subarctic Pacific, the endemic copepod Neocalanus plumchrus enters diapause as C5,
mostly in June. The resting individuals scatter downward from 500 to 2000 m and
begin slowly maturing in August, sustaining a roughly constant number of adults from
September through January. As stated above, they spawn in place without feeding.
Individuals probably do not spawn for longer than a month, but population
reproduction is continuous over about 5 months. In years observed to date,
development is only successful toward the end of that reproductive period. The timing
of peak abundance of copepodites feeding near the surface in spring can vary by 6
weeks or more, so presumably the metering out of reproduction makes some young
available to grow whenever growth is favored. This seems like a population
“strategy”, but there are individual-based selection mechanisms that could make it
work: sometimes early spawners succeed, sometimes late ones do. Another species,
Neocalanus flemingeri, is sympatric with N. plumchrus, but less abundant. It grows
from February through May, then descends to depth and matures to adulthood. The
males stop part way down and are joined by the females. Mating occurs and the males
die off. The females carry a large mass of oil, and they are the diapause stage. Ovary
ripening and spawning are narrowly focused in February, and the cycle repeats. Every
species examined has a somewhat different mode in the diapause and reproductive
phases. Calanus hyperboreus in the Arctic Ocean appears to diapause twice, placing
parts of its life cycle in each of three summer production seasons. There are five large,
particle-feeding copepods in Antarctic waters. Calanoides acutus and likely
Rhincalanus gigas have winter resting stages with features of diapause similar to
those of subarctic species: abundant wax accumulation, reduced activity and
metabolism, and ontogenetic down-migration. On the other hand, Calanus
propinquus, Calanus simillimus, and Metridia gerlachi appear only to slow their usual
activities in winter, which may be feasible because temperature actually varies rather
little seasonally. More numerous small species, including Microcalanus pygmaeus,
Ctenocalanus citer, and Oithona similis, also remain active.
(^) We do know something about life-cycle timing and resting periods in euphausiids
and chaetognaths. There are strong pauses in growth during winter periods, so that
peaks in size–frequency distributions do not move. The characterizations of rest
phases are not as sharp as for copepods, however. It looks as if in some seasons they
simply stand by, waiting for things to get better, but that may be because we do not
know how to observe them, and other groups (ostracods, pteropods, appendicularia,
... ) are very little studied in this regard. Most life-history studies, even of copepods,
consider species to be epipelagic during growth. The few studies for mesopelagic
species indicate continuous population processes, with reproducing individuals
present all year. However, in seasonal regions, some mesopelagic zooplankton, such
as the copepod Gaidius variabilis (Yamaguchi & Ikeda 2000), accelerate reproduction
after organic matter from the spring production pulse sinks through their habitat.