but cold adaptation is expected in antarctic krill. Maturity comes as early as 36 mm
total length, with initial clutches numbering ∼450 eggs. Clutches increase with body
growth, and females of 56 mm produce ∼5000 eggs per clutch on average.
Maintenance of live antarctic krill suggests 9 to 10 spawning episodes over two
summer months, a mean of ∼2500 eggs per spawning and total output of about 22,000
eggs per female per season. Ross and Quetin point out that these results are in contrast
to earlier work in which it was assumed that females produced one brood per year;
they warn of the danger in guessing what animals do in the field from characteristics
of preserved specimens. The parallel with the chaetognath case below is exact.
(^) The material and energetic outputs involved in spawning are substantial, amounting
to ∼40% of body mass in each clutch (Nicol et al. 1995), substantial growth to
accumulate every six days. Quetin and Ross (2001) quantified the interannual
variability of Antarctic krill fecundity near Palmer Peninsula. The reproducing
fraction of the female stock varied from 10 to 98% over 7 years, corresponding to
variations of food availability that depended, in turn, on timing of sea-ice retreat in
spring. A collation of development-time data from free-spawning euphausiids
(Pinchuk & Hopcroft 2006) shows (Fig. 8.6) a remarkably consistent relation to
temperature. The prolonged development at antarctic temperatures allows sinking of
Euphausia cystallophorias and E. superba eggs to great depths before hatching.
Fig. 8.6 Inverse exponential relationship of embryonic development time with
temperature for various species of polar and subpolar euphausiids. At least this subset
of euphausiids share a common relationship of development rate to temperature. E. =
Euphausia spp. T. = Thysanoessa.
(^) (From Pinchuk & Hopcroft 2006.)
European workers often refer to Meganyctiphanes norvegica as northern krill,
which are abundant in coastal waters from Norway to the Ligurian Sea and are found
off North America from the Gulf of Maine northward. Cuzin-Roudy and Bucholz