extension (Plate 10.1). In the Oyashio, the coherence is harder to see. The strong,
coherent flow probably explains the strong drop-off in abundance of transition zone
species at this western extreme. But just east of 150°E, lots of back-eddying sets in;
the Kuroshio breaks up. In fact, to the north of 36°N the eddy activity is high almost
to the coast of Japan. All the way from 150°E to the Emperor Seamounts, there is very
strong eddy activity to return plankton upstream. Mesoscale features reaching 1000 m
down the water column and roughly of 250 km scale alternate all across north–south
hydrographic sections in this vicinity. Net throughput is on the order of 15–20 cm s−1,
while eddy velocities to move animals upstream are of the same order, in agreement
with the drifter tracks. Since there is no stock input in the inflow, it must be return of
stock via recurring back-eddies that allows population maintenance. Transition-zone
endemics peter out at the farthest upstream end, exactly as one would expect.
Enhancement of production by deep seasonal mixing, which does not occur in either
subarctic or subtropical waters to the north and south, is certainly important in
providing transition endemics with the high rates of population increase needed to
compensate for the advective losses.
(^) The distribution of Calanus finmarchicus (copepod; Plate 10.2), Meganyctiphanes
norwegica (euphausiid) and some other species in the North Atlantic Current (NAC)
from New England to Iceland to the Norwegian Sea likely has a similar explanation.
The through-flow of the NAC, that sustains the warmth of northern Europe, has
upper-ocean return flows along the northern limbs of three large cyclonic gyres
(Norwegian Sea, Irminger Sea, and the slope water from New York to the Grand
Banks). Exchanges between adjacent gyres are thought to connect planktonic
populations across the whole region with subarctic conditions (Bucklin et al. 2000).
Bucklin and colleagues applied gene analysis (methods for reading of gene sequences
are summarized in Box 2.4) and found a suggestion of genetic differentiation in C.
finmarchicus based on proportions of small differences in a 72 base-pair sequence
from a mitochondrial pseudogene. A pseudogene is a copy in nuclear DNA of the
mtDNA. However, the dominant haplotype is the same across the whole range (save
for one sample of 10 specimens off Iceland). Recently, Provan et al. (2010) found no
evidence for geographical differentiation of C. finmarchicus in either nuclear
microsatellite DNA or mitochondrial cytochrome B genes. The degree of population-
genetic separation appears to be somewhat greater in M. norvegica (Papetti et al.
2005), which has a much wider range, including the eastern boundary current and
even the Mediterranean. On the other hand, the mtDNA sequence studied by Papetti et
al. has two major haplotypes, two minor ones, and 31 rare haplotypes. All the
populations have substantial proportions of the major and some of the minor forms,
except for those to the far southeast. Inter-gyre exchange among the northern stocks
could either be substantial or the major haplotypes represent coalitions of previously
separate populations and have no current selective significance.