2 Clutch size is greater for larger females: in C. finmarchicus 45 eggs per
clutch at 2.5 mm prosome length, increasing to 120 eggs at 3.3 mm.
3 Egg production is mostly late at night, sometimes extending into morning.
4 Inter-clutch interval is set by interaction of food availability and
temperature, which thus determines egg production rate.
5 Egg production rate for the population as a whole can be determined for a
stock by estimating the fraction of females approaching spawning readiness
(“ripe and semi-ripe”) and multiplying by the mean clutch size. This can be
done with preserved samples.Fig. 8.2 Egg production by Calanus pacificus (egg carbon as a fraction of female
carbon) from Puget Sound measured during the day after capture, plotted against
vertically integrated water-column chlorophyll.
(^) (After Runge 1985.)
(^) The apparent functional relation with chlorophyll doesn’t always hold up. Plourde
and Runge (1993) showed a long delay of any reproduction in the St Lawrence
estuary until abundant phytoplankton showed up in late March–early April. However,
the subsequent reproduction of this stock did not seem to depend very tightly upon
chlorophyll abundance; probably the diet switched to microzooplankton. Runge often
expresses egg production as a fraction of female body mass (carbon, usually) per day
(Fig. 8.2). This measure tends to increase almost smoothly with temperature, about a
doubling in Calanus sp. between 5° and 10°C, mostly due to shorter inter-clutch
intervals. Ambler (1986) demonstrated egg production rates in well-fed Acartia tonsa
as great as 1.6 body masses per day at 28°C. A great deal of mesozooplankton
production goes into eggs.
(^) Niehoff (2000), working with C. finmarchicus from a Norwegian fjord, confirmed
that, in this genus, feeding is requisite for continued egg production (Fig. 8.3). In
contrast, several Neocalanus species of the northern Pacific do not support egg
production with current nutrition. In fact, the adults have no functional feeding limbs.