Euphausiids, too, divide into free-spawning (e.g. Euphausia, Thysanoessa) and egg-
carrying (e.g. Nematoscelis, Nyctiphanes) groups. Available fecundity data are mostly
from free-spawners, for example the species of Euphausia (Ross et al. 1982; Gómez-
Gutiérrez et al. 2007). Euphausiids close to spawning have a large, colored ovary
loaded with so-called Stage IV oocytes. In E. pacifica, of the subarctic Pacific,
spawning occurs within a day (at most two) after the egg mass appears darkly blue or
purple and is bulky enough to spread the carapace laterally. Similar criteria apply (a
more grayish color) to the antarctic E. superba (Ross & Quetin 1984). After
spawning, E. superba exhibits a clear space in the ovary, which also helps to evaluate
field spawning activity. Ross et al. (1982) distinguished “ripe” from Stage-IV females
of E. pacifica as those that spawn within 24 hours after capture. In their Puget Sound
study site, the fraction of females with Stage-IV oocytes rose to 100% in late April,
stayed high through May, then tapered off. Ripe females were about half of those in
Stage-IV. Ross estimated the inter-brood interval as the inverse of the fraction of
females spawning during the day after capture, obtaining a 2–3 day interval. Egg
number per spawning episode is highly variable, with greater maxima at greater body
size (Fig. 8.5a). Some, but only some, of the variability results from availability of
food (Fig. 8.5b), with clutch size usually greater at high chlorophyll concentrations.
However, low chlorophyll levels only sometimes correspond with low clutch size,
suggesting that other foods can replace phytoplankton; indeed all filter-feeding
euphausiids are omnivores. Euphausia pacifica spawns from April through September
in Oregon (USA) waters, and a female might produce a total of 45 clutches in that
time (Gómez-Gutiérrez et al. 2007). If she grows across the size-range of spawning
females (13 to 25 mm length) and produces the average egg outputs for her increasing
sizes, she could produce over 5800 eggs.
Fig. 8.5 (a) Initial after-collection brood sizes of Euphausia pacifica from Oregon
(USA) coastal waters. The line indicates maximum egg numbers for 95% of the
observations; 5% of females had larger broods. (b) Average brood sizes for variable
numbers of females from different collection sites plotted vs. near-surface chlorophyll
concentration.
(^) Data from J. Gómez-Gutiérrez (see Gómez-Gutiérrez et al. 2007.)