with their parasites, die from their parasitoids), Collinia, can infect several species of
euphausiids, at least occasionally causing mass mortality.
Life-History Variations
(^) Zooplankters, like many other animals, vary their activities and reproduction
according to the season; the pattern of a particular species or population is termed its
phenology. Many phenologies include a prolonged resting phase or diapause in each
generation or in the generations of some seasons, others do not. Diapause is not found
only at high latitudes, but also in monsoonal areas like the Arabian Sea. Some
zooplankton at high latitudes may take more than one growing season to mature and
thus live several years, requiring two or even three diapauses. Like most aspects of the
biology of zooplankton, life-history patterns are best known for the reliably collected
crustaceans, particularly copepods. Their abundance changes are great enough to
provide a strong signal against the severe sampling noise that afflicts abundance
estimation. Finally, we can rear or at least maintain copepods in the laboratory.
Copepods exhibit three basic life-history patterns. First, some tropical species simply
hatch, pass through successive naupliar and copepodite stages, mature, mate, and
spawn. That cycle is then repeated by the offspring. Individuals can be found in all
stages of the cycle at any given time. Similarly continuous patterns have been
demonstrated in chaetognaths.
(^) Second, copepod species for which the range is inshore over shallow bottoms are in
many cases absent from the water column during some part of the year. Those
inhospitable seasons are avoided by producing eggs adapted for a period of rest in the
sediment before they hatch. Workers puzzled surprisingly long about the periods of
total absence before these resting eggs were discovered in several Black Sea species
by Sazhina (1968). At the beginning of more favorable seasons, the resting eggs
develop, hatch, and the nauplii re-enter the water column, a substantial stock
developing very quickly from the sediment “egg bank”. Females ready to spawn in
periods of favorable conditions produce eggs which hatch immediately, termed
subitaneous eggs. Common genera exhibiting this pattern are Acartia, Centropages,
and Labidocera. In Acartia, individual females can switch from spawning subitaneous
eggs to spawning resting eggs, and they can switch back. The cues are changes of
water temperature, which cycles more evenly in water bodies than in the atmosphere.
Acartia hudsonica produces resting eggs when the temperature rises above a threshold
(Sullivan & McManus 1986); A. tonsa and A. californiensis produce them when the
temperature drops below a threshold (e.g. Johnson 1980). Development and hatching
occur when conditions again cool, in A. hudsonica, or warm, in A. californiensis.
Development of resting eggs of the latter species, in which the active population is
restricted to the upper reaches of estuaries, remains suppressed until salinity rises in