V I I . DESERT FISHES
429
ity would be more appropriate. Such is suggested by data presented by
Lowe et al (1967) for the desert pupfish. In addition to its remarkable
ability to survive low dissolved oxygen, that species apparently moved less
than any other used in an experiment in catastrophic selection. Three other
species, two minnows and a sucker, shared the experimental tank. The
sucker had physiological tolerance to low oxygen second only to the pupfish. Because of its heavy body that required considerable effort to move,
its increased movements under stress, and its inferior mouth utilization of
atmospheric oxygen was hampered. Hence the sucker was the least capable
of surviving of all species tested.
Minimum tolerances and LD 50 have been determined for several fish
species, some of which live in deserts. The lowest minima known, 0.13
mg/liter and 0.22 mg/liter oxygen, respectively, are for the desert pupfish
(Lowe et al., 1967). Many other species of Cyprinodon most likely survive
similarly low concentrations, but they are yet to be studied. These species
dive into the reducing, odoriferous mud bottoms of their habitats, as has
been mentioned before. Since hydrogen sulfide is readily oxidized, but occurs
in considerable volume in the malodorous sediments, such substrates must
be essentially devoid of oxygen. The fish frequently bury themselves completely, but sometimes remain with their mouths or entire heads exposed.
Fish in the last position may obtain oxygen through normal respiratory
movements, or abnormally when the opercles are blocked. Some individuals, however, have been found 2 cm below the mud-water interface
and certainly could not obtain water from above the mud surface. Blazka
(1958) and Eckberg (1962) reported an ability of crucian carp (Carassius
carassius) to survive for long periods during winter under anaerobic conditions. It seems likely that Cyprinodon may share such an ability.
Minimum levels of environmental oxygen, along with ambient temperatures when available, in which desert fishes are known to have survived
are summarized in Tables I and II. These data obviously provide no indication of survival (or resistance) times at the recorded levels, and such
may be an important consideration that allows survival through a period
of temporary environmental stress that would prove lethal if it were
sustained.
Diel changes in oxygen content of open spring waters rarely involve decreases below critical levels (Deacon and Wilson, 1967; Arnold, 1970),
although they may sometimes be of a magnitude sufficient to elicit behavioral adjustments in fishes (Section IV,A,5). However, in smaller
pools, changes from below 10% saturation to more than 150% may obtain; oxygen depletion most likely occurs at the mud-water interface or
beneath beds of aquatic plants, at least at night. In the latter habitat, bottoms often indicate reducing conditions at the mud-water interface
429
ity would be more appropriate. Such is suggested by data presented by
Lowe et al (1967) for the desert pupfish. In addition to its remarkable
ability to survive low dissolved oxygen, that species apparently moved less
than any other used in an experiment in catastrophic selection. Three other
species, two minnows and a sucker, shared the experimental tank. The
sucker had physiological tolerance to low oxygen second only to the pupfish. Because of its heavy body that required considerable effort to move,
its increased movements under stress, and its inferior mouth utilization of
atmospheric oxygen was hampered. Hence the sucker was the least capable
of surviving of all species tested.
Minimum tolerances and LD 50 have been determined for several fish
species, some of which live in deserts. The lowest minima known, 0.13
mg/liter and 0.22 mg/liter oxygen, respectively, are for the desert pupfish
(Lowe et al., 1967). Many other species of Cyprinodon most likely survive
similarly low concentrations, but they are yet to be studied. These species
dive into the reducing, odoriferous mud bottoms of their habitats, as has
been mentioned before. Since hydrogen sulfide is readily oxidized, but occurs
in considerable volume in the malodorous sediments, such substrates must
be essentially devoid of oxygen. The fish frequently bury themselves completely, but sometimes remain with their mouths or entire heads exposed.
Fish in the last position may obtain oxygen through normal respiratory
movements, or abnormally when the opercles are blocked. Some individuals, however, have been found 2 cm below the mud-water interface
and certainly could not obtain water from above the mud surface. Blazka
(1958) and Eckberg (1962) reported an ability of crucian carp (Carassius
carassius) to survive for long periods during winter under anaerobic conditions. It seems likely that Cyprinodon may share such an ability.
Minimum levels of environmental oxygen, along with ambient temperatures when available, in which desert fishes are known to have survived
are summarized in Tables I and II. These data obviously provide no indication of survival (or resistance) times at the recorded levels, and such
may be an important consideration that allows survival through a period
of temporary environmental stress that would prove lethal if it were
sustained.
Diel changes in oxygen content of open spring waters rarely involve decreases below critical levels (Deacon and Wilson, 1967; Arnold, 1970),
although they may sometimes be of a magnitude sufficient to elicit behavioral adjustments in fishes (Section IV,A,5). However, in smaller
pools, changes from below 10% saturation to more than 150% may obtain; oxygen depletion most likely occurs at the mud-water interface or
beneath beds of aquatic plants, at least at night. In the latter habitat, bottoms often indicate reducing conditions at the mud-water interface