106
GORDON A. RILEY
indicated earlier, there are likely to be micro-concentrations of food in
convective downdrafts that are not clearly demonstrated by routine
sampling procedures.
Most of the mid-depth organisms in this area perform long diurnal
migrations of some hundreds of metres. Varying food habits are represented. The herbivorous fraction presumably gets most of its nourishment in the surface layer at night. At the daytime level there is
probably little more particulate food available than in the deep sea,
although there may be layering in small discontinuities in the main
thermocline as indicated earlier.
Food relations of bathypelagic animals have been discussed by
many authors, but this remains as one of the most poorly understood
problems in biological oceanography. Early investigators wrote that
deep sea animals are fed by a " rain of detritus from the surface layer )',
a phrase that has been quoted too often, for it is a superficiality that
tends to sweep the problem under the rug. It is presumably a simplistic
expression of the valid concept that all deep sea life is supported
directly or indirectly by surface production, but the statement was
made with essentially no knowledge of the quantity or food value of
detritus sinking from the surface, or of the further transformations
that must occur in order to develop the observed assemblage of deep
sea animals.
A considerable proportion of this assemblage is of course carnivorous
or necrophagous. Wheeler (1967) has found that dead zooplankton
constitutes a significant fraction of total net catches in deep water.
This is not generally the case in near-surface catches, where presumably
the population is sufficiently concentrated so that any animals dying a
natural death will be consumed very quickly. The implication in
Wheeler's finding is that bathypelagic organisms which die a natural
death may not be found and eaten immediately and in the meantime
will sink into deeper water. Even in the case of capture of living prey,
vertical movement of animals and random capture at different depths
can lead to a sort of " biological diffusion " of food materials from middepths down into the sparser deep waters, a concept mentioned by
Riley (1951) and developed more fully by Vinogradov (1962).
Some bathypelagic copepods have foliaceous appendages which
appear to be suitable for a filter feeding habit. Animals with mouthparts of this type probably are more common than has been suspected,
because most of the collections of deep water zooplankton have been
made with coarse nets that do not catch small copepods effectively.
However, it remains an open question as to whether indiscriminate
filter feeding in the bathypelagic zone will supply an adequate amount
GORDON A. RILEY
indicated earlier, there are likely to be micro-concentrations of food in
convective downdrafts that are not clearly demonstrated by routine
sampling procedures.
Most of the mid-depth organisms in this area perform long diurnal
migrations of some hundreds of metres. Varying food habits are represented. The herbivorous fraction presumably gets most of its nourishment in the surface layer at night. At the daytime level there is
probably little more particulate food available than in the deep sea,
although there may be layering in small discontinuities in the main
thermocline as indicated earlier.
Food relations of bathypelagic animals have been discussed by
many authors, but this remains as one of the most poorly understood
problems in biological oceanography. Early investigators wrote that
deep sea animals are fed by a " rain of detritus from the surface layer )',
a phrase that has been quoted too often, for it is a superficiality that
tends to sweep the problem under the rug. It is presumably a simplistic
expression of the valid concept that all deep sea life is supported
directly or indirectly by surface production, but the statement was
made with essentially no knowledge of the quantity or food value of
detritus sinking from the surface, or of the further transformations
that must occur in order to develop the observed assemblage of deep
sea animals.
A considerable proportion of this assemblage is of course carnivorous
or necrophagous. Wheeler (1967) has found that dead zooplankton
constitutes a significant fraction of total net catches in deep water.
This is not generally the case in near-surface catches, where presumably
the population is sufficiently concentrated so that any animals dying a
natural death will be consumed very quickly. The implication in
Wheeler's finding is that bathypelagic organisms which die a natural
death may not be found and eaten immediately and in the meantime
will sink into deeper water. Even in the case of capture of living prey,
vertical movement of animals and random capture at different depths
can lead to a sort of " biological diffusion " of food materials from middepths down into the sparser deep waters, a concept mentioned by
Riley (1951) and developed more fully by Vinogradov (1962).
Some bathypelagic copepods have foliaceous appendages which
appear to be suitable for a filter feeding habit. Animals with mouthparts of this type probably are more common than has been suspected,
because most of the collections of deep water zooplankton have been
made with coarse nets that do not catch small copepods effectively.
However, it remains an open question as to whether indiscriminate
filter feeding in the bathypelagic zone will supply an adequate amount