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JAMES E. DEACON AND W. L. MINCKLEY
often-stated correlation between darkened méninges of the brain, darker
dorsal surfaces of the head, or blackened peritonea of adult fishes, with
incident radiation [the "protective adaptations against the light" of
Nikolsky (1963)] seems problematical. Darkened peritonea also appear
to correlate with a long digestive tract and an herbivorous diet (Hubbs
and Black, 1947; Smith, 1966).
5. Temperature
Conditions of constant high temperature in springs and of widely fluctuating temperatures in other desert waters may introduce stresses to
which only a few fish species have successfully adapted. Problems of
thermal adaptations and acclimation in aquatic poikilotherms have been
discussed by many authors, and reviews by Fry (1947, 1957), Brett
(1956), Prosser (1958), Prosser and Brown (1961), and Kinne (1963)
summarize much of the information available and establish terminology
and general concepts. All fishes have a range of temperature within which
they are capable of survival, and they usually have a somewhat narrower
range where maximum respiratory efficiency is maintained. The optima for
a species may change with seasonal trends, through gradual acclimation.
Prior acclimation determines in part the upper and lower limits of tolerance,
and the full range of acclimation temperatures, along with consequent shifts
in maximum and minimum lethal temperatures, defines the zone of thermal
tolerance. Such zones have been determined for relatively few fishes, but
they provide a most convenient method for comparing thermal labilities.
In general, Brett (1956) showed that among North American fishes,
Salmonidae have the narrowest and Ictaluridae the broadest zones of thermal tolerance of groups studied to that date. Poeciliids exhibited a tolerance nearly equal to that of ictalurids, while cyprinids, with the exception
of the Asiatic goldfish (Carassius auratus), which had the broadest zone
of all species tested (see also Fry et ai, 1942), were generally intermediate.
Another measure of thermal capabilities is the incipient lethal temperature, defined as the temperature that 50% of a population can withstand
for an infinite period of time. Levels above or below these upper and lower
limits may be tolerated for short periods, and such resistance time is exceedingly important in deserts since it permits fishes to enter (or temporarily persist in) temperatures that eventually would be lethal.
Some workers, notably Heath (1962, 1967) and Lowe and Heath
(1969), have used the critical thermal maximum as a measure of temperature tolerance in fishes. This technique subjects experimental populations
that have been acclimated at a constant temperature to a gradual increase
or decrease until death ensues.
JAMES E. DEACON AND W. L. MINCKLEY
often-stated correlation between darkened méninges of the brain, darker
dorsal surfaces of the head, or blackened peritonea of adult fishes, with
incident radiation [the "protective adaptations against the light" of
Nikolsky (1963)] seems problematical. Darkened peritonea also appear
to correlate with a long digestive tract and an herbivorous diet (Hubbs
and Black, 1947; Smith, 1966).
5. Temperature
Conditions of constant high temperature in springs and of widely fluctuating temperatures in other desert waters may introduce stresses to
which only a few fish species have successfully adapted. Problems of
thermal adaptations and acclimation in aquatic poikilotherms have been
discussed by many authors, and reviews by Fry (1947, 1957), Brett
(1956), Prosser (1958), Prosser and Brown (1961), and Kinne (1963)
summarize much of the information available and establish terminology
and general concepts. All fishes have a range of temperature within which
they are capable of survival, and they usually have a somewhat narrower
range where maximum respiratory efficiency is maintained. The optima for
a species may change with seasonal trends, through gradual acclimation.
Prior acclimation determines in part the upper and lower limits of tolerance,
and the full range of acclimation temperatures, along with consequent shifts
in maximum and minimum lethal temperatures, defines the zone of thermal
tolerance. Such zones have been determined for relatively few fishes, but
they provide a most convenient method for comparing thermal labilities.
In general, Brett (1956) showed that among North American fishes,
Salmonidae have the narrowest and Ictaluridae the broadest zones of thermal tolerance of groups studied to that date. Poeciliids exhibited a tolerance nearly equal to that of ictalurids, while cyprinids, with the exception
of the Asiatic goldfish (Carassius auratus), which had the broadest zone
of all species tested (see also Fry et ai, 1942), were generally intermediate.
Another measure of thermal capabilities is the incipient lethal temperature, defined as the temperature that 50% of a population can withstand
for an infinite period of time. Levels above or below these upper and lower
limits may be tolerated for short periods, and such resistance time is exceedingly important in deserts since it permits fishes to enter (or temporarily persist in) temperatures that eventually would be lethal.
Some workers, notably Heath (1962, 1967) and Lowe and Heath
(1969), have used the critical thermal maximum as a measure of temperature tolerance in fishes. This technique subjects experimental populations
that have been acclimated at a constant temperature to a gradual increase
or decrease until death ensues.