136
THE BIOLOGY O F EUPHAUSIIDS
tends to take place in the deeper water strata of the Firth of Clyde and
there is an upward movement of nauplii, metanauplii and calyptopes
so that the layers of maximum abundance of the early furciliae tend to
be in the near surface layers. The " developmental ascent " is by no
means well defined in the Clyde because a large proportion of eggs with
nauplii inside is present in the surface layers and a proportion of all
larval stages of these species is always present close to the bottom. The
developmental ascent (Pig. 51), however, is probably a feature of the
early development of all species of euphausiids but is much more pronounced in oceanic areas than in shallow coastal areas. Lacroix (1961),
for instance, studying the early larvae of Thysanoessa raschii and
Meganyctiphanes norvegica in Chaleur Bay, 70 m deep, describes the
vertical distribution of these larvae but no sign of a developmental
ascent is indicated in the results of his analysis. There is considerable
variation in the behaviour of individuals of a species within a population. For example, gravid females of M . norvegica live close to the
bottom a t a depth of about 150 m during the day in the Firth of Clyde
but migrate towards the surface a t night and tend to lay their eggs in
the surface layers. Some females, however, must lay their eggs close to
the bottom because new laid eggs are present a t all depths. Similarly,
Thysanoessa raschii was found, on average, to lay its eggs at a deeper
level in the surface layers of the Clyde than Meganyctiphanes norvegica
but Ponomareva (1959a) finds that the majority of the eggs of
Thysanoesssa raschii and T . inermis are laid in the upper 25 m of the
water column. An examination of the distribution of the unsegmented
eggs found by Marr (Fig. 50) suggests that Euphausia superba may lay
its eggs within the depth range 0-500 m. Thus, there is probably
considerable local variation in the spawning behaviour of any one
species and this will affect the vertical distribution of the earliest larval
stages. The nauplius and metanauplius do not feed but subsist on the
yolk still present in their bodies. The first calyptopis of all species has
functional mouthparts, and presumably requires food, and since the
main source of this food is in the surface layers it must travel into
these layers.
Several studies of the vertical distribution of the calyptopes and
furciliae have been made. Mauchline (1959, 1965a) and Lacroix (1961)
have studied the diurnal vertical migration of the calyptopes and
furciliae of Meganyctiphanes norvegica and Thysanoessa raschii in
considerable detail and these results are very pertinent to any discussion
of the vertical distribution of larvae of all species during daylight hours.
A diurnal vertical migration is the term applied to the movement of the
organisms towards the surface a t night and their return to deeper
THE BIOLOGY O F EUPHAUSIIDS
tends to take place in the deeper water strata of the Firth of Clyde and
there is an upward movement of nauplii, metanauplii and calyptopes
so that the layers of maximum abundance of the early furciliae tend to
be in the near surface layers. The " developmental ascent " is by no
means well defined in the Clyde because a large proportion of eggs with
nauplii inside is present in the surface layers and a proportion of all
larval stages of these species is always present close to the bottom. The
developmental ascent (Pig. 51), however, is probably a feature of the
early development of all species of euphausiids but is much more pronounced in oceanic areas than in shallow coastal areas. Lacroix (1961),
for instance, studying the early larvae of Thysanoessa raschii and
Meganyctiphanes norvegica in Chaleur Bay, 70 m deep, describes the
vertical distribution of these larvae but no sign of a developmental
ascent is indicated in the results of his analysis. There is considerable
variation in the behaviour of individuals of a species within a population. For example, gravid females of M . norvegica live close to the
bottom a t a depth of about 150 m during the day in the Firth of Clyde
but migrate towards the surface a t night and tend to lay their eggs in
the surface layers. Some females, however, must lay their eggs close to
the bottom because new laid eggs are present a t all depths. Similarly,
Thysanoessa raschii was found, on average, to lay its eggs at a deeper
level in the surface layers of the Clyde than Meganyctiphanes norvegica
but Ponomareva (1959a) finds that the majority of the eggs of
Thysanoesssa raschii and T . inermis are laid in the upper 25 m of the
water column. An examination of the distribution of the unsegmented
eggs found by Marr (Fig. 50) suggests that Euphausia superba may lay
its eggs within the depth range 0-500 m. Thus, there is probably
considerable local variation in the spawning behaviour of any one
species and this will affect the vertical distribution of the earliest larval
stages. The nauplius and metanauplius do not feed but subsist on the
yolk still present in their bodies. The first calyptopis of all species has
functional mouthparts, and presumably requires food, and since the
main source of this food is in the surface layers it must travel into
these layers.
Several studies of the vertical distribution of the calyptopes and
furciliae have been made. Mauchline (1959, 1965a) and Lacroix (1961)
have studied the diurnal vertical migration of the calyptopes and
furciliae of Meganyctiphanes norvegica and Thysanoessa raschii in
considerable detail and these results are very pertinent to any discussion
of the vertical distribution of larvae of all species during daylight hours.
A diurnal vertical migration is the term applied to the movement of the
organisms towards the surface a t night and their return to deeper