402 JOHNSON AND BR1NTOX [CHAP. 18 



Groups of four related species occur in each of the three major euphausiid 

 genera that inhabit the upper layers — Euphausia (2 groups), Kematoscelis 

 (1 group), Stylocheiron (1 group). These may be explained by postulating 

 long-term isolation of populations, split up from the once-widespread distribu- 

 tion of each parent species, into the four major subtropical oceans — the Atlantic, 

 North Pacific, South Pacific and Indian Oceans. Each quartet of species con- 

 tains three central and one equatorial Pacific-Indian Ocean species. For 

 example, Euphausia brevis, E. mutica and E. recurva are central, while E. 

 diomediae is equatorial. The central species have, in most cases, re-established 

 themselves in the other central regions already occupied by sibling species 

 when these other regions became accessible during Pleistocene periods of 

 latitudinal oscillations of isotherms. The equatorial species remained bound to 

 the equatorial water-masses. 



Three closely related mesopelagic species occur in each of three euphausiid 

 genera: the il Thysanopoda orientalis group", the ''Stylocheiron maximum 

 group" and the three species comprising the genus Nematobrachion. All occupy 

 depths to 1000 m, mainly in the zone 45°N-45°S. Differentiation from the three 

 parental forms may have followed availability of the three isolated mesopelagic 

 habitats — the low-mid-latitudes of the Atlantic, Pacific and Indian Oceans. 



Furthermore, the ten endemic antarctic and subantarctic species of Eu- 

 phausia and Thysanoessa may have differentiated within the southern zones of 

 the Atlantic, Pacific and Indian Oceans when circumpolar stocks were split up 

 by the southern continents during cool epochs in the cycles of change in ocean 

 climate. 



In this way, latitudinal thermal fluctuations occurring simultaneously within 

 a series of intercommunicating oceans could have led to the rise of groups of 

 euphausiid species. Invasion of equatorial waters and of passageways around 

 the southern tij>s of continents can account for the present sympatric occur- 

 rence of species that initially differentiated as allopatric forms. Earlier trans- 

 gressions may have brought about the present sympatric occurrence of related 

 genera. Successive climatic revolutions might, therefore, be accompanied by 

 an increasing rate of species formation were it not for the probability that 

 few environmental changes have been of sufficient magnitude or geographical 

 extent to allow the differentiation of species in the partitioned stocks. In 

 addition, successive epochs may not always be enough separated in time to 

 permit allopatric differentiation before coalescence again occurs, either around 

 the tips of continents or across the tropics. Of course, environments may be 

 so altered by climatic change that those species unable to adapt to the changing 

 conditions become extinct. 



Less may be inferred about the possible effectiveness of oceanographic 

 barriers in isolating planktonic populations from one another. The findings of 

 geographical races or subspecies suggests the possibility that morphological 

 differentiation may arise in segments of the overall population of a species, in 

 each of a series of communicating oceanographic regions. For example, several 

 Pacific variants of the euphausiid Stylocheiron affine are distinguished on the 



