430
JAMES E. DEACON AND W. L MINCKLEY
throughout the day, and such substrates are generally avoided by other
than certain invertebrates (chironomid dipterans, some oligochaetes)
capable of essentially anaerobic existence. The ova of cyprinodontids that
are deposited within such bottom sediments must be capable of resisting
considerable periods of anoxia.
In clear desert pools and in springs, gas production by algae and other
photosynthetic organisms in or on bottom sediments may be very high
(Minckley and Itzkowitz, 1967; Arnold, 1970). However, this may not
contribute oxygen to the water, and may actually tend to reduce supersaturations by creating vertical turbulence, or by modifying other gases
through phenomena of partial pressure adjustments in the rising bubbles.
Accumulation of gases in sediments have been known to be an indirect
cause of mortality of pupfishes. Ingested bubbles caused the fish to float,
where they were exposed to sunburn and to predators (Minckley and Itzkowitz, 1967). Algal flotation in periods of high insolation may "raft" food
organisms (including algae) into areas where they are more available to
fishes as food, or may provide cover and additional substrate for certain
isolated populations, such as the Devil's Hole pupfish (James, 1969).
The springfishes, Crenichthys baileyi and C. nevadae, have been examined more thoroughly than other desert fishes with regard to their oxygen relations. Minimum environmental oxygen levels to which these species
are exposed range from 0.5 mg/liter upward (Table II). Extended exposure
of C. nevadae to oxygen levels below 0.7 mg/liter at 32°C is lethal, and
normal activity at that temperature appears to be suppressed by oxygen
levels below about 2 mg/liter (Hubbs et al., 1967). Such amounts therefore approximate the level of no excess activity and the incipient limiting
point, respectively, for C. nevadae at about 32°C. Similar levels are suggested for C. baileyi by Hubbs and Hettler (1964), Hubbs et al (1967),
and Deacon and Wilson (1967). Data presented by Sumner^and Sargent
(1940), Sumner and Lanham (1942), and Brues (1928, 1932) on environmental conditions show that C. baileyi has been living under the conditions of low oxygen and high temperature for a long period of time (and
probably for millenia). The fish move relative to dissolved oxygen content
of the waters, and therefore remain in relatively amenable conditions
through behavioral regulations (Section IV,A,5).
Crenichthys appears to have a markedly enlarged head, which is broad
in the opercular region, and this is especially noticeable in larger specimens. Such enlargement accommodates exceptionally well-developed and
proportionately large gills, an apparent advantage to a fish living in a lowoxygen environment. Measurements of gill surface area in C. baileyi range
from about 200 to 850 mm
2
/gm body weight. By comparison, the streamdwelling minnow Lepidomeda mollispinis has a gill surface area ranging
JAMES E. DEACON AND W. L MINCKLEY
throughout the day, and such substrates are generally avoided by other
than certain invertebrates (chironomid dipterans, some oligochaetes)
capable of essentially anaerobic existence. The ova of cyprinodontids that
are deposited within such bottom sediments must be capable of resisting
considerable periods of anoxia.
In clear desert pools and in springs, gas production by algae and other
photosynthetic organisms in or on bottom sediments may be very high
(Minckley and Itzkowitz, 1967; Arnold, 1970). However, this may not
contribute oxygen to the water, and may actually tend to reduce supersaturations by creating vertical turbulence, or by modifying other gases
through phenomena of partial pressure adjustments in the rising bubbles.
Accumulation of gases in sediments have been known to be an indirect
cause of mortality of pupfishes. Ingested bubbles caused the fish to float,
where they were exposed to sunburn and to predators (Minckley and Itzkowitz, 1967). Algal flotation in periods of high insolation may "raft" food
organisms (including algae) into areas where they are more available to
fishes as food, or may provide cover and additional substrate for certain
isolated populations, such as the Devil's Hole pupfish (James, 1969).
The springfishes, Crenichthys baileyi and C. nevadae, have been examined more thoroughly than other desert fishes with regard to their oxygen relations. Minimum environmental oxygen levels to which these species
are exposed range from 0.5 mg/liter upward (Table II). Extended exposure
of C. nevadae to oxygen levels below 0.7 mg/liter at 32°C is lethal, and
normal activity at that temperature appears to be suppressed by oxygen
levels below about 2 mg/liter (Hubbs et al., 1967). Such amounts therefore approximate the level of no excess activity and the incipient limiting
point, respectively, for C. nevadae at about 32°C. Similar levels are suggested for C. baileyi by Hubbs and Hettler (1964), Hubbs et al (1967),
and Deacon and Wilson (1967). Data presented by Sumner^and Sargent
(1940), Sumner and Lanham (1942), and Brues (1928, 1932) on environmental conditions show that C. baileyi has been living under the conditions of low oxygen and high temperature for a long period of time (and
probably for millenia). The fish move relative to dissolved oxygen content
of the waters, and therefore remain in relatively amenable conditions
through behavioral regulations (Section IV,A,5).
Crenichthys appears to have a markedly enlarged head, which is broad
in the opercular region, and this is especially noticeable in larger specimens. Such enlargement accommodates exceptionally well-developed and
proportionately large gills, an apparent advantage to a fish living in a lowoxygen environment. Measurements of gill surface area in C. baileyi range
from about 200 to 850 mm
2
/gm body weight. By comparison, the streamdwelling minnow Lepidomeda mollispinis has a gill surface area ranging