V I I . DESERT FISHES
411
Some of the higher temperatures tolerated by poikilothermous vertebrates have been recorded for desert fishes, and most field maxima
correspond well to those recorded by various techniques under laboratory
conditions. Numerous records of high temperatures in nature are doubtful
(Miller, 1949b), and it seems likely that any tolerance higher than about
45 °C should be seriously questioned and carefully examined. However,
variations in resistance times of fishes are great and maximum recorded
temperatures are generally highest for the most eurythermal species or
populations (Table I). Maximum constant temperatures occupied by reproducing populations of fishes, presumably comparable to the incipient
lethal temperatures of laboratory studies, rarely exceed 35°C.
A number of pitfalls exist for the unwary investigator of high temperature tolerances of fishes in nature, and description of some of these serves
also to define the ecology of extreme habitats. Vast differences exist in
densities of water of differing temperatures, and stratified flows often may
be detected in springs or their outlets. Miller (1949b), for example, found
a molly (Mollinesia [ = Poecilia] sphenops) at Lake Amatitlân, Guatemala, living in waters of 33.3°-35.6°C. Temperatures as hot as 49°C
were recorded in water flowing over them at the surface, and local people
were convinced that the fish were living at "boiling" water temperatures.
In outflows of hot springs, temperatures are typically warmest in the
main current and noticably cooler along the banks. Fishes will move upstream along the shoreline, and temperatures in that area will often be
1°-3°C or more cooler than in the channel. The reverse situation also
often occurs with springhead fishes that move downstream in the channel
of larger outflows. Temperatures along the shorelines in these situations may
be substantially lower than any recorded before for the species. This is
especially true in larger rios of the Cuatro Ciénegas basin, where Cyprinodon bifasciatus moves long distances from the springs in summer, but dies
or moves back into the immediate area of thermal water in winter. It is
interesting that a similar dispersion relative to minor temperature differences is demonstrated by thermophilic algae and other microorganisms in
hot springs (Brock, 1967).
Shading may also create microdifferences in water temperatures (Young
and Zimmerman, 1956) that are used to advantage by fishes. Barlow
(1958a) found one shaded area of less than 100 cm
2
in an isolated pool
along the Salton Sea, California, that remained a full 2°C cooler than water
in full sunlight. An undescribed Cyprinodon from Cottonball Marsh in
Death Valley seems to retreat beneath shelves of encrusting salts, where
water temperatures may be a few degrees cooler than in the open water
of their severe habitat. Air temperatures in that area range as high as
55°C, and the species may consistently inhabit the most severe habitat
411
Some of the higher temperatures tolerated by poikilothermous vertebrates have been recorded for desert fishes, and most field maxima
correspond well to those recorded by various techniques under laboratory
conditions. Numerous records of high temperatures in nature are doubtful
(Miller, 1949b), and it seems likely that any tolerance higher than about
45 °C should be seriously questioned and carefully examined. However,
variations in resistance times of fishes are great and maximum recorded
temperatures are generally highest for the most eurythermal species or
populations (Table I). Maximum constant temperatures occupied by reproducing populations of fishes, presumably comparable to the incipient
lethal temperatures of laboratory studies, rarely exceed 35°C.
A number of pitfalls exist for the unwary investigator of high temperature tolerances of fishes in nature, and description of some of these serves
also to define the ecology of extreme habitats. Vast differences exist in
densities of water of differing temperatures, and stratified flows often may
be detected in springs or their outlets. Miller (1949b), for example, found
a molly (Mollinesia [ = Poecilia] sphenops) at Lake Amatitlân, Guatemala, living in waters of 33.3°-35.6°C. Temperatures as hot as 49°C
were recorded in water flowing over them at the surface, and local people
were convinced that the fish were living at "boiling" water temperatures.
In outflows of hot springs, temperatures are typically warmest in the
main current and noticably cooler along the banks. Fishes will move upstream along the shoreline, and temperatures in that area will often be
1°-3°C or more cooler than in the channel. The reverse situation also
often occurs with springhead fishes that move downstream in the channel
of larger outflows. Temperatures along the shorelines in these situations may
be substantially lower than any recorded before for the species. This is
especially true in larger rios of the Cuatro Ciénegas basin, where Cyprinodon bifasciatus moves long distances from the springs in summer, but dies
or moves back into the immediate area of thermal water in winter. It is
interesting that a similar dispersion relative to minor temperature differences is demonstrated by thermophilic algae and other microorganisms in
hot springs (Brock, 1967).
Shading may also create microdifferences in water temperatures (Young
and Zimmerman, 1956) that are used to advantage by fishes. Barlow
(1958a) found one shaded area of less than 100 cm
2
in an isolated pool
along the Salton Sea, California, that remained a full 2°C cooler than water
in full sunlight. An undescribed Cyprinodon from Cottonball Marsh in
Death Valley seems to retreat beneath shelves of encrusting salts, where
water temperatures may be a few degrees cooler than in the open water
of their severe habitat. Air temperatures in that area range as high as
55°C, and the species may consistently inhabit the most severe habitat