preparations. KOCH and HEUTS in 1942 (quoted by FONTAINE, 1956) found that
feeding thyroid gland to one species of stickleback, Gasterosteus aculeatus, decreased its ability to withstand transfer into fresh water, while in another species,
Pygosteus pungitius, the salinity tolerance was increased. SMITH (1956) observed
that administration of thyroid hormone (thyroxine) increased the ability of trout
to survive in salt solutions, while antithyroid drugs decreased this ability. FONTAINE and his collaborators (see FONTAINE, 1956) have also found that antithyroid
drugs decrease the salinity tolerance of various marine fish. On the other hand,
HOAR, BLACK, and BLACK (1951) (quoted by FONTAINE, 1956) found that keeping
juvenile salmon in thyroxine solutions did not alter their ability to osmoregulate
in salt water. The physiological significance of the various observations on the
thyroid gland is not clearcut and the evidence that it is involved in osmoregulation
has been treated with some suspicion. SMITH (1956) commented that the doses of
thyroxine that he used were excessive, and found that in nature there was no correlation between the thyroid activity of the trout and their ability to osmoregulate.
In addition the administration of thyrotrophin did not alter the trout's salinity tolerance . As commented upon by others, the actions of antithyroid drugs may have
actions other than on the thyroid gland for such substances may exhibit non-specific toxicities, as are readily apparent when they are given to mammals. Although
the thyroid probably does not have a direct effect on osmoregulation in fish, it
could interact with other physiological factors, which together could influence euryhalinity in the fish.
The pituitary, as will be described in more detail later, can influence osmoregulation in fish and the whole gamut of pituitary hormones has been tested
on fish placed in solutions of differing salt concentration. The most dramatic effect
found is the action of mammalian prolactin in restoring the ability of hypophysectomized killifish, Fundulus beteroclitus, to survive in fresh water (PICKFORD and
PHILLIPS, 1959). Other pituitary hormones, including corticotrophin, are ineffective. Injections of large doses of corticosteroids, including cortisol and aldosterone,
were not found to affect survival, or electrolyte composition, of several species of
teleosts after they were placed in solutions of increased salt concentration
(EDELMAN, YOUNG, and HARRIS, 1960). SMITH (1956) demonstrated that pituitary
growth hormone, when injected into trout, produced a dramatic increase in the ability to survive in salt solutions. The effective dose was quite small, but these interesting observations do not appear to have received the subsequent consideration
that they deserve. As growth and differentiation appear to play an important role
in the ability of salmonids to osrnoregulate in the sea, the action of this hormone
may be the result of an increased rate of development, possibly an accelerated maturation of the branchial 'chloride secreting cells' .
The frequent failure to demonstrate any dramatic changes in the salinity tolerances of various fishes after they have been injected with large doses of endocrine
extracts does not necessarily indicate that these hormones are not involved in such
changes. The hormone preparations are sometimes those that are only found in
tetrapods and may not be identical in their structure to the piscine secretions. Even
if such a hormone is present in fishes, its endogenous levels may already be maximal
at the time of the injection, so that the extra hormone will have no added effect.
In addition, physiological adaptations to solutions of different osmotic con231
feeding thyroid gland to one species of stickleback, Gasterosteus aculeatus, decreased its ability to withstand transfer into fresh water, while in another species,
Pygosteus pungitius, the salinity tolerance was increased. SMITH (1956) observed
that administration of thyroid hormone (thyroxine) increased the ability of trout
to survive in salt solutions, while antithyroid drugs decreased this ability. FONTAINE and his collaborators (see FONTAINE, 1956) have also found that antithyroid
drugs decrease the salinity tolerance of various marine fish. On the other hand,
HOAR, BLACK, and BLACK (1951) (quoted by FONTAINE, 1956) found that keeping
juvenile salmon in thyroxine solutions did not alter their ability to osmoregulate
in salt water. The physiological significance of the various observations on the
thyroid gland is not clearcut and the evidence that it is involved in osmoregulation
has been treated with some suspicion. SMITH (1956) commented that the doses of
thyroxine that he used were excessive, and found that in nature there was no correlation between the thyroid activity of the trout and their ability to osmoregulate.
In addition the administration of thyrotrophin did not alter the trout's salinity tolerance . As commented upon by others, the actions of antithyroid drugs may have
actions other than on the thyroid gland for such substances may exhibit non-specific toxicities, as are readily apparent when they are given to mammals. Although
the thyroid probably does not have a direct effect on osmoregulation in fish, it
could interact with other physiological factors, which together could influence euryhalinity in the fish.
The pituitary, as will be described in more detail later, can influence osmoregulation in fish and the whole gamut of pituitary hormones has been tested
on fish placed in solutions of differing salt concentration. The most dramatic effect
found is the action of mammalian prolactin in restoring the ability of hypophysectomized killifish, Fundulus beteroclitus, to survive in fresh water (PICKFORD and
PHILLIPS, 1959). Other pituitary hormones, including corticotrophin, are ineffective. Injections of large doses of corticosteroids, including cortisol and aldosterone,
were not found to affect survival, or electrolyte composition, of several species of
teleosts after they were placed in solutions of increased salt concentration
(EDELMAN, YOUNG, and HARRIS, 1960). SMITH (1956) demonstrated that pituitary
growth hormone, when injected into trout, produced a dramatic increase in the ability to survive in salt solutions. The effective dose was quite small, but these interesting observations do not appear to have received the subsequent consideration
that they deserve. As growth and differentiation appear to play an important role
in the ability of salmonids to osrnoregulate in the sea, the action of this hormone
may be the result of an increased rate of development, possibly an accelerated maturation of the branchial 'chloride secreting cells' .
The frequent failure to demonstrate any dramatic changes in the salinity tolerances of various fishes after they have been injected with large doses of endocrine
extracts does not necessarily indicate that these hormones are not involved in such
changes. The hormone preparations are sometimes those that are only found in
tetrapods and may not be identical in their structure to the piscine secretions. Even
if such a hormone is present in fishes, its endogenous levels may already be maximal
at the time of the injection, so that the extra hormone will have no added effect.
In addition, physiological adaptations to solutions of different osmotic con231