Fig. 10.15 Present distribution of eastern tropical Pacific and Arabian Sea populations
of Euphausia distinguenda compared to the real distribution of Euphausia diomediae,
which Brinton proposes as the hypothetical distribution of E. distinguenda in the
event of global cooling.
(^) (After Brinton 1962.)
(^) Abraham Fleminger (e.g. Fleminger & Hulsemann 1974) pursued this last class of
mechanisms, proposing that for circumglobal tropical and temperate–tropical species
the continents must serve as barriers to dispersal between the Atlantic and the Indo-
Pacific, and sometimes between the Indian and Pacific Oceans. During an interglacial
warming, tropical–subtropical species like Clausocalanus arcuicornis (Fig. 10.2b) can
spread around the Cape of Good Hope into the Atlantic. Subsequent cooling then
squeezes the distribution until the Atlantic and Indo-Pacific populations are not in
communication, and the stocks could speciate. When warm conditions again prevail,
the process is completed by formation of biological mating barriers, and in some cases
both forms spread to circumglobal distributions. Withdrawal of water from the oceans
during major glacial episodes would make the landmass along the arc from Malaysia
to Australia more complete, a second barrier, allowing the development of three
species from one original form in a single glacial cycle by separating stocks in each of
the Atlantic, Indian, and Pacific Oceans. There are many epiplanktonic copepod
genera with three or four groups of three or four similar species. Thus the separate-
ocean mechanism (in combination with Brinton’s equatorial–subtropical one) could
have had two to four chances to operate, in line with the number of major Pleistocene