124
THE BIOLOGY OF EUPHAUSIIDS
furciliae of Meganyctiphanes norvegica in the Clyde, concluded that
certain forms of furciliae were dominant in his samples and that some
of the postulated stages in the development of this species were being
omitted in this sea area. Lebour (1926b) had previously suggested that
this took place in the development of euphausiids but did not have the
direct evidence which Macdonald published in the following year. All
later papers were written in the light of these new concepts and the
previously complicated development of these animals became considerably simplified (Rustad, 1934; Fraser, 1936; Einarsson, 1945 ; Sheard,
1953).
The known or hypothetical paths of development of species of
euphausiids are illustrated in Fig. 47. The species have been grouped in
genera and the development within genera will be compared. Nothing
is known about the development of Bentheuphausia amblyops. Species
of Thysanopoda are generally oceanic in distribution and many are
bathypelagic. The development of these species is relatively rigid, six
of the species having a third calyptopis which moults to a furcilia with
non-setose pleopods, this to one with a combination of non-setose and
setose pleopods, and this in turn to a larva with five pairs of setose
pleopods (three moults). Thy8anopoda egregia has an additional moult
because the larva with three pairs of non-setose pleopods moults to one
with three pairs of setose and one pair of non-setose pleopods and this
larva in turn moults to the form with four pairs of setose and one pair
of non-setose pleopods (four moults). The remaining two species,
T. aequalis and T . tricuspidata show various pathways of development.
This is especially surprising in the case of T . tricuspidata because this
species is oceanic in distribution and, as will be seen, there is usually a
fairly close correlation between a more stereotyped pathway of development and a more oceanic distribution. The species showing the most
variable patterns of development are usually distributed throughout
coastal or continental slope areas. Lebour examined the development of
T. aequalis in the eastern Mediterranean and around Bermuda, both
coastal areas (Fig. 47). Consequently, with the exception of T . tricuspidata, the more oceanic the distribution of a species of the genus
Thysanopoda the less variation in its development is found.
The next three genera, Meganyctiphanes, Nyctiphanes and Pseudeuphausia, consist of species whose main geographical distribution is
restricted to coastal or continental shelf or slope waters. A large
variation in the development of the early furciliae is found, the most
detailed study being that of Sheard (1953) on Nyctiphanes australis but
similar variation is present in the development of Meganyctiphanes
norvegica and Thysanoessa raschii (Mauchline, 1959, 1965a) and in the
THE BIOLOGY OF EUPHAUSIIDS
furciliae of Meganyctiphanes norvegica in the Clyde, concluded that
certain forms of furciliae were dominant in his samples and that some
of the postulated stages in the development of this species were being
omitted in this sea area. Lebour (1926b) had previously suggested that
this took place in the development of euphausiids but did not have the
direct evidence which Macdonald published in the following year. All
later papers were written in the light of these new concepts and the
previously complicated development of these animals became considerably simplified (Rustad, 1934; Fraser, 1936; Einarsson, 1945 ; Sheard,
1953).
The known or hypothetical paths of development of species of
euphausiids are illustrated in Fig. 47. The species have been grouped in
genera and the development within genera will be compared. Nothing
is known about the development of Bentheuphausia amblyops. Species
of Thysanopoda are generally oceanic in distribution and many are
bathypelagic. The development of these species is relatively rigid, six
of the species having a third calyptopis which moults to a furcilia with
non-setose pleopods, this to one with a combination of non-setose and
setose pleopods, and this in turn to a larva with five pairs of setose
pleopods (three moults). Thy8anopoda egregia has an additional moult
because the larva with three pairs of non-setose pleopods moults to one
with three pairs of setose and one pair of non-setose pleopods and this
larva in turn moults to the form with four pairs of setose and one pair
of non-setose pleopods (four moults). The remaining two species,
T. aequalis and T . tricuspidata show various pathways of development.
This is especially surprising in the case of T . tricuspidata because this
species is oceanic in distribution and, as will be seen, there is usually a
fairly close correlation between a more stereotyped pathway of development and a more oceanic distribution. The species showing the most
variable patterns of development are usually distributed throughout
coastal or continental slope areas. Lebour examined the development of
T. aequalis in the eastern Mediterranean and around Bermuda, both
coastal areas (Fig. 47). Consequently, with the exception of T . tricuspidata, the more oceanic the distribution of a species of the genus
Thysanopoda the less variation in its development is found.
The next three genera, Meganyctiphanes, Nyctiphanes and Pseudeuphausia, consist of species whose main geographical distribution is
restricted to coastal or continental shelf or slope waters. A large
variation in the development of the early furciliae is found, the most
detailed study being that of Sheard (1953) on Nyctiphanes australis but
similar variation is present in the development of Meganyctiphanes
norvegica and Thysanoessa raschii (Mauchline, 1959, 1965a) and in the