V I I . DESERT FISHES
405
bottom-dwelling fish-food organisms, or may physically smother fish eggs
or larvae (Starrett, 1950a, 1951).
Other adaptations to, or against, suspended solids in natural waters include development of barbels and other types of cutaneous sense organs
for increase in the proportion of nonvisual sensory input (Moore, 1950;
Branson, 1966), sometimes a considerable reduction in the exposed surface of the eye (Moore, 1950), development of special surfaces within
the eye for concentration of light (Moore, 1944a), and reductions in
squamation, or development of a leathery skin with minute, embedded
scales, the last two presumably in response to the abrasive nature of certain highly turbid waters (Hubbs, 1940, 1941a).
Morphology of the brain in fishes also varies according to their mode
of feeding (Miller and Evans, 1965; Davis and Miller, 1967), which is
related to the level of light in their environment that is, in turn, a partial
function of turbidity. It is interesting in this respect that the morphology of
brains of fishes from relatively constant habitats, whether they are perennially turbid or typically clear, tends to be far less variable than that from
fishes living in markedly variable habitats (Davis and Miller, 1967). This
tendency was found within the species level when subspecies of wide-ranging forms were compared, and among species.
As with flooding, tremendous increases in turbidity pose severe, immediate problems for fishes, and they similarly tend to avoid it or to move
to areas where current assists in minimizing its physical effects (Trautman,
1957).
3. Drought and Desiccation
Downstream displacement of fishes by flood, recorded in some large
rivers of more mesic zones (Viosca, in Trautman, 1957), may have fatal
results under desert conditions. Fish populations from streams tributary
to closed-basin lakes or playas, or to now dry channels of formerly large
rivers, may be left on dry flats or injected into salinities of lethal
concentrations.
In the Namib and Kalahari Deserts of southern Africa, rare flows of
water in ancient river beds carry fishes frcm the southwest African Plateau
and western Transvaal Highveld into arid zones, where they typically
perish as the rivers again desiccate (Jubb, 1969). Included in these pioneering species are three cyprinids, genus Barbus, a clariid catfish, and
two cichlids. None of these have special adaptations for survival, other
than their broad, ecological tolerances, but the clariid has epibranchial organs that allow it to persist under "most austere conditions" (Jubb, 1969).
Only the catfish and the cichlids maintain small, relatively permanent,
stunted populations on the desert floor (Jubb, cited in Cole, 1968).
405
bottom-dwelling fish-food organisms, or may physically smother fish eggs
or larvae (Starrett, 1950a, 1951).
Other adaptations to, or against, suspended solids in natural waters include development of barbels and other types of cutaneous sense organs
for increase in the proportion of nonvisual sensory input (Moore, 1950;
Branson, 1966), sometimes a considerable reduction in the exposed surface of the eye (Moore, 1950), development of special surfaces within
the eye for concentration of light (Moore, 1944a), and reductions in
squamation, or development of a leathery skin with minute, embedded
scales, the last two presumably in response to the abrasive nature of certain highly turbid waters (Hubbs, 1940, 1941a).
Morphology of the brain in fishes also varies according to their mode
of feeding (Miller and Evans, 1965; Davis and Miller, 1967), which is
related to the level of light in their environment that is, in turn, a partial
function of turbidity. It is interesting in this respect that the morphology of
brains of fishes from relatively constant habitats, whether they are perennially turbid or typically clear, tends to be far less variable than that from
fishes living in markedly variable habitats (Davis and Miller, 1967). This
tendency was found within the species level when subspecies of wide-ranging forms were compared, and among species.
As with flooding, tremendous increases in turbidity pose severe, immediate problems for fishes, and they similarly tend to avoid it or to move
to areas where current assists in minimizing its physical effects (Trautman,
1957).
3. Drought and Desiccation
Downstream displacement of fishes by flood, recorded in some large
rivers of more mesic zones (Viosca, in Trautman, 1957), may have fatal
results under desert conditions. Fish populations from streams tributary
to closed-basin lakes or playas, or to now dry channels of formerly large
rivers, may be left on dry flats or injected into salinities of lethal
concentrations.
In the Namib and Kalahari Deserts of southern Africa, rare flows of
water in ancient river beds carry fishes frcm the southwest African Plateau
and western Transvaal Highveld into arid zones, where they typically
perish as the rivers again desiccate (Jubb, 1969). Included in these pioneering species are three cyprinids, genus Barbus, a clariid catfish, and
two cichlids. None of these have special adaptations for survival, other
than their broad, ecological tolerances, but the clariid has epibranchial organs that allow it to persist under "most austere conditions" (Jubb, 1969).
Only the catfish and the cichlids maintain small, relatively permanent,
stunted populations on the desert floor (Jubb, cited in Cole, 1968).