 

 the Atlantic pool may be at least partially iso- 

 lated with respect to Pontelliiia whereas the 

 Indian and Pacific pools are confluent is sug- 

 gested by morphological differences in the 

 plumata populations rejioited above. 



The circulation systems and physical condi- 

 tions known to maintain these lenses of eutro- 

 phic tropical water and the pools of oligotrophic 

 tropical -subtropical waters are the obvious 

 mechanisms sustaining the geographical dis- 



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