11. HYDROSTATIC PRESSURE
453
Bathypelagic fishes are often more uniformly dark in color than
mesopelagic forms. Many groups are not known to be luminescent.
Species restricted to depths below 1000 meters universally lack a gasfilled swim bladder ( Marshall, 1960).
Not all meso- and bathypelagic fishes are vertically migratory, at least
as adults. However, these groups include many forms which have epipelagic eggs and larvae; hence, they must normally make at least one
extended migration all the way up to the surface layers (Mead et al.,
1964 ) .
Many kinds of meso- and bathypelagic fishes, mostly predatory species,
have greatly reduced skeletons and trunk musculature, as compared
with epipelagic fishes. Marshall (1960) and Denton (1961) argue that
these reductions are primarily adaptations of buoyancy control mechanisms in response to restricted food supplies in the deep-sea midwater
environment. The systematic distribution of these reductions correlates
well with the absence of gas-filled swim bladders, the maintenance of
which both authors consider to be energetically expensive. If this hypothesis is correct, most of the major structural differences between epi-,
meso-, and bathypelagic fishes would appear to be primarily responses
to environmental factors other than hydrostatic pressure.
Much the same situation may hold for abyssal and hadal benthic
fishes as well. These latter fishes are generally larger and more solidly
constructed than the pelagic forms. Associated with this circumstance
is the occurrence of gas-filled swim bladders in more than half the
species, including some which occur to depths of 5000 meters (Marshall,
1960). The food supply on and near the bottom appears to be much
greater than in midwater (although still much less than in shallow
water environments). Many other morphological features of these
fishes seem explicable in terms of requirements for food and mate location, species recognition, and reproduction (Mead et al., 1964).
Except for hydrostatic pressure, the physical features of the abyssal
environment are very uniform (Menzies, 1965). In all oceans, at latitudes
between 50"N and 58"S, the temperature range at depths below 2000
meters is only from -0.6' to 38°C. Salinities are universally near 3436,.
with variations of less than 0.496, from this figure. Sunlight does not
penetrate below 1000 meters, but measurable bioluminescent light intensities occur at most greater depths. Oxygen concentrations are generally high, anaerobic conditions being restricted to a few enclosed or
semi-enclosed basins.
The abyssal environment has generally bcen considercd to ha\7e bcen
as uniform in its properties over time as it spatially is now. Russian
ichthyologists especially have based theories of the evolutionary history
of the deep-sea fauna upon this presumption (Rass, 1959). However,
453
Bathypelagic fishes are often more uniformly dark in color than
mesopelagic forms. Many groups are not known to be luminescent.
Species restricted to depths below 1000 meters universally lack a gasfilled swim bladder ( Marshall, 1960).
Not all meso- and bathypelagic fishes are vertically migratory, at least
as adults. However, these groups include many forms which have epipelagic eggs and larvae; hence, they must normally make at least one
extended migration all the way up to the surface layers (Mead et al.,
1964 ) .
Many kinds of meso- and bathypelagic fishes, mostly predatory species,
have greatly reduced skeletons and trunk musculature, as compared
with epipelagic fishes. Marshall (1960) and Denton (1961) argue that
these reductions are primarily adaptations of buoyancy control mechanisms in response to restricted food supplies in the deep-sea midwater
environment. The systematic distribution of these reductions correlates
well with the absence of gas-filled swim bladders, the maintenance of
which both authors consider to be energetically expensive. If this hypothesis is correct, most of the major structural differences between epi-,
meso-, and bathypelagic fishes would appear to be primarily responses
to environmental factors other than hydrostatic pressure.
Much the same situation may hold for abyssal and hadal benthic
fishes as well. These latter fishes are generally larger and more solidly
constructed than the pelagic forms. Associated with this circumstance
is the occurrence of gas-filled swim bladders in more than half the
species, including some which occur to depths of 5000 meters (Marshall,
1960). The food supply on and near the bottom appears to be much
greater than in midwater (although still much less than in shallow
water environments). Many other morphological features of these
fishes seem explicable in terms of requirements for food and mate location, species recognition, and reproduction (Mead et al., 1964).
Except for hydrostatic pressure, the physical features of the abyssal
environment are very uniform (Menzies, 1965). In all oceans, at latitudes
between 50"N and 58"S, the temperature range at depths below 2000
meters is only from -0.6' to 38°C. Salinities are universally near 3436,.
with variations of less than 0.496, from this figure. Sunlight does not
penetrate below 1000 meters, but measurable bioluminescent light intensities occur at most greater depths. Oxygen concentrations are generally high, anaerobic conditions being restricted to a few enclosed or
semi-enclosed basins.
The abyssal environment has generally bcen considercd to ha\7e bcen
as uniform in its properties over time as it spatially is now. Russian
ichthyologists especially have based theories of the evolutionary history
of the deep-sea fauna upon this presumption (Rass, 1959). However,