feeding mechanics are described in Chapter 7.
(^) Sexes are separate in the euphausiids and, since mating is apparently required for
each spawning (or at least quite often), males and females are found together in
roughly equal numbers. Sperm transfer is by spermatophore, with the two anterior
pairs of pleopods (abdominal limbs) modified in the male for manipulating these
sperm sacs. Some genera carry the eggs in masses on the thoracic legs (e.g.
Nematocelis), others release them into the water (e.g. Euphausia). Hatching produces
a nauplius or “metanauplius”, which shortly molts to a calyptopis stage, a limpet-
shaped larva with a stick-like abdomen. Later larvae with stalked eyes outside the
carapace and one or more abdominal legs are called furcilia. Furcilia grow
progressively larger and add limbs through an indeterminate number of molts, such
that more stages can be added or stages can be skipped, to produce juveniles (e.g.
Knight 1984). Juveniles and adults molt on a regular, temperature-determined
schedule, regardless of either food availability or achieved growth. Without food, they
continue to molt but to progressively smaller sizes. However, growth adds elements to
the compound eye, and they are not lost during size reduction (Sun et al. 1995). Trace
metals have been shown to accumulate in the exoskeleton and are discarded at molt,
so perhaps regular molting is a form of excretion or toxin elimination. It may also be
adaptive during intervals of food shortage to repackage the body in a smaller shell,
thus sustaining a compact, robust body format despite weight loss.
(^) Euphausiids have photophores along the midline on the ventral side of the body.
These produce countershading illumination to fill in the silhouette from below. Many
species are strong diel vertical migrators. Species of at least Euphausia and
Thysanoessa can gather into swarms or schools. The prime example is Euphausia
superba, the Antarctic krill, which lives in massive shoals at older ages. These are
true “schools”, with individuals maintaining close discipline with respect to spacing
and common orientation (more about E. superba in Chapter 10). In Thysanoessa, the
massing is more properly called swarming, with individuals moving alternately
toward and away from the swarm center. Swarms in that genus, and perhaps in
Euphausia pacifica, seem to be an adaptation for mating.
(^) Systematic work on krill includes a CD-ROM expert system for identification
(including larval stages) by Brinton et al. (1999/2000). It includes references to
virtually the entire literature to that date on Euphausiacea, and a wealth of systematic
and biological information. Sampling has been globally extensive. With the exception
of cryptic species that may be identified by molecular genetics and possibly a few
more bathypelagic forms (like Bentheuphausia amblyops), the systematics are nearly
complete. There are 86 species in 11 genera.