tetrapods at least one amphibian, the crab-eating frog, Rana cancrivora , has returned to a marine environment, and even more numerous examples of such a transition can be seen among the reptiles that have marine and freshwater as well as
terrestrial representatives. The biological adaptations accompanying such transitions are sometimes very similar in different groups, providing examples of parallel
evolution. In other instances , more novel and unique mechanisms have evolved.
Thus both the marine frog, Rana cancrivora , and the sharks and rays, maintain
their body fluids hyperosmotic to sea-water by accumulating urea . On the other
hand marine reptiles, like the loggerhead turtle, Caretta caretta , have body fluids
similar in concentration to their terrestrial relatives which is hypotonic to sea-water,
but they can excrete salt as a highly concentrated solution from a modified orbital
gland. Rana cancrivora has evolved a parallel mechanism to that of the sharks and
rays, while the reptiles have utilized a novel mechanism not seen in their phyletic
forbears . Both similarities and diversities in osmoregulatory mechanisms are found
within, and between, the major phyletic groups of vertebrates.
The vertebrates have occupied most of the earth's geographic areas, being sparse
in the cold terrestrial polar regions, and not as numerous in hot dry deserts as in
the wetter tropical zones . The desert regions, where the supply of water may be
limiting to life, make up about one third (50 million square kilometers) of the land
surface of the earth (SCHMIDT-NIELSEN, 1964a). Despite the potential osmoregulatory problems, vertebrates do live in even the extremely dry parts of such
desert areas, and exhibit interesting physiological and behavioural patterns consistent with their life there. The seas, even in polar regions, have a diverse vertebrate
fauna of fish, birds and mammals. Major geographic limitations exist for the various
vertebrate groups and will be discussed later, but in the instance of the Amphibia
and reptiles, they are mainly dictated by temperature rather than by water.
Geographic dispersal, followed by genetic isolation, has played an important
tole in the process of evolution. Both the ability and inability to osmoregulate in
different situations have played a part in breaking and maintaining the physical bar -
riers involved. Deserts can retard the dispersal of animals (DARLINGTON, 1957),
as shown by the distribution of the Amphibia in Australia. DARWIN (1859) considered the sea to be the major physical barrier to dispersal, a view that is still current (DARLINGTON, 1957). The sea constitutes the most effective barrier to the
movement of the freshwater fishes, and DARWIN noted the absence of Amphibia
and terrestrial mammals from most oceanic islands, where reptiles are frequent inhabitants. This probably reflects the osmoregulatory powers of the different
groups. Reptiles with their less permeable integument, considerable tolerance to
internal osmotic change, extrarenal salt excretion and marked independence of environmental temperature would appear to be better suited to prolonged oceanic
voyages than freshwater and terrestrial Amphibia and mammals. At Duke University I kept a freshwater turtle, Pseudemys scripta, for 30 days in sea-water. At the
end of this time it was sluggish and had a plasma sodium concentration of nearly
300 rn-equiv/l, but on being returned to fresh water it resumed its normal life. Prolon ged oceanic voyages are thus conceivable, even in contempory species of freshwater-terrestrial reptiles , a tribute to their osmoregulatory capabilities.
2
terrestrial representatives. The biological adaptations accompanying such transitions are sometimes very similar in different groups, providing examples of parallel
evolution. In other instances , more novel and unique mechanisms have evolved.
Thus both the marine frog, Rana cancrivora , and the sharks and rays, maintain
their body fluids hyperosmotic to sea-water by accumulating urea . On the other
hand marine reptiles, like the loggerhead turtle, Caretta caretta , have body fluids
similar in concentration to their terrestrial relatives which is hypotonic to sea-water,
but they can excrete salt as a highly concentrated solution from a modified orbital
gland. Rana cancrivora has evolved a parallel mechanism to that of the sharks and
rays, while the reptiles have utilized a novel mechanism not seen in their phyletic
forbears . Both similarities and diversities in osmoregulatory mechanisms are found
within, and between, the major phyletic groups of vertebrates.
The vertebrates have occupied most of the earth's geographic areas, being sparse
in the cold terrestrial polar regions, and not as numerous in hot dry deserts as in
the wetter tropical zones . The desert regions, where the supply of water may be
limiting to life, make up about one third (50 million square kilometers) of the land
surface of the earth (SCHMIDT-NIELSEN, 1964a). Despite the potential osmoregulatory problems, vertebrates do live in even the extremely dry parts of such
desert areas, and exhibit interesting physiological and behavioural patterns consistent with their life there. The seas, even in polar regions, have a diverse vertebrate
fauna of fish, birds and mammals. Major geographic limitations exist for the various
vertebrate groups and will be discussed later, but in the instance of the Amphibia
and reptiles, they are mainly dictated by temperature rather than by water.
Geographic dispersal, followed by genetic isolation, has played an important
tole in the process of evolution. Both the ability and inability to osmoregulate in
different situations have played a part in breaking and maintaining the physical bar -
riers involved. Deserts can retard the dispersal of animals (DARLINGTON, 1957),
as shown by the distribution of the Amphibia in Australia. DARWIN (1859) considered the sea to be the major physical barrier to dispersal, a view that is still current (DARLINGTON, 1957). The sea constitutes the most effective barrier to the
movement of the freshwater fishes, and DARWIN noted the absence of Amphibia
and terrestrial mammals from most oceanic islands, where reptiles are frequent inhabitants. This probably reflects the osmoregulatory powers of the different
groups. Reptiles with their less permeable integument, considerable tolerance to
internal osmotic change, extrarenal salt excretion and marked independence of environmental temperature would appear to be better suited to prolonged oceanic
voyages than freshwater and terrestrial Amphibia and mammals. At Duke University I kept a freshwater turtle, Pseudemys scripta, for 30 days in sea-water. At the
end of this time it was sluggish and had a plasma sodium concentration of nearly
300 rn-equiv/l, but on being returned to fresh water it resumed its normal life. Prolon ged oceanic voyages are thus conceivable, even in contempory species of freshwater-terrestrial reptiles , a tribute to their osmoregulatory capabilities.
2