5. VERTICAL DISTRIBUTION AND MIGRATION
161
not very conclusive in the case of Thysanoessa inermis. The results for
T. raschii are extraordinary because Ponomareva found the largest
animals at the shallowest depths. In the Firth of Clyde, although no
regular spatial distribution of size classes of this species is present
throughout the diurnal vertical migration, there is a tendency for the
largest animals of this species to be caught in the deepest areas of the
Clyde during the day while smaller animals may be taken in shallower
water closer inshore. Baker (1959) shows that larger Euphausia
triacantha occur deeper in the sea than the smaller ones, and there is
evidence that adolescent Thysanopoda acutifrons live above adult
T . acutifrons but more conclusive data are required for this species
(Einarsson, 1945).
Cushing (1951) suggests that " some animals become more sensitive
to light as they grow older and others become less so ". He presents
evidence from other groups, mainly Copepoda, and some contradictory
results are quoted. The evidence for this layering of size-groups of
euphausiids is much less contradictory; indeed, except for the data
quoted by Ponomareva (1959b) for Thysanoessa raschii (Table V), there
are strong reasons for suggesting that this may be a feature of the
vertical distribution of many species of euphausiids. Exceptions, of
course, are no doubt inevitable in any such attempted generalization
and probably the most notable exception will be found to be Euphausia
superba, a species which lives in the surface layers. We are not now
comparing the vertical distribution of larvae to that of adults but the
differences in the distributions of different sizes (or ages) of the adults
themselves. Naturally, the vertical distribution of the young adolescents is likely, in a deep-living species like Thysanopoda acutifrons that
produces larvae that live in the surface layers, to be influenced by the
distribution of the late larval stages. This assumption is made because
there is no reason at present for suggesting any major physiological or
other differences between late larvae and early adolescents. This being
so, radical changes in the behaviour of late larval stages when they
become adolescents would not be expected. Consequently, when larval
stages are surface-living organisms and adults are deep-living, a gradual
change in the layer of maximum occurrence of a batch of larvae or
adolescents could be expected as they approach maturity. Such a
regular change of the habitat of a shallow, coastal living species
could probably not be expected where the vertical scale of distribution
is small and where there may be mixing of deeper and shallower layers
of water leading to almost uniform vertical distribution of larval stages.
This would also apply to oceanic species which live, as Euphausia
superba does, in a surface layer of restricted vertical dimensions.
161
not very conclusive in the case of Thysanoessa inermis. The results for
T. raschii are extraordinary because Ponomareva found the largest
animals at the shallowest depths. In the Firth of Clyde, although no
regular spatial distribution of size classes of this species is present
throughout the diurnal vertical migration, there is a tendency for the
largest animals of this species to be caught in the deepest areas of the
Clyde during the day while smaller animals may be taken in shallower
water closer inshore. Baker (1959) shows that larger Euphausia
triacantha occur deeper in the sea than the smaller ones, and there is
evidence that adolescent Thysanopoda acutifrons live above adult
T . acutifrons but more conclusive data are required for this species
(Einarsson, 1945).
Cushing (1951) suggests that " some animals become more sensitive
to light as they grow older and others become less so ". He presents
evidence from other groups, mainly Copepoda, and some contradictory
results are quoted. The evidence for this layering of size-groups of
euphausiids is much less contradictory; indeed, except for the data
quoted by Ponomareva (1959b) for Thysanoessa raschii (Table V), there
are strong reasons for suggesting that this may be a feature of the
vertical distribution of many species of euphausiids. Exceptions, of
course, are no doubt inevitable in any such attempted generalization
and probably the most notable exception will be found to be Euphausia
superba, a species which lives in the surface layers. We are not now
comparing the vertical distribution of larvae to that of adults but the
differences in the distributions of different sizes (or ages) of the adults
themselves. Naturally, the vertical distribution of the young adolescents is likely, in a deep-living species like Thysanopoda acutifrons that
produces larvae that live in the surface layers, to be influenced by the
distribution of the late larval stages. This assumption is made because
there is no reason at present for suggesting any major physiological or
other differences between late larvae and early adolescents. This being
so, radical changes in the behaviour of late larval stages when they
become adolescents would not be expected. Consequently, when larval
stages are surface-living organisms and adults are deep-living, a gradual
change in the layer of maximum occurrence of a batch of larvae or
adolescents could be expected as they approach maturity. Such a
regular change of the habitat of a shallow, coastal living species
could probably not be expected where the vertical scale of distribution
is small and where there may be mixing of deeper and shallower layers
of water leading to almost uniform vertical distribution of larval stages.
This would also apply to oceanic species which live, as Euphausia
superba does, in a surface layer of restricted vertical dimensions.