11. HYDROSTATIC PRESSURE
451
played a significant selective role in the evolutionary development of
the many bizarre and specialized patterns of structure and natural
history shown by deep water fishes.
B. Freshwater Forms
The only freshwater lake which is of sufficient geological age, of
sufficient depth and area, and sufficiently aerobic in its deeper parts to
have evolved a truly deep water fish fauna (both pelagic and benthic)
is Lake Baikal. Kozhov (1963) provides the most thorough description
of Baikal available in English. Baikal presently supports 50 species of
fishes representing nine families. One of these families, the cottoid
Comephoridae, is endemic.
Kozhov (1963) calls abyssal those parts of Baikal with depths of 250
meters or more. The two species of Comephoridae are both pelagic and
apparently frequently enter the abyssal zone, reaching depths well in
excess of 500 meters. Both species attain lengths of 18-20 cm, are scaleless and dull glassy and translucent in appearance. They have large
heads and well-developed lateral lines.
The benthic abyssal fish fauna of Baikal is composed entirely of
cottids. The upper abyssal subzone (250500 meters) is occupied by at
least 10 species belonging to five genera. The lower abyssal subzone
(500 meters and below) contains three species, representing two genera,
which appear not to occur in shallower areas. Several of the upper
abyssal species occasionally enter the lower subzone.
The abyssal cottids generally have flabby bodies covered with loose,
thin skin. They have reduced eyes and well-developed lateral lines. Their
bodies are generally whitish or light pale yellow in color, without spots.
Adults of these species generally attain maximum sizes of 10-20 cm.
The abyssal zone of Baikal has temperatures of 3.4"-3.6"C all year
round. Light is practically absent. None of the abyssal fishes is known
to be luminescent. The oxygen content of the deepest water layers is
high, usually 75-90% of saturation. As is the case in the deep oceans, the
biomass of abyssal benthic organisms is only a small fraction of that
in shallower zones.
C. Marine Forms
Deep-sea fishes occur in all oceans and most major adjacent seas.
Grey (1956) and Wolff (1960) list most species known to occur at
depths greater than 2000 meters. They are an extremely diverse group,
451
played a significant selective role in the evolutionary development of
the many bizarre and specialized patterns of structure and natural
history shown by deep water fishes.
B. Freshwater Forms
The only freshwater lake which is of sufficient geological age, of
sufficient depth and area, and sufficiently aerobic in its deeper parts to
have evolved a truly deep water fish fauna (both pelagic and benthic)
is Lake Baikal. Kozhov (1963) provides the most thorough description
of Baikal available in English. Baikal presently supports 50 species of
fishes representing nine families. One of these families, the cottoid
Comephoridae, is endemic.
Kozhov (1963) calls abyssal those parts of Baikal with depths of 250
meters or more. The two species of Comephoridae are both pelagic and
apparently frequently enter the abyssal zone, reaching depths well in
excess of 500 meters. Both species attain lengths of 18-20 cm, are scaleless and dull glassy and translucent in appearance. They have large
heads and well-developed lateral lines.
The benthic abyssal fish fauna of Baikal is composed entirely of
cottids. The upper abyssal subzone (250500 meters) is occupied by at
least 10 species belonging to five genera. The lower abyssal subzone
(500 meters and below) contains three species, representing two genera,
which appear not to occur in shallower areas. Several of the upper
abyssal species occasionally enter the lower subzone.
The abyssal cottids generally have flabby bodies covered with loose,
thin skin. They have reduced eyes and well-developed lateral lines. Their
bodies are generally whitish or light pale yellow in color, without spots.
Adults of these species generally attain maximum sizes of 10-20 cm.
The abyssal zone of Baikal has temperatures of 3.4"-3.6"C all year
round. Light is practically absent. None of the abyssal fishes is known
to be luminescent. The oxygen content of the deepest water layers is
high, usually 75-90% of saturation. As is the case in the deep oceans, the
biomass of abyssal benthic organisms is only a small fraction of that
in shallower zones.
C. Marine Forms
Deep-sea fishes occur in all oceans and most major adjacent seas.
Grey (1956) and Wolff (1960) list most species known to occur at
depths greater than 2000 meters. They are an extremely diverse group,