pounds from the renal tubule is a very efficient process (SMITH, 1936). Similarly
the gills of chondrichthyean fish are also relatively impermeable to urea and trimethylamine oxide. The possible actions of hormones, particularly the neurohypophysial peptides, in changing renal urea secretion in chondrichthyeans have been
suggested to me by HANS HELLER, but this interesting possibility has not been investigated.
s. Adaptations in the Osmoregulation of Teleost Fish during
Transfers between Fresh Water and Sea-water
Many fish can adapt to life in either fresh water or the sea; they are euryhaline.
Migrations, such as are often associated with breeding, involve the movement of
the fish from the sea into rivers and back into the sea again. Salmon, trout and lampreys breed in fresh water where their young undergo initial growth and development before returning to the sea. Others, like eels, breed in the ocean but subsequently spend a large part of their life in fresh water. Some species, like the trout,
Salmo gairdneri, and the marine lamprey, Petromyzon marinus, have landlocked
populations that have discarded such migratory habits, and undergo their entire
life cycle in fresh water. Certain other species like the flounder, killifish and toadfish
occupy coastal waters near the mouths of rivers and streams, where they experience
variations in the salinity of the water in which they live. There is not only a variety
of euryhaline fishes, but they exhibit diverse morphological development and
physiological conditions associated with different stages of their life period. These
include the juvenile freshwater ' parr' and premigatory 'silver' eels, as well as the
adult fishes in various stages of their breeding cycle. Many such fish cannot withstand direct transfers from fresh water to sea-water during all stages of their development, or even at any season of the year. When such a migration is made, several
days are required to adjust completely to the new osmotic conditions. Our knowledge of the physiological changes that accompany the development of euryhalinity
and the acute changes that occur at the time of the transition are still incomplete,
but it is often considered likely that hormones may have a role to play.
CONTE, WAGNER and their collaborators (CONTE and WAGNER, 1965; CONTE
et al., 1966) have made some interesting comparisons of the migratory behaviour,
and ability to adapt to sea-water, between the steelhead trout, Salmo gairdneri, and
the coho salmon, Oncorhynchus kisutch. Populations of these fish breed and undergo their initial development in the rivers of the northwest of the United States .
When very young juveniles of these salmonids are placed in sea-water they die,
but subsequently as they grow larger they can adapt to such solutions. It is well
known that this often occurs at about the time when they metamorphose from a
'parr' to a 'smelt', and this also corresponds to their seaward migration. It was
found that it is not this metamorphosis per se that results in their ability to adapt
to sea-water, but rather their size. When they attain a length of about 15 cm they
can adapt to sea-water and indeed in the juvenile 'parr' coho salmon studied, this
was usually seen 6 or 7 months before they metamorphosed into 'smelts'. This suggests that surface area relative to the body weight may be important, and that physiological and morphological features may develop that are associated with the gen229
the gills of chondrichthyean fish are also relatively impermeable to urea and trimethylamine oxide. The possible actions of hormones, particularly the neurohypophysial peptides, in changing renal urea secretion in chondrichthyeans have been
suggested to me by HANS HELLER, but this interesting possibility has not been investigated.
s. Adaptations in the Osmoregulation of Teleost Fish during
Transfers between Fresh Water and Sea-water
Many fish can adapt to life in either fresh water or the sea; they are euryhaline.
Migrations, such as are often associated with breeding, involve the movement of
the fish from the sea into rivers and back into the sea again. Salmon, trout and lampreys breed in fresh water where their young undergo initial growth and development before returning to the sea. Others, like eels, breed in the ocean but subsequently spend a large part of their life in fresh water. Some species, like the trout,
Salmo gairdneri, and the marine lamprey, Petromyzon marinus, have landlocked
populations that have discarded such migratory habits, and undergo their entire
life cycle in fresh water. Certain other species like the flounder, killifish and toadfish
occupy coastal waters near the mouths of rivers and streams, where they experience
variations in the salinity of the water in which they live. There is not only a variety
of euryhaline fishes, but they exhibit diverse morphological development and
physiological conditions associated with different stages of their life period. These
include the juvenile freshwater ' parr' and premigatory 'silver' eels, as well as the
adult fishes in various stages of their breeding cycle. Many such fish cannot withstand direct transfers from fresh water to sea-water during all stages of their development, or even at any season of the year. When such a migration is made, several
days are required to adjust completely to the new osmotic conditions. Our knowledge of the physiological changes that accompany the development of euryhalinity
and the acute changes that occur at the time of the transition are still incomplete,
but it is often considered likely that hormones may have a role to play.
CONTE, WAGNER and their collaborators (CONTE and WAGNER, 1965; CONTE
et al., 1966) have made some interesting comparisons of the migratory behaviour,
and ability to adapt to sea-water, between the steelhead trout, Salmo gairdneri, and
the coho salmon, Oncorhynchus kisutch. Populations of these fish breed and undergo their initial development in the rivers of the northwest of the United States .
When very young juveniles of these salmonids are placed in sea-water they die,
but subsequently as they grow larger they can adapt to such solutions. It is well
known that this often occurs at about the time when they metamorphose from a
'parr' to a 'smelt', and this also corresponds to their seaward migration. It was
found that it is not this metamorphosis per se that results in their ability to adapt
to sea-water, but rather their size. When they attain a length of about 15 cm they
can adapt to sea-water and indeed in the juvenile 'parr' coho salmon studied, this
was usually seen 6 or 7 months before they metamorphosed into 'smelts'. This suggests that surface area relative to the body weight may be important, and that physiological and morphological features may develop that are associated with the gen229