4. THE LARVAE
115
into a metanauplius without moulting (Komaki, 1967b). I n the
majority of euphausiid species, however, the nauplius swims freely in
the sea by means of its antennae and mandibles. The antennules are
usually not involved directly in swimming but are held straight out in
front of the animal. The nauplius has no mouth and does not feed but
obtains its nourishment from the remainder of the yolk present in its
body. The antennae and mandibles have no setae for filtering food from
the water as do those of the nauplii of copepods and cirripedes (Gauld,
1959). There are usually a first and a second nauplius in the development of the species, the second nauplius being closely similar in form
t o the first but a little larger in size and usually having a distinct median
eye present. The antennules are uniramous, the antennae and mandibles
biramous. The nauplii of all species are so similar that it is frequently
impossible to separate them.
The metanauplius (Fig. 43) is recognized by its two pairs of limbs,
the uniramous antennules and the biramous antennae ; the mandibles
are reduced and this larva, like the previous two, has no functional
feeding appendages and so, presumably, is still obtaining nourishment
from the remainder of the yolk. The metanauplius is larger than the
second nauplius and has an abdomen protruding from the carapace
which envelops the anterior part of the body. The nauplius eye is
present. The abdomen has a series of distal spines on the developing
telson. The antennules of the metanauplius are usually unsegmented
but the antennae are segmented or show segmentation developing. The
jaw or corpus mandibularis of the mandible can also, at times, be seen
developing under the cuticle of the bud-like mandibles of this stage as
can the developmental stages of the maxillules and maxillae. The
metanauplii of some species can be distinguished from those of others
by the presence or absence and form of the spines on the telson and the
form of the carapace but, in most cases, specific identification of these
larvae is still almost impossible. Fraser (1936) has shown that the size
of the metanauplius can vary. He found that the range in length of the
metanauplius of Euphausia superba taken between 500 and 750m
depth was 0.90-1-05 mm as compared to a range of 0.84-0.98 mm in
larvae taken at the same place but at depths between 750 and 1 000 m ;
he compares these ranges to the range of 0.72-0.82 mm found for the
metanauplius by Ruud (1932). No information on the variation in size
of the metanauplii of other species is available because of the difficulty
of identifying them in the plankton.
The metanauplius moults to the first calyptopis, the first of the three
larvae in the next phase of development (Fig. 44), during which the
abdomen and the eyes develop. The first calyptopis has functional
115
into a metanauplius without moulting (Komaki, 1967b). I n the
majority of euphausiid species, however, the nauplius swims freely in
the sea by means of its antennae and mandibles. The antennules are
usually not involved directly in swimming but are held straight out in
front of the animal. The nauplius has no mouth and does not feed but
obtains its nourishment from the remainder of the yolk present in its
body. The antennae and mandibles have no setae for filtering food from
the water as do those of the nauplii of copepods and cirripedes (Gauld,
1959). There are usually a first and a second nauplius in the development of the species, the second nauplius being closely similar in form
t o the first but a little larger in size and usually having a distinct median
eye present. The antennules are uniramous, the antennae and mandibles
biramous. The nauplii of all species are so similar that it is frequently
impossible to separate them.
The metanauplius (Fig. 43) is recognized by its two pairs of limbs,
the uniramous antennules and the biramous antennae ; the mandibles
are reduced and this larva, like the previous two, has no functional
feeding appendages and so, presumably, is still obtaining nourishment
from the remainder of the yolk. The metanauplius is larger than the
second nauplius and has an abdomen protruding from the carapace
which envelops the anterior part of the body. The nauplius eye is
present. The abdomen has a series of distal spines on the developing
telson. The antennules of the metanauplius are usually unsegmented
but the antennae are segmented or show segmentation developing. The
jaw or corpus mandibularis of the mandible can also, at times, be seen
developing under the cuticle of the bud-like mandibles of this stage as
can the developmental stages of the maxillules and maxillae. The
metanauplii of some species can be distinguished from those of others
by the presence or absence and form of the spines on the telson and the
form of the carapace but, in most cases, specific identification of these
larvae is still almost impossible. Fraser (1936) has shown that the size
of the metanauplius can vary. He found that the range in length of the
metanauplius of Euphausia superba taken between 500 and 750m
depth was 0.90-1-05 mm as compared to a range of 0.84-0.98 mm in
larvae taken at the same place but at depths between 750 and 1 000 m ;
he compares these ranges to the range of 0.72-0.82 mm found for the
metanauplius by Ruud (1932). No information on the variation in size
of the metanauplii of other species is available because of the difficulty
of identifying them in the plankton.
The metanauplius moults to the first calyptopis, the first of the three
larvae in the next phase of development (Fig. 44), during which the
abdomen and the eyes develop. The first calyptopis has functional