384 JOHNSON AND BRINTON [CHAP. 18 



from other populations of the same morphological type. The apparently broad 

 character of other ranges has prompted study leading also to the recognition 

 of complexes of regional species or subspecies. For example, Salpa fusiformis 

 Cuvier, long recognized as cosmopolitan and variable in form, is now seen to 

 consist of four species, two tropical-subtropical, one living south of the sub- 

 tropical convergence, and one limited to the Pacific sector of the Antarctic 

 (Foxton, 1961). Such forms thereby become meaningful in the framework of 

 oceanic ecology. The discussion in this section will be concerned mainly with 

 the euphausiacian Crustacea and is based on earlier work by Brinton (1962a). 



Many of the zooplankton species became known from collections made by 

 exploratory expeditions in the late 19th and early 20th centuries. Classification 

 of plankton at the level of the subspecies or geographical race, requiring 

 exhaustive systematic and ecological data, is a recent consequence of sampling 

 by comprehensive oceanwide surveys and of morphometric comparisons of 

 specimens from different regions. Investigations carried out by various nations 

 in the Atlantic and Antarctic Oceans, and by North American, Japanese and 

 Russian surveys in the Pacific, 1950-1960, provided material from nearly the 

 full extent of the distributional ranges of many species. These surveys have 

 employed improved sampling techniques making possible better quantitative 

 estimates. 



The water-mass boundary is recognized as a zone of discontinuity frequently 

 associated with sharp horizontal gradients in temperature. The distributions 

 of subspecific forms (subspecies, races, ecophenotypes), as well as species, may 

 correspond with physical zones. In other instances, two or more closely related 

 forms live in adjoining parts of one well defined water-mass, forming an allopatric 

 complex. Nevertheless, each distribution derives its identity from a system of 

 circulation. The biological evolution of passively drifting organisms is believed 

 to have proceeded according to the availability of habitats that can conserve 

 a stock. 



Furthermore, the vertical range of certain planktonic species may correspond 

 with the thickness of the physical water-mass and its contained currents. For 

 example, in temperate and tropical latitudes where the euphausiid and sergestid 

 (decapod) crustaceans perform diurnal vertical migrations between the surface 

 and 300-800 m of depth, the water-masses are identified according to the 

 temperature-salinity characteristic between depths of 150 and 1000 or more 

 meters. The migrations and water-masses are mainly in the mesopelagic zone, a 

 stratum between about 150 to about 700 m. Above this lies the epipelagic or 

 photic zone and below it the bathypelagic and abyssopelagic zones. Bruun (1957) 

 discusses the temperature relations for this zonation and Hedgpeth (1957) has 

 reviewed certain works pertinent to the latitudinal aspects of the epipelagic. 



The geographical ranges of species inhabiting the epipelagic zone appear in 

 many cases to be limited by near-surface isotherms, while boundaries of 

 mesopelagic species can be shown to conform with isotherms lying at some 

 greater depth within the vertical range of the species. Inasmuch as many 

 species have an extensive diurnal range that carries them through a vertical 



