4. THE LARVAE
137
water in the morning. During daylight hours, most species of larvae
live below the immediate surface layers but when darkness approaches
they move towards the surface. They have a preference for an environment of a given light intensity (isolume) and, of course, as the sun
moves to a lower position in the sky this layer of light intensity moves
upwards. When darkness falls, then levels of illumination in the sea
are below the levels of the isolumes preferred by many species, but the
animals seem to maintain themselves in the surface layers throughout
the night until the approach of daylight when the isolumes are again
present and the animals follow them downwards as the sun moves
higher in the sky. The whole problem of vertical migration of euphausiids will be discussed later because much more information is available
on the behaviour of the adults than on that of the larvae. The larvae
react quite quickly to changes in light intensity; when the sun is
obscured by cloud they approach the surface but move to deeper layers
when the sun is once again bright. Consequently, there seems little
doubt that light intensity is the factor controlling the vertical movements of the larvae as well as the vertical movements of the adults.
Lacroix (1961) shows that the nauplii and metanauplii of Meganyctip h n e s norvegica and Thysanoessa raschii live in the uppermost 15m
of a water column of total vertical extent of 70 m in Chaleur Bay, Gulf
of St. Lawrence. No obvious diurnal migration of these larvae was
found and Mauchline could detect none in these larvae in the Clyde.
Comparable larvae of Euphausia superba live a t greater depths and no
diurnal migration was observed but simply a continuous movement
towards the surface layers (Marr, 1962). This was also reflected in the
analysis of the distribution of the first calyptopis of E . superba.
The second calyptopis and all later larvae of all species examined
usually show indications of a t least a restricted diurnal vertical migration (Fig. 52). There is evidence in Lacroix’s data of a vertical migration
of the calyptopes and Mauchline found that there is an aggregation of
these larvae and also of early furciliae immediately under the surface
film of the water column a t night; a net towed so that it breaks the
surface catches immense numbers of larvae whereas a t a depth of
2-3m these concentrations are absent, The source of these larvae,
that is whether they had migrated from a considerable depth or whether
they were simply surface aggregations of individuals which had been
spread throughout the upper part of the water column during daylight
hours, is unknown. The vertical extent of the migration of any one
individual, of the species of calyptopes so far examined (Fig. 52), is
probably restricted to a range of 50 m or less. The larvae are distributed
through a wide vertical range during the day and the population, as a
137
water in the morning. During daylight hours, most species of larvae
live below the immediate surface layers but when darkness approaches
they move towards the surface. They have a preference for an environment of a given light intensity (isolume) and, of course, as the sun
moves to a lower position in the sky this layer of light intensity moves
upwards. When darkness falls, then levels of illumination in the sea
are below the levels of the isolumes preferred by many species, but the
animals seem to maintain themselves in the surface layers throughout
the night until the approach of daylight when the isolumes are again
present and the animals follow them downwards as the sun moves
higher in the sky. The whole problem of vertical migration of euphausiids will be discussed later because much more information is available
on the behaviour of the adults than on that of the larvae. The larvae
react quite quickly to changes in light intensity; when the sun is
obscured by cloud they approach the surface but move to deeper layers
when the sun is once again bright. Consequently, there seems little
doubt that light intensity is the factor controlling the vertical movements of the larvae as well as the vertical movements of the adults.
Lacroix (1961) shows that the nauplii and metanauplii of Meganyctip h n e s norvegica and Thysanoessa raschii live in the uppermost 15m
of a water column of total vertical extent of 70 m in Chaleur Bay, Gulf
of St. Lawrence. No obvious diurnal migration of these larvae was
found and Mauchline could detect none in these larvae in the Clyde.
Comparable larvae of Euphausia superba live a t greater depths and no
diurnal migration was observed but simply a continuous movement
towards the surface layers (Marr, 1962). This was also reflected in the
analysis of the distribution of the first calyptopis of E . superba.
The second calyptopis and all later larvae of all species examined
usually show indications of a t least a restricted diurnal vertical migration (Fig. 52). There is evidence in Lacroix’s data of a vertical migration
of the calyptopes and Mauchline found that there is an aggregation of
these larvae and also of early furciliae immediately under the surface
film of the water column a t night; a net towed so that it breaks the
surface catches immense numbers of larvae whereas a t a depth of
2-3m these concentrations are absent, The source of these larvae,
that is whether they had migrated from a considerable depth or whether
they were simply surface aggregations of individuals which had been
spread throughout the upper part of the water column during daylight
hours, is unknown. The vertical extent of the migration of any one
individual, of the species of calyptopes so far examined (Fig. 52), is
probably restricted to a range of 50 m or less. The larvae are distributed
through a wide vertical range during the day and the population, as a