probably reflect changes in the general metabolism of the fish, rather than their
osmoregulation (CHESTER JONES and PHILUPS, 1960). Exercise and various nonspecific stresses can result in elevated corticosteroid levels in the blood of trout
and salmon (HILL and FROMM, 1968; FAGERLUND, 1967). When smolts of salmon,
Salmo salar, are transferred from fresh water to sea-water the interrenals exhibit
histological signs of increased activity (OLIVEREAU, 1966). ARVY et al. (1959) found
a reduction in the neurosecretory products of the neurohypophysis in various
fishes transferred into solutions of increased salt concentration. When rainbow
trout, Salmo gairdneri, are shifted from fresh water into sea-water there is a decrease in the amount of peptides stored in the neurohypophysis and this returns
to normal after about 3 h (CARLSON and HOLMES, 1962). LEDERIS (1964) also
found such a change in trout treated in this manner, but found that the isotocin
levels in the gland were unchanged while the amount of vasotocin decreased by
50%. Neurohypophysial peptides, like corticosteroids, can be released in response
to non-specific stresses so that the significance of such changes in relation to osmoregulation should be interpreted with caution. We (BENTLEY and FOLLETT,
1963) could not detect any change in the vasotocin content of the neurohypophysis
of lampreys transferred from fresh water to sea-water. When North American eels
are moved from fresh water to sea-water there is also no significant change of the
vasotocin content in the pituitary after 3 days adaptation (BENTLEY, 1971b).
Changes in the histological appearance of the adenohypophysial eta cells, which
secrete prolactin (or 'paralactin') are also prominent when euryhaline fish are transferred between sea-water and fresh water. These cells are more active when the
fish are in fresh water than when they are in sea-water. This has been observed
in Fundulus heteroclitus (BALL and PICKFORD, 1964), Poecilia latipinna (OLIVEREAU and BALL, 1964), Tilapia mossambica (DHARMAMBA and NISHIOKA, 1968)
and Anguilla anguilla (OLIVEREAU and OLIVEREAU, 1968). The 'paralactin' content
of the pituitary of Poecilia is more than doubled when the fish are moved from
fresh water to sea-water (ENSOR and BALL, 1968a). On the other hand, the teleost
fish, Mugil cephalus, caught in freshwater ponds in Israel , had a much greater
content of 'paralactin' in their pituitary than those collected from the sea or 'hypersaline' lagoons (BLANC-LIVNI and ABRAHAM, 1970). The differences possibly
reflect the relative degrees of adaptation of the two species to their environments
or seasonal variations in their condition. As we shall see, 'paralactin' has a dramatic
effect on the adaptation of such euryhaline fish to fresh water. The
adenohypophysial somatotrophic cells of eels kept in fresh water are also strongly
stimulated (OLIVEREAU, 1967) but it is unknown whether their secretion is involved
in osmotic adaptation.
a) Effects 0/Extirpation 0/the Pituitary and Interrenals. A number of experiments
have been carried out on the effects of extirpation of various endocrine tissues on
the ability of teleost fish to adapt to fresh water and sea-water. FONTAINE, CALLAMAND, and OLIVEREAU (1949) found that the euryhalinity of the eel was not affected following hypophysectomy and this has since been confirmed. However,
the ability of a number of teleosts to survive transfer from sea-water into fresh water
is abolished by this operation. Even in the fresh water eel a closer examination has
shown that hypophysectomy results in an accelerated rate of sodium loss (see Table
239
osmoregulation (CHESTER JONES and PHILUPS, 1960). Exercise and various nonspecific stresses can result in elevated corticosteroid levels in the blood of trout
and salmon (HILL and FROMM, 1968; FAGERLUND, 1967). When smolts of salmon,
Salmo salar, are transferred from fresh water to sea-water the interrenals exhibit
histological signs of increased activity (OLIVEREAU, 1966). ARVY et al. (1959) found
a reduction in the neurosecretory products of the neurohypophysis in various
fishes transferred into solutions of increased salt concentration. When rainbow
trout, Salmo gairdneri, are shifted from fresh water into sea-water there is a decrease in the amount of peptides stored in the neurohypophysis and this returns
to normal after about 3 h (CARLSON and HOLMES, 1962). LEDERIS (1964) also
found such a change in trout treated in this manner, but found that the isotocin
levels in the gland were unchanged while the amount of vasotocin decreased by
50%. Neurohypophysial peptides, like corticosteroids, can be released in response
to non-specific stresses so that the significance of such changes in relation to osmoregulation should be interpreted with caution. We (BENTLEY and FOLLETT,
1963) could not detect any change in the vasotocin content of the neurohypophysis
of lampreys transferred from fresh water to sea-water. When North American eels
are moved from fresh water to sea-water there is also no significant change of the
vasotocin content in the pituitary after 3 days adaptation (BENTLEY, 1971b).
Changes in the histological appearance of the adenohypophysial eta cells, which
secrete prolactin (or 'paralactin') are also prominent when euryhaline fish are transferred between sea-water and fresh water. These cells are more active when the
fish are in fresh water than when they are in sea-water. This has been observed
in Fundulus heteroclitus (BALL and PICKFORD, 1964), Poecilia latipinna (OLIVEREAU and BALL, 1964), Tilapia mossambica (DHARMAMBA and NISHIOKA, 1968)
and Anguilla anguilla (OLIVEREAU and OLIVEREAU, 1968). The 'paralactin' content
of the pituitary of Poecilia is more than doubled when the fish are moved from
fresh water to sea-water (ENSOR and BALL, 1968a). On the other hand, the teleost
fish, Mugil cephalus, caught in freshwater ponds in Israel , had a much greater
content of 'paralactin' in their pituitary than those collected from the sea or 'hypersaline' lagoons (BLANC-LIVNI and ABRAHAM, 1970). The differences possibly
reflect the relative degrees of adaptation of the two species to their environments
or seasonal variations in their condition. As we shall see, 'paralactin' has a dramatic
effect on the adaptation of such euryhaline fish to fresh water. The
adenohypophysial somatotrophic cells of eels kept in fresh water are also strongly
stimulated (OLIVEREAU, 1967) but it is unknown whether their secretion is involved
in osmotic adaptation.
a) Effects 0/Extirpation 0/the Pituitary and Interrenals. A number of experiments
have been carried out on the effects of extirpation of various endocrine tissues on
the ability of teleost fish to adapt to fresh water and sea-water. FONTAINE, CALLAMAND, and OLIVEREAU (1949) found that the euryhalinity of the eel was not affected following hypophysectomy and this has since been confirmed. However,
the ability of a number of teleosts to survive transfer from sea-water into fresh water
is abolished by this operation. Even in the fresh water eel a closer examination has
shown that hypophysectomy results in an accelerated rate of sodium loss (see Table
239