130
THE BIOLOGY OF EUPHAUSIIDS
similar species N . atlantica and it is probable that the larvae described
by Lebour belong to that species. Nematoscelis atlantica, N . microps
and N . tenella follow the same dominant path of development
and few variant forms are found, even when the larvae have been
caught in coastal waters.
The species in the genus Xtylocheiron are all oceanic in their distribution although populations can be found close to coastal regions.
The development in the species examined so far is relatively rigid
although variant forms have been reported (Fig. 47). Dominant forms
have been indicated in three species but there is inconclusive evidence
for these and further investigations may modify these results.
There is no information about the development of Tessarabrachion
oculatum or the three species in the genus Nematobrachion.
The variation in these early furciliae is not peculiar to them.
Variation is present in the development of calyptopes but it is not so
easily noticed or measured. Similar variation is present in the later
furciliae and here it is most noticeable in the detailed states of development of the thoracic limbs but, because the task of defining such
variation is so tedious, less attention has been paid to this variation
than to that inherent in the development of the pleopods. Heegaard
(1 948), working with early larvae of Meganyctiphanes norvegica, showed
that “. . . not only the pleopods but any organ of the animal may
vary more or less within one and the same stage;” and he concludes
“. . . that in the neritic Euphausiacea there can be no question of fixed
larval stages as in the higher Crustacea and Insecta, but that their
growth and development must rather be considered as almost continuous, only retarded and partly interrupted by the necessary number
of ecdyses.” Broad (1957a,b) found that larvae of the decapods,
Palaemonetes pugio Holthuis and P. vulgaris, which were of the same
size but had not necessarily moulted the same number of times, were
alike, whereas larvae which had moulted the same number of times
were not necessarily alike. This is exactly the case with the euphausiids.
Broad carried out feeding experiments with larvae of both decapod
species and concluded that the frequency of moulting, and so the
number of larval stages, and the duration of the larval life varied
according to the quantity of food available. The frequency of
moulting and the rate of development were both reduced when food
was scarce. No such experiments have been done with euphausiid
larvae.
The later furciliae are usually distinguished from one another by the
presence of additional abdominal photophores and the sequential
reduction in the numbers of terminal and lateral spines on the telson.
THE BIOLOGY OF EUPHAUSIIDS
similar species N . atlantica and it is probable that the larvae described
by Lebour belong to that species. Nematoscelis atlantica, N . microps
and N . tenella follow the same dominant path of development
and few variant forms are found, even when the larvae have been
caught in coastal waters.
The species in the genus Xtylocheiron are all oceanic in their distribution although populations can be found close to coastal regions.
The development in the species examined so far is relatively rigid
although variant forms have been reported (Fig. 47). Dominant forms
have been indicated in three species but there is inconclusive evidence
for these and further investigations may modify these results.
There is no information about the development of Tessarabrachion
oculatum or the three species in the genus Nematobrachion.
The variation in these early furciliae is not peculiar to them.
Variation is present in the development of calyptopes but it is not so
easily noticed or measured. Similar variation is present in the later
furciliae and here it is most noticeable in the detailed states of development of the thoracic limbs but, because the task of defining such
variation is so tedious, less attention has been paid to this variation
than to that inherent in the development of the pleopods. Heegaard
(1 948), working with early larvae of Meganyctiphanes norvegica, showed
that “. . . not only the pleopods but any organ of the animal may
vary more or less within one and the same stage;” and he concludes
“. . . that in the neritic Euphausiacea there can be no question of fixed
larval stages as in the higher Crustacea and Insecta, but that their
growth and development must rather be considered as almost continuous, only retarded and partly interrupted by the necessary number
of ecdyses.” Broad (1957a,b) found that larvae of the decapods,
Palaemonetes pugio Holthuis and P. vulgaris, which were of the same
size but had not necessarily moulted the same number of times, were
alike, whereas larvae which had moulted the same number of times
were not necessarily alike. This is exactly the case with the euphausiids.
Broad carried out feeding experiments with larvae of both decapod
species and concluded that the frequency of moulting, and so the
number of larval stages, and the duration of the larval life varied
according to the quantity of food available. The frequency of
moulting and the rate of development were both reduced when food
was scarce. No such experiments have been done with euphausiid
larvae.
The later furciliae are usually distinguished from one another by the
presence of additional abdominal photophores and the sequential
reduction in the numbers of terminal and lateral spines on the telson.