SECT. 4] BIOLOGICAL SPECIES, WATER-MASSES AND CURRENTS 397 



E. longirostris and E. lucens live in subantarctic water bounded by the Antarctic 

 and Subtropical Convergences. This habitat is comparable in both latitude and 

 temperature to the subarctic habitat of E. pacifica, a species related to the E. 

 lucens-E. vallentini line of the Antarctic Ocean. A thorough study of E. tri- 

 acantha (Baker, 1959) demonstrated a close relationship between maximum 

 density of this circumpolar species and the position of the Subantarctic Con- 

 vergence. Smaller numbers were found on either side of the convergence, 

 within the belt of the West Wind Drift. David (1955) described separate races 

 of Sagitta gazellae: a "large southern" form is antarctic, while a "small 

 northern" form is subantarctic. 



G. Distribution and Speciation 



Reference was made in the preceding section to forms that may or may not 

 be sufficiently distinct genetically to be regarded as species. The importance of 

 colonization of neighboring waters as compared with absolute geographical 

 isolation in the differentiation of zooplankton species is an open question. 



It may be noted, however, that there are several instances in which forms of 

 uncertain taxonomic rank live on opposite sides of convergences or other 

 oceanographic boundaries, while there are few instances in which closely 

 related, well defined species are found in such adjoining regions of the high seas. 

 The forms of Sagitta gazellae, Thysanoessa longipes, and Limacina helicina and 

 the Vanadis antarctica-V . longissima and Thysanoessa aequalis-T. subaequalis 

 "species pairs" are instances in which the extent of reproductive isolation 

 between closely related forms is not clear. These, and the intergrading forms of 

 the euphausiid Stylocheiron affine, discussed in later paragraphs, may be 

 sympatric complexes in which morphological differences between populations 

 are caused by environmental differences across the geographical range of a 

 single genotype. Oceanic forms clearly recognized to be sibling species, as in the 

 cases mentioned above of Nematoscelis difficilis-N '. megalops and Euphausia 

 distinguenda-E . sibogae, are often more widely separated geographically. 



In view of the uncertain influence of water-mass boundaries in evolutionary 

 processes, only the three north-south continental land-masses that partition 

 the seas into the Atlantic, Pacific and Indian Oceans can be recognized as 

 absolute barriers between pelagic populations of low latitudes. Certain warm- 

 water species (e.g. E. brevis, Fig. 5a) are today split up by these barriers into 

 isolated populations living in the separate oceans. 



Similarly, the antitropical pattern of distribution, in which parts of a range 

 are separated by a tropical or subtropical belt, can establish absolute isolation 

 of northern and southern population elements. 



The paired roles of continental and tropical barriers in the differentiation of 

 euphausiid species are considered by Brinton (1962b). In that discussion, iso- 

 therms that agree with the limits of Recent distributions are used to extrapolate 

 the limits of hypothetical distributions that may have existed during past 

 epochs when the oceans were warmer or cooler. 



