requires much higher population density. It may also force mate-seeking females to
move as much as males, leading to more equal predatory mortality rates for the sexes.
Indeed, Acartia species generally have more equal adult sex ratios than, say,
Pseudocalanus. Females of many copepod genera (e.g. Calanus) and likely
chaetognaths, store sperm from one or a few matings to fertilize all the eggs that they
will produce over a long spawning interval. Females of other groups (e.g. Acartia and
Temora among copepods) require recurring mating. The latter strategy, common in
free-spawning euphausiids, requires greater proportions of males and a persistent
association of the sexes that may promote long-term swarming. A few groups of
zooplankters release sperm into the water, and those must make their way to free ova
or to females bearing unfertilized ova. Close grouping during release is often involved
in raising the fertilization success rate. For example, a subarctic Pacific salp,
Cyclosalpa bakeri, migrates from considerable daytime depths shortly after sundown,
right to the sea surface. This is not a feeding migration, as no mucous feeding webs
are in place, but a trip to a readily identified location for mating. Sperm are released
from older male individuals and pumped through the tests of younger individuals with
ripe ova (Purcell & Madin 1991).
(^) As for several other topics, fecundity has been most extensively studied in
crustaceans, although there are sketchy data for other groups. Very gently collect
some adult females from the ocean, ladle ,or pipette them into containers of the water
(and food) they came from, hold the temperature steady, wait a day, then count the
eggs in the container. It’s easy, but immediately there are problems. Cooped up close
to their eggs, the females can eat them. So, since most eggs sink, a mesh screen is
placed under the females so the eggs can sink out of reach. Or, the females are held in
flat petri dishes. The eggs rest on the bottom, and the females cannot move easily to
get them. This seems to give the highest fecundity estimates. These methods have
many variants with investigator-specific and species-specific details. First, of course,
we will examine some copepod data.
Copepod Reproductive Rates
(^) Jeffrey Runge started a continuing wave of interest in egg production rates (EPR) with
papers (e.g. Runge 1984, 1985) on the fecundity of Calanus pacificus in Puget Sound,
Washington. The key thing sparking interest was his demonstration that egg
production depends upon relatively recent feeding, such that a correlation exists
between measures of available food (the ever-accessible chlorophyll concentration)
and egg output. Runge has also studied egg production in C. finmarchicus, the
dominant large copepod of the subarctic Atlantic. The general conclusions are:
(^1) Egg production depends upon available food (Fig. 8.2), in a hyperbolic
fashion.