frogs , Rana pipiens, are placed in sea-water they die in 30 to 60 min . SCHMIDTNIELSEN and I have found that under these conditions they lose water through the
skin, but the main reason for their demi se is not water loss, but a massive accumulation of salt that takes place through their skin and as a result of drinking. Even when
these frogs are placed in saline solutions similar in concentration to their body fluids
(0.7% sodium chloride) they often do not survive for longer than 24 h. In more
dilute solutions they live for longer periods, gaining weight due to the accumulation
of salt and water (ADOLPH, 1933). Nevertheless, some anurans do habitually enter
brackish water and at least one, the crab-eating frog, Rana cancrivora, lives in seawater. The European green toad, Bu/o viridis, has been found in ponds where the
salt concentration, while less than that of sea-water, is a substantial 2% (GISLEN
and KAURI, 1959). There are other reports of amphibians being found in brackish
water, but it is often difficult to judge how typical such an event is in the life of
the animal. The sight or sound of an approaching naturalist can conceivably strike
sufficient terror into a frog to prompt it, unwittingly, to leap into a pool of salt
water. GORDON (1962) collected Bu/o viridis in different areas of Europe ranging
from Naples to Belgrade and found that these toads, in common with Roumanian
ones previously studied by STOICOVICI and PORA (1951), could live for extended
periods of time in solutions with a sodium chloride content of 2% or, sometimes,
even more. The most basically important osmotic adjustment Bu/o viridis and
Rana cancrivora make towards their life in saline solutions, is to initiate and
maintain the concentrations of their body fluids at levels that are always, sometimes
only slightly, hyperosmotic to the bathing fluids . This elevation in the osmotic
pressure of the body 's fluids results from an elevation of the sodium levels and ,
particularly in the instance of Rana cancrivora, an accumulation of urea (Table
6.2). The tadpoles of Rana cancrivora, however, like all amphibian larvae cannot
form urea, but utili ze a completely different mechanism for their life in sea-water.
They maintain their body fluids , like those in bony fishes, at a concentration which
is hypoosmotic to sea-water, possibly excreting excess salt through their gills
(GORDON and TUCKER, 1965).
When amphibians utilize their inherent ability to live on dry land, the y are po -
tentially subject to an osmotic stress that is initiated in a manner different from
that in sea-water, that is the concentration of their body fluids as a result of evaporation through the integument. Evaporation through the skin is characteristically
rapid in the Amphibia as compared to other tetrapods. Nevertheless, amphibians,
especially anurans, have successfully occupied arid areas in which high environmental temperatures and a low content of water vapour in the air, considerably
facilitates evaporation. Such species utilize a number of mechanisms that somewhat
mitigate these conditions. Like all vertebrates living in such places they have
adopted patterns of behaviour that reduce the times of exposure to the extremes
of the environment; they seek refuge in cracks in rocks, and burrow, often deeply,
into the soil. In addition a variety of ph ysiological processes affect the osmoregulation of such amphibians.
The amphibians, like other tetrapods, are aided in their osmoregulation by the
action of some hormones, the most important being those secreted by the neurohypophysis and the adrenocortical tissues.
165
skin, but the main reason for their demi se is not water loss, but a massive accumulation of salt that takes place through their skin and as a result of drinking. Even when
these frogs are placed in saline solutions similar in concentration to their body fluids
(0.7% sodium chloride) they often do not survive for longer than 24 h. In more
dilute solutions they live for longer periods, gaining weight due to the accumulation
of salt and water (ADOLPH, 1933). Nevertheless, some anurans do habitually enter
brackish water and at least one, the crab-eating frog, Rana cancrivora, lives in seawater. The European green toad, Bu/o viridis, has been found in ponds where the
salt concentration, while less than that of sea-water, is a substantial 2% (GISLEN
and KAURI, 1959). There are other reports of amphibians being found in brackish
water, but it is often difficult to judge how typical such an event is in the life of
the animal. The sight or sound of an approaching naturalist can conceivably strike
sufficient terror into a frog to prompt it, unwittingly, to leap into a pool of salt
water. GORDON (1962) collected Bu/o viridis in different areas of Europe ranging
from Naples to Belgrade and found that these toads, in common with Roumanian
ones previously studied by STOICOVICI and PORA (1951), could live for extended
periods of time in solutions with a sodium chloride content of 2% or, sometimes,
even more. The most basically important osmotic adjustment Bu/o viridis and
Rana cancrivora make towards their life in saline solutions, is to initiate and
maintain the concentrations of their body fluids at levels that are always, sometimes
only slightly, hyperosmotic to the bathing fluids . This elevation in the osmotic
pressure of the body 's fluids results from an elevation of the sodium levels and ,
particularly in the instance of Rana cancrivora, an accumulation of urea (Table
6.2). The tadpoles of Rana cancrivora, however, like all amphibian larvae cannot
form urea, but utili ze a completely different mechanism for their life in sea-water.
They maintain their body fluids , like those in bony fishes, at a concentration which
is hypoosmotic to sea-water, possibly excreting excess salt through their gills
(GORDON and TUCKER, 1965).
When amphibians utilize their inherent ability to live on dry land, the y are po -
tentially subject to an osmotic stress that is initiated in a manner different from
that in sea-water, that is the concentration of their body fluids as a result of evaporation through the integument. Evaporation through the skin is characteristically
rapid in the Amphibia as compared to other tetrapods. Nevertheless, amphibians,
especially anurans, have successfully occupied arid areas in which high environmental temperatures and a low content of water vapour in the air, considerably
facilitates evaporation. Such species utilize a number of mechanisms that somewhat
mitigate these conditions. Like all vertebrates living in such places they have
adopted patterns of behaviour that reduce the times of exposure to the extremes
of the environment; they seek refuge in cracks in rocks, and burrow, often deeply,
into the soil. In addition a variety of ph ysiological processes affect the osmoregulation of such amphibians.
The amphibians, like other tetrapods, are aided in their osmoregulation by the
action of some hormones, the most important being those secreted by the neurohypophysis and the adrenocortical tissues.
165