Atlantic, newly emerged and mated females (termed the G 0 generation) move near to
the surface to feed and spawn. Food is required to make eggs (G 1 generation),
although spawning can begin well before the spring bloom. Timing of G 0 maturation
from diapause by C. finmarchicus varies strongly by region (Planque & Batten 2000).
In the Gulf of Maine it starts in late December; off Scotland it begins in February;
west of Iceland and in the southern Labrador Sea it is even later. Late winter and
spring see completion of one generation, which at least partly matures in many
subregions, but not off northern Norway, where all but a few percent of G 1 enter
diapause. The G 1 females produce G 2 , which completes growth by a time varying
from May to August and mostly enters diapause as C5, sometimes as C4. In some
areas part of G 2 also matures and produces G 3 , which may or may not survive
significantly to reach diapause. Possibly, G 3 is the source of most of the C4
individuals included in the final, November diapause stock.
(^) Variation for Canadian Atlantic waters has been nicely summarized by Johnson et
al. (2008) from multi-year time-series of samples (Plate 8.2). The consistency within
subregions is striking, as are differences in timing among subregions. Adults emerge
from diapause in December off Newfoundland and on the shelf south of Nova Scotia,
similarly to the Gulf of Maine, in some years allowing a strong G 2 generation. In the
Lower St Lawrence (west of Nova Scotia), diapause breaks much later, although that
time-series has long winter sampling intervals. The St Lawrence series usually shows
a greater proportion of C4 in the diapause stock.
(^) We would like to know which changes in the habitat inform an individual that it
should enter diapause or mature immediately. Unfortunately, experiments have not
answered this, since C. finmarchicus will not enter diapause in containers, even in
very large, so-called mesocosms. Arousal from diapause appears to depend upon
internal time-keeping, although exactly how that works remains unclear.
(^) Not only diapause timing, but depths occupied during rest, are variable. Calanus
finmarchicus spends its diapause at ∼450 m depth south of Georges Bank, but well
above the ∼300 m bottom in the Gulf of Maine north of Georges Bank. In basins on
the Nova Scotian Shelf, it is found just above the bottom. In deep areas (>2000 m) of
the eastern North Atlantic, the diapause stock is widely dispersed, mostly between
400 and 1200 to 1600 m (Heath et al. 2000). In Norway, the depth of diapause varies
strongly from one fjord to another. Dale et al. (1999) have suggested that this
variation is a response to vertical changes in predation pressure from place to place.
Either layered predators remove diapause stock everywhere that we do not find it, or
the copepods avoid the layers with predators. We don’t have a test to distinguish these
hypotheses.
(^) Other species show other patterns. In the Gulf of Alaska and west across the