132
THE BIOLOGY O F EUPHAUSTIDS
course, involves the reproductive organs and the secondary sexual
characters.
Eggs are usually laid by euphausiids in the spring and the larvae
live in an environment relatively rich in phytoplankton and zooplankton. Few data exist on the diet of larval euphausiids. The mouthparts
of the nauplius and metanauplius are not functional and so they presumably do not feed. The first calyptopis has functional mouthparts
and these larvae of Meganyctiphanes norvegica and Thysanoessa raschii
in the Clyde filter-feed on phytoplankton and suspended material in
the water. Stomach contents of early furciliae of these species consist of
bottom material, diatoms, filamentous algae, and dinoflagellates. Lebour
(1922) records the presence of fragments of Mollusca, the diatoms,
Coscinodiscus, Navicula, Paralia, and of coccolithophores in the stomachs
of unidentified euphausiid larvae. Later furciliae of Meganyctiphanes
norvegica and Thysanoessa raschii have remains of crustaceans, probably
of copepodite stages of copepods, also present among the stomach
contents. Consequently, the few data available suggest that the diet
of the larval stages may be restricted because the food is obtained by
filter feeding but the later stages have a more omnivorous diet somewhat
similar to that of the adults. The mouthparts of larvae of all species
are, as far as is known, similar as regards their setation and so have the
same potential for filter feeding. Detailed knowledge, however, of the
mouthparts of larvae of Stylocheiron species is not available; the
mouthparts of the adults of this genus are the least adapted t o filter
feeding (Mauchline, 1967b).
The vertical distribution of the larvae in the sea varies from species
to species and is further complicated because, in most of the species
examined, some of the larval stages, especially the more advanced,
perform a diurnal vertical migration. The bathymetric distribution of
the earliest larval stages-nauplii, metanauplii, and calyptopes-is
influenced to a considerable degree, of course, by the depth a t which
the adult females lay their eggs and by the depth a t which pseudometanauplii free themselves from the egg masses carried by females of
certain genera. The vertical distribution of the larvae of Euphausia
superba is described in detail by Marr (1962) and the depth of water
over which this species spawns is usually much greater than that over
which the populations of Meganyctiphanes norvegica and Thysanoessa
raschii, studied by Lacroix (1961) and Mauchline (1959, 1965a) spawn.
The eggs of euphausiids are denser than sea water and so they tend to
sink when laid freely in the sea. The deeper the water column over which
the eggs are laid, the more obvious are the results of the sinking of the
eggs on the distributional patterns, within the water column, of the
THE BIOLOGY O F EUPHAUSTIDS
course, involves the reproductive organs and the secondary sexual
characters.
Eggs are usually laid by euphausiids in the spring and the larvae
live in an environment relatively rich in phytoplankton and zooplankton. Few data exist on the diet of larval euphausiids. The mouthparts
of the nauplius and metanauplius are not functional and so they presumably do not feed. The first calyptopis has functional mouthparts
and these larvae of Meganyctiphanes norvegica and Thysanoessa raschii
in the Clyde filter-feed on phytoplankton and suspended material in
the water. Stomach contents of early furciliae of these species consist of
bottom material, diatoms, filamentous algae, and dinoflagellates. Lebour
(1922) records the presence of fragments of Mollusca, the diatoms,
Coscinodiscus, Navicula, Paralia, and of coccolithophores in the stomachs
of unidentified euphausiid larvae. Later furciliae of Meganyctiphanes
norvegica and Thysanoessa raschii have remains of crustaceans, probably
of copepodite stages of copepods, also present among the stomach
contents. Consequently, the few data available suggest that the diet
of the larval stages may be restricted because the food is obtained by
filter feeding but the later stages have a more omnivorous diet somewhat
similar to that of the adults. The mouthparts of larvae of all species
are, as far as is known, similar as regards their setation and so have the
same potential for filter feeding. Detailed knowledge, however, of the
mouthparts of larvae of Stylocheiron species is not available; the
mouthparts of the adults of this genus are the least adapted t o filter
feeding (Mauchline, 1967b).
The vertical distribution of the larvae in the sea varies from species
to species and is further complicated because, in most of the species
examined, some of the larval stages, especially the more advanced,
perform a diurnal vertical migration. The bathymetric distribution of
the earliest larval stages-nauplii, metanauplii, and calyptopes-is
influenced to a considerable degree, of course, by the depth a t which
the adult females lay their eggs and by the depth a t which pseudometanauplii free themselves from the egg masses carried by females of
certain genera. The vertical distribution of the larvae of Euphausia
superba is described in detail by Marr (1962) and the depth of water
over which this species spawns is usually much greater than that over
which the populations of Meganyctiphanes norvegica and Thysanoessa
raschii, studied by Lacroix (1961) and Mauchline (1959, 1965a) spawn.
The eggs of euphausiids are denser than sea water and so they tend to
sink when laid freely in the sea. The deeper the water column over which
the eggs are laid, the more obvious are the results of the sinking of the
eggs on the distributional patterns, within the water column, of the