168
THE BIOLOGY OF EUPHAWSIIDS
Euphausia pacijica feed mostly in the evening and night during the
winter and spring when they are feeding more actively in the Sea of
Japan. The general conclusions which the present authors draw from
the small amount of data available are that these animals show little
diurnal change in feeding intensities, that they do not migrate vertically
every 24 hr for the exclusive reason of obtaining food, and that they
eat what is available in the part of the water column in which they are
resident at that time. This means, in effect, that there is a change in
their diet when they migrate upwards a t night because organisms such
as tintinnids and dinoflagellates occur mostly in the surface layers and
are fed on by euphausiids which in the depths during the day are
frequently feeding on pelagic organisms or, if in continental slope areas,
on bottom material and/or organisms living close to the sea bed. These
diurnal changes in diet will be discussed in the chapter on food and
feeding methods of these organisms.
What other reasons would cause the evolutionary development of a
diurnal vertical migration? Reviews of the vertical migrations of
marine crustaceans in particular have been presented recently by
Cushing (1951), Hardy (1956), and Bainbridge (1961); David (1961),
Wynne-Edwards (1962) and McLaren (1963) consider the adaptive or
evolutionary significance of these migrations. McLaren criticizes the
ideas of the previous five authors and it might be best to examine his
criticisms and also the theory he himself presents to explain the significance of this phenomenon. Diurnal vertical migration is not
a prerogative of euphausiids but is performed by representatives
of most groups within the plankton; further, certain bottom
living organisms also perform a vertical migration. However,
we consider that the significance of the migration to one species
need not necessarily be the same to another. It may be that
it evolved through the same pressures acting on the species but
secondary adaptations may now outweigh the original reasons for its
development.
Most workers agree that light and water temperature are the
parameters of the environment having the greatest effect on vertical
migrations of organisms. Light appears to be more important than
temperature. Huntsman (1925) and Mauchline (unpublished) have
shown that Meganyctiphanes norvegica and Thysanoessa raschii kept in
darkness live much longer than when kept in the light. As McLaren
points out, this lethality of sunlight would only act in the immediate
surface waters where other factors, such as visibility to predators, would
probably be of more importance to the organism in escaping. McLaren
admits that prey organisms are probably still visible to their predators
THE BIOLOGY OF EUPHAWSIIDS
Euphausia pacijica feed mostly in the evening and night during the
winter and spring when they are feeding more actively in the Sea of
Japan. The general conclusions which the present authors draw from
the small amount of data available are that these animals show little
diurnal change in feeding intensities, that they do not migrate vertically
every 24 hr for the exclusive reason of obtaining food, and that they
eat what is available in the part of the water column in which they are
resident at that time. This means, in effect, that there is a change in
their diet when they migrate upwards a t night because organisms such
as tintinnids and dinoflagellates occur mostly in the surface layers and
are fed on by euphausiids which in the depths during the day are
frequently feeding on pelagic organisms or, if in continental slope areas,
on bottom material and/or organisms living close to the sea bed. These
diurnal changes in diet will be discussed in the chapter on food and
feeding methods of these organisms.
What other reasons would cause the evolutionary development of a
diurnal vertical migration? Reviews of the vertical migrations of
marine crustaceans in particular have been presented recently by
Cushing (1951), Hardy (1956), and Bainbridge (1961); David (1961),
Wynne-Edwards (1962) and McLaren (1963) consider the adaptive or
evolutionary significance of these migrations. McLaren criticizes the
ideas of the previous five authors and it might be best to examine his
criticisms and also the theory he himself presents to explain the significance of this phenomenon. Diurnal vertical migration is not
a prerogative of euphausiids but is performed by representatives
of most groups within the plankton; further, certain bottom
living organisms also perform a vertical migration. However,
we consider that the significance of the migration to one species
need not necessarily be the same to another. It may be that
it evolved through the same pressures acting on the species but
secondary adaptations may now outweigh the original reasons for its
development.
Most workers agree that light and water temperature are the
parameters of the environment having the greatest effect on vertical
migrations of organisms. Light appears to be more important than
temperature. Huntsman (1925) and Mauchline (unpublished) have
shown that Meganyctiphanes norvegica and Thysanoessa raschii kept in
darkness live much longer than when kept in the light. As McLaren
points out, this lethality of sunlight would only act in the immediate
surface waters where other factors, such as visibility to predators, would
probably be of more importance to the organism in escaping. McLaren
admits that prey organisms are probably still visible to their predators