region of closed flow. Natural selection for sufficiently wide tolerance of
environmental variation to live throughout an entire circuit must be strong. For sound
physical reasons, the regions with semi-closed circulation are largely the same as
water masses defined by TS diagrams. Moreover, because water masses are internally
mixing, they tend to be more homogeneous than larger regions. Thus, the expansion
of tolerance to encompass the whole range of conditions for a water mass is a
reasonably obvious direction for evolution.
(^) In confirmation of this notion, there are many patterns of distribution along the
continental coasts, for example, the California Current distribution of Nyctiphanes
simplex (Eu) (Fig. 10.12). Actually, the N. simplex pattern extends south along the
American coasts to south of the equator, and it is abundant around the Galapagos
Islands. Planktonic populations occur in places where coastal effects on the
circulation create small gyres or intermittent return flows.
Fig. 10.12 North Pacific distribution of Nyctiphanes simplex, a euphausiid. Another
population is found in the vicinity of the Galapagos Islands.
(^) (After Brinton 1967a.)