4. THE LARVAE
131
There are four abdominal photophores, one to each of the first four
abdominal segments in species of all genera except Bentheuphausia and
Stylocheiron. No photophores are present in Bentheuphausia amblyops
and Stylocheiron species have a photophore present on the first abdominal
segment only. I n some species, e.g. Thysanoessa raschii (Mauchline,
1965a), the pleopods become setose and during the following four larval
stages one abdominal photophore is developed per stage ; when the full
complement of abdominal photophores is present then the reduction in
the number of spines on the telson begins. I n other species, probably
the majority, the photophores develop in the same manner but reduction of the spines on the telson takes place a t the same time. The spines
are usually reduced in the order 7:5:3:1, although in Euphausia
triacantha the three spined stage is usually omitted (Baker, 1959);
larvae with even numbers of spines occur, usually as variant forms. As
in the case of the development of the pleopods, there is a strong correlation between the size of the larva and the number of spines, larvae
with four spines being, on average, larger than larvae with five (Fraser,
1936; Sheard, 1953; Mauchline, 1959, 1965a). The two species comprising the genus Pseudeuphausia appear to be exceptional because the
terminal spines are reduced to one in number by the time the furcilia
acquires five pairs of setose pleopods. Sheard (1953) has tables showing
the number of terminal and lateral spines on the telsons of the different
larvae o f each species.
The larval stages graduate into the adolescent stages of the species
without any very definite delineation between the two phases of
development. A larva is usually designated " adolescent " once it has
five pairs of setose pleopods, all luminescent organs, and the numbers
of lateral and terminal spines on the telson reduced to the adult number.
By that stage, the thoracic appendages are very nearly, if not, adult in
form but the maxillules and maxillae may still be more similar to those
of the early larvae than to those of the adults. There is little obvious
change in the form of the mandibles, maxillules, and maxillae between
the stage found in the calyptopes (Fig. 44) and those found in the third
to fourth furciliae-that is, a t the stage when all the pleopods are present
and setose (Mauchline, 1967b). I n the following two or three furciliae
the pseudexopod of the maxillule replaces the larval exopod, and the
maxilla develops its exopod. Concurrently, the mandible acquires a
mandibular palp in species where such an organ is present in the adult
and so the transition between the larval mouthparts, illustrated in
Fig. 44, and the adult mouthparts, illustrated in Figs 61 and 62, takes
place but is completed during the course of development of the adolescent animals. Most of the development of the adolescent animals, of
131
There are four abdominal photophores, one to each of the first four
abdominal segments in species of all genera except Bentheuphausia and
Stylocheiron. No photophores are present in Bentheuphausia amblyops
and Stylocheiron species have a photophore present on the first abdominal
segment only. I n some species, e.g. Thysanoessa raschii (Mauchline,
1965a), the pleopods become setose and during the following four larval
stages one abdominal photophore is developed per stage ; when the full
complement of abdominal photophores is present then the reduction in
the number of spines on the telson begins. I n other species, probably
the majority, the photophores develop in the same manner but reduction of the spines on the telson takes place a t the same time. The spines
are usually reduced in the order 7:5:3:1, although in Euphausia
triacantha the three spined stage is usually omitted (Baker, 1959);
larvae with even numbers of spines occur, usually as variant forms. As
in the case of the development of the pleopods, there is a strong correlation between the size of the larva and the number of spines, larvae
with four spines being, on average, larger than larvae with five (Fraser,
1936; Sheard, 1953; Mauchline, 1959, 1965a). The two species comprising the genus Pseudeuphausia appear to be exceptional because the
terminal spines are reduced to one in number by the time the furcilia
acquires five pairs of setose pleopods. Sheard (1953) has tables showing
the number of terminal and lateral spines on the telsons of the different
larvae o f each species.
The larval stages graduate into the adolescent stages of the species
without any very definite delineation between the two phases of
development. A larva is usually designated " adolescent " once it has
five pairs of setose pleopods, all luminescent organs, and the numbers
of lateral and terminal spines on the telson reduced to the adult number.
By that stage, the thoracic appendages are very nearly, if not, adult in
form but the maxillules and maxillae may still be more similar to those
of the early larvae than to those of the adults. There is little obvious
change in the form of the mandibles, maxillules, and maxillae between
the stage found in the calyptopes (Fig. 44) and those found in the third
to fourth furciliae-that is, a t the stage when all the pleopods are present
and setose (Mauchline, 1967b). I n the following two or three furciliae
the pseudexopod of the maxillule replaces the larval exopod, and the
maxilla develops its exopod. Concurrently, the mandible acquires a
mandibular palp in species where such an organ is present in the adult
and so the transition between the larval mouthparts, illustrated in
Fig. 44, and the adult mouthparts, illustrated in Figs 61 and 62, takes
place but is completed during the course of development of the adolescent animals. Most of the development of the adolescent animals, of