V I I . DESERT FISHES
431
from about 50 to 200 mm
2
/gm body weight. Comparisons of individuals
of the two species having the same weight indicate that C. baileyi consistently has 50-200% more gill surface area than does the minnow. Small
individuals of both species have proportionately more gill surface area than
larger individuals.
This last relationship probably exists in most fishes and may partially
explain the general trend that juvenile to subadult fish may survive adversity longer than large, mature individuals. Comparisons of survival in
nature have not been made. However, preliminary laboratory data suggest
that the range of 0.9-1.2 mg/liter dissolved oxygen at 15-20°C approximates the level of no excess activity for Lepidomeda. Survival, in fact
swimming activity, of C. nevadae in 0.8 mg/liter dissolved oxygen at
32.3°C (Hubbs et al., 1967), and the indication that C. baileyi lives in
Mormon Spring, Nevada, at very low levels of oxygen concentration
( < ~ 1.0 mg/liter; Deacon and Wilson, 1967), suggests that Crenichthys
would undergo little stress under a combination of oxygen and temperature
that is fatal to Lepidomeda. A situation such as this appears to obtain
in some Nevada springs now infested with introduced poeciliids.
Crenichthys lives essentially undisturbed in the low-oxygen, high-temperature springheads, and the poeciliids occur only downflow, in less severe
conditions (Hubbs and Hettler, 1964).
b. Carbon Dioxide. Hutchinson (1957) discussed the complex chemical
relations of carbon dioxide in water, and in 1967 he reviewed its role in
the phytoplankton. After several workers established that free carbon dioxide in water was of primary significance to fishes, data were reviewed and
evaluated by Doudoroff and Katz (1950), Doudoroff (1957), and Jones
(1964). Desert habitats, as with most open waters, rarely contain concentrations of free carbon dioxide sufficiently high to create problems for
fishes. The highest contents usually are in springs, where in most instances
high dissolved solids (especially carbonates) tends to buffer any direct
effects of the gas. Cole (1968), however, reported presence of free carbon
dioxide in amounts up to 600 mg/liter in Montezuma Well, Arizona, a
large limnocrene, and suggested that the absence of fishes might relate to the
high concentrations present. According to local testimony, fishes introduced into the well die quickly. Doudoroff (1957) considered
~ 2 0 0 mg/liter as the approximate upper lethal concentration for moderately susceptible freshwater fishes.
An inverse relationship exists between oxygen and carbon dioxide tolerance in fishes (Jones, 1964); the ability of a fish to utilize oxygen seems
to decrease as concentration of carbon dioxide increases. Levels of free
carbon dioxide necessary to cause a marked change in oxygen utilization
range widely, from a partial pressure of ~ 1 0 mm Hg in trout to more
431
from about 50 to 200 mm
2
/gm body weight. Comparisons of individuals
of the two species having the same weight indicate that C. baileyi consistently has 50-200% more gill surface area than does the minnow. Small
individuals of both species have proportionately more gill surface area than
larger individuals.
This last relationship probably exists in most fishes and may partially
explain the general trend that juvenile to subadult fish may survive adversity longer than large, mature individuals. Comparisons of survival in
nature have not been made. However, preliminary laboratory data suggest
that the range of 0.9-1.2 mg/liter dissolved oxygen at 15-20°C approximates the level of no excess activity for Lepidomeda. Survival, in fact
swimming activity, of C. nevadae in 0.8 mg/liter dissolved oxygen at
32.3°C (Hubbs et al., 1967), and the indication that C. baileyi lives in
Mormon Spring, Nevada, at very low levels of oxygen concentration
( < ~ 1.0 mg/liter; Deacon and Wilson, 1967), suggests that Crenichthys
would undergo little stress under a combination of oxygen and temperature
that is fatal to Lepidomeda. A situation such as this appears to obtain
in some Nevada springs now infested with introduced poeciliids.
Crenichthys lives essentially undisturbed in the low-oxygen, high-temperature springheads, and the poeciliids occur only downflow, in less severe
conditions (Hubbs and Hettler, 1964).
b. Carbon Dioxide. Hutchinson (1957) discussed the complex chemical
relations of carbon dioxide in water, and in 1967 he reviewed its role in
the phytoplankton. After several workers established that free carbon dioxide in water was of primary significance to fishes, data were reviewed and
evaluated by Doudoroff and Katz (1950), Doudoroff (1957), and Jones
(1964). Desert habitats, as with most open waters, rarely contain concentrations of free carbon dioxide sufficiently high to create problems for
fishes. The highest contents usually are in springs, where in most instances
high dissolved solids (especially carbonates) tends to buffer any direct
effects of the gas. Cole (1968), however, reported presence of free carbon
dioxide in amounts up to 600 mg/liter in Montezuma Well, Arizona, a
large limnocrene, and suggested that the absence of fishes might relate to the
high concentrations present. According to local testimony, fishes introduced into the well die quickly. Doudoroff (1957) considered
~ 2 0 0 mg/liter as the approximate upper lethal concentration for moderately susceptible freshwater fishes.
An inverse relationship exists between oxygen and carbon dioxide tolerance in fishes (Jones, 1964); the ability of a fish to utilize oxygen seems
to decrease as concentration of carbon dioxide increases. Levels of free
carbon dioxide necessary to cause a marked change in oxygen utilization
range widely, from a partial pressure of ~ 1 0 mm Hg in trout to more