156
THE BIOLOGY OF EUPHAUSIIDR
species. Aitken (1960) examined the composition of a surface swarm in
Loch Strangford in Ireland and found the sex ratio within the swarm
to be normal but from his histogram of the distribution of body lengths
it seems to have been predominantly of I and I1 group animals. Surface
swarms of Thysanoessa raschii, T . inermis, T . gregaria, T . longicaudata,
Nyctiphanes australis, and Euphausia krohnii and E . pacijica have been
recorded so that this is a relatively common feature in the behaviour of
euphausiids. I n several instances, e.g. Meganyctiphanes norvegica in
L. Strangford (Aitken, 1960), hydrographic features may have caused
the accumulations but more frequently no cause is so far known.
Euphausia superba, if Marr is correct, is exceptional in this respect
because this is a common feature of its behaviour. At present, we are
only concerned with the fact that E . superba is not continuously distributed throughout its horizontal and vertical environment but is
aggregated into discrete swarms or " patches ". This species appears
to aggregate in swarms throughout every known phase of its development, from the nauplii to the adults, the swarms being autonomous and
moving vertically and horizontally without apparently mixing very
much with each other. Such pronounced patchiness is as yet not
known to occur in other species of euphausiids. There are certain
sea areas where dense populations of a species usually occur, others
where they rarely occur, and attempts a t quantitative sampling can
produce irregular results suggestive of patchy distribution but, compared to E . superba, other species appear to be much more regularly
distributed. This is illustrated by the fact that diurnal vertical
migration of other species can be determined quite easily and EL
repetition of the investigation usually produces very similar results.
One conclusion often quoted is that of Savage (1926) that the main
migration of Nyctiphanes couchii a t night was downwards, although
a small percentage of the population moved upwards to the surface.
This is probably a wrong conclusion from his data and arises because
of errors made in the estimation of the vertical distribution of the
population during the day, a large proportion of the population being
so close to the bottom that his nets did not sample them sufficiently.
The speed at which euphausiids swim upwards towards the surface
can be estimated. Hardy and Bainbridge (1954) constructed an
apparatus in which it was possible to observe and record the vertical
movements of planktonic organisms through several hundred metres.
The apparatus was in the form of an upright hollow wheel which thus
formed a circular tank in the vertical plane and the animals were placed
in it at the side. When they swam upwards or downwards the wheel was
rotated so that the animals remained stationary relative to the environ-
THE BIOLOGY OF EUPHAUSIIDR
species. Aitken (1960) examined the composition of a surface swarm in
Loch Strangford in Ireland and found the sex ratio within the swarm
to be normal but from his histogram of the distribution of body lengths
it seems to have been predominantly of I and I1 group animals. Surface
swarms of Thysanoessa raschii, T . inermis, T . gregaria, T . longicaudata,
Nyctiphanes australis, and Euphausia krohnii and E . pacijica have been
recorded so that this is a relatively common feature in the behaviour of
euphausiids. I n several instances, e.g. Meganyctiphanes norvegica in
L. Strangford (Aitken, 1960), hydrographic features may have caused
the accumulations but more frequently no cause is so far known.
Euphausia superba, if Marr is correct, is exceptional in this respect
because this is a common feature of its behaviour. At present, we are
only concerned with the fact that E . superba is not continuously distributed throughout its horizontal and vertical environment but is
aggregated into discrete swarms or " patches ". This species appears
to aggregate in swarms throughout every known phase of its development, from the nauplii to the adults, the swarms being autonomous and
moving vertically and horizontally without apparently mixing very
much with each other. Such pronounced patchiness is as yet not
known to occur in other species of euphausiids. There are certain
sea areas where dense populations of a species usually occur, others
where they rarely occur, and attempts a t quantitative sampling can
produce irregular results suggestive of patchy distribution but, compared to E . superba, other species appear to be much more regularly
distributed. This is illustrated by the fact that diurnal vertical
migration of other species can be determined quite easily and EL
repetition of the investigation usually produces very similar results.
One conclusion often quoted is that of Savage (1926) that the main
migration of Nyctiphanes couchii a t night was downwards, although
a small percentage of the population moved upwards to the surface.
This is probably a wrong conclusion from his data and arises because
of errors made in the estimation of the vertical distribution of the
population during the day, a large proportion of the population being
so close to the bottom that his nets did not sample them sufficiently.
The speed at which euphausiids swim upwards towards the surface
can be estimated. Hardy and Bainbridge (1954) constructed an
apparatus in which it was possible to observe and record the vertical
movements of planktonic organisms through several hundred metres.
The apparatus was in the form of an upright hollow wheel which thus
formed a circular tank in the vertical plane and the animals were placed
in it at the side. When they swam upwards or downwards the wheel was
rotated so that the animals remained stationary relative to the environ-