(1999) have shown that M. norvegica has a strong relationship between its egg
production and molt cycles. A group of eggs matures for spawning just before cells of
the epidermis detach from the old exoskeleton. Spawning itself occurs in two roughly
equal bursts (and sometimes a smaller third) during the premolt phase in which new
skin and setae form. Once the ovary is clear of large eggs, yolk formation
(vitellogenesis) begins to enlarge a new lot, and that continues past the shedding of
the old skin and on through an entire second molt cycle. Thus, the female alternates
between a spawning molt cycle and a vitellogenic molt cycle. New small oocytes are
produced continuously, restocking a layer from which those undergoing vitellogenesis
are drawn. Molt cycles at a given temperature are shorter (with variation) for animals
during the Mediterranean spawning season from January to May than during the
summer–fall period of reproductive quiescence. Not only are spawning and molting
cycles coordinated, most euphausiids molt at night, and M. norvegica, which is often
a diel vertical migrator, spawns shortly before dawn. Thus, all the strong periodicities
in its biology are coupled: spawning, molting, and vertical migration. Similar
coupling will probably be found for more euphausiids.
(^) Sac-spawning euphausiids extrude their eggs into paired sacs that are attached to
and surround the posterior thoracic legs. In the small, mostly subtropical species
Nyctiphanes simplex, the egg count increases with female size from ∼32 at 9.5 mm to
∼70 at 12.2 mm (Lavaniegos 1995). Development to hatching takes 5 days at 16°C,
and the mother’s ovary is engaged in oogenesis as development of an attached brood
proceeds. Females molt soon after a brood hatches and then fill a new pair of egg
sacs, the total cycle time being 7 to 12 days for most individuals (Gómez-Gutiérrez et
al. 2010). The commitment of growth to egg production is considerably less than for
free-spawning species, but certainly with some energetic cost for carrying the eggs
and moving water past them for oxygen and waste exchange.
Chaetognath Egg Production
(^) Chaetognaths usually get abraded in net collections, so reliable fecundity data for
oceanic forms are elusive. Nagasawa (1984) worked with Sagitta crassa, a small,
robust, coastal species that she collected in a salt pond connected to Tokyo Bay by a
sluice gate. She kept them in dishes and fed them Acartia clausi from the same
source. Only three specimens from a collection of 100 held until death were
significantly reproductive. Spawning by those three was cyclic, increasing then
decreasing over periods of ∼10 days, but not corresponding to cycles in feeding
activity. Lifetime production for the longest-lived specimen of S. crassa was 952 eggs
over 33 days, a mean of 29 eggs per day, cycling from zero to almost 90 (Fig. 8.7).
Fig. 8.7 Cycling of daily egg production rate in Sagitta crassa collected from Tokyo
Bay and fed to repletion on Acartia copepodites.