398 JOHNSON AND BRINTON [CHAP. 18 



The positions of ancient water-mass habitats are not mapped as easily. The 

 persistence of belts of oceanic productivity, as measured in the sediments by 

 Arrhenius (1955), suggests that some of the major currents have retained their 

 positions since the close of the Tertiary. If the zones of the water-masses have 

 similarly persisted, it is unlikely that plankton distributions have migrated in 

 strict harmony with postulated latitudinal migrations of isotherms. When 

 ocean-wide warming or cooling compelled a species to occupy a new geographical 

 range, still tolerable with respect to temperature, salinity and food, this new 

 environment had also to include a sufficiently closed current system to main- 

 tain the population. Nevertheless, temperature, reflecting obvious responses 

 of the ocean to climatic change, was necessarily used in the euphausiid 

 study. 



Of the species with disjunct ranges, those with the widest latitudinal range- 

 split occupy the two transition zones (zones of sinking isohaline and isothermal 

 surfaces, Fig. 7). Given sufficient time, the separated populations of Thysa- 

 noessa gregaria would be expected to undergo independent selective adaptations. 

 The species pair Nematoscelis difficilis-N. megalops is an example of anti- 

 tropical evolutionary divergence already in an early stage. Compared with 

 N. difficilis-N. megalops, Thysanoessa gregaria ranges somewhat farther into 

 warm waters, by 1°C, interpreting 100-m isotherms as limiting. The northern 

 and southern populations of T. gregaria are morphologically indistinguishable. 

 They may have exchanged genetic material across the tropics more recently 

 than the precursor of Nematoscelis difficilis-N. megalops. 



Reconstructed Pleistocene distributions for Thysanoessa gregaria (Fig. 9) 

 are based on 2|°C warming, or cooling, at a depth of 200 m. Hubbs (1952) 

 suggested that if the surface near-shore waters of the eastern Pacific were 8°C 

 cooler in the winter than during present winters and 3°C cooler in the summer, 

 continuity and exchange between fish faunas now antitropically separated 

 would have been possible. The 7°C and 11°C isotherms at 200 m are associated 

 with the limits of the Recent range of T. gregaria. The temperature change 

 required to cool the warm waters west of Central America enough to allow 

 equatorial transgression by T. gregaria would have made the California 

 Current extremely cold. The bisubtropical transition-zone fauna would then 

 have moved far offshore into central waters, so that it would no longer have 

 access to prevailing currents entering the eastern equatorial basin. The northern 

 and southern populations might, therefore, have coalesced far offshore in the 

 zone of 0°-12°N (Fig. 9c). With 2°-3°C of cooling, the coastal fauna in the 

 region 20°-30°N would have consisted of subarctic species {Euphausia pacifica, 

 Thysanoessa longipes, Tessarabrachion oculatus). These occur only in the nor- 

 thern hemisphere. It is probable that they have never transgressed the tropics. 

 Similarly, the Peru Current species Euphausia mucronata and all subantarctic 

 euphausiids appear to have never crossed the equator. 



Thus, during moderately cool epochs, either the transition-zone populations 

 would have coalesced at the equator, or, if the species had previously been 

 present in only one hemisphere, access to a new habitat in the opposite hemi- 



