The solute content of the plasma in representatives of the main groups of fishes
has been measured (Table 7.2). The bony fishes that live in the sea generally have
higher solute levels than those of fish in fresh water. Euryhaline fish, like the flounder, salmon and eel, have a plasma osmolarity that is about 20% greater in the sea
than in fresh water, and this is the result of an elevated sodium concentration. Such
marine fish are considerably hypoosmotic, 2 to 3-fold less, to the sea-water that
bathes them . The only exception among the Osteichthyes appears to be the relict
crossopterygian (originally a predominantly freshwater group) Latimeria whose
plasma is slightly hyperosmotic to sea-water, due mainly to a retention of urea in
the fluids. It is interesting that the closest living phyletic relatives of this fish are
the lungfishes, and one of these, Protopterus, can also tolerate large amounts of
urea in its body fluids . The marine chondrichthyean fishes, which like the
crossopterygians are thought to have originated in fresh water, also maintain their
body fluids slightly hyperosmotic to sea-water by reta ining urea as well as some
sodium (H. SMITH, 1936). Chondrichthyeans that live in fresh water, like the sawfish Pristis, have a much lower plasma concentration due to a reduction in the levels
of both urea and sodium (Table 7.2). The agnathans exhibit both types of osmotic
constitution, the lampreys are hypoosmotic to sea-water, while the hagfishes are
slightly hyperosrnotic. The latter group, however, does not retain urea for this purpose, but has high salt concentrations in its body fluids.
We can predict the osmotic stresses on the various fishes from the osmotic concentrations in their body fluids. In fresh water all fishes tend to gain water by osmosis while at the same time they may be expected to lose solutes by diffusion.
In the sea, chondrichthyeans, Latimeria and the hagfishes, all gain water by osmosis, and (except for the latter) may be expected to accumulate sodium by diffusion . The marine bony fishes-and lampreys, which are hypoosrnotic to sea-water,
lose water osmotically and gain sodium. Measurements of water and solute balance
in the different groups of fishes indicate that such osmotic changes are occurring,
but they differ widely and are adequately compensated for by physiological adjustments.
1. The Piscine Endocrines
The fishes possess tissues that are homologous to the endocrine glands of tetrapods
and that form similar biological products (see for instance: BERN and NANDI, 1964;
BERN, 1967). Whether these all have an endocrine function is uncertain, but in man y
instances this seems likely. The precise molecular structures of the products of such
tissues in fish often differ somewhat from those of the secretions from the homologous tissues in other groups of vertebrates. This is particularly apparent among
the proteinaceous hormones which may exhibit different potencies, pharmacological activities and immunological behaviour from those in Other vertebrates. An
orderly pattern in their distribution may, nevertheless, be apparent. In add ition,
while the fish seem to have all the endocrine tissues that have been identified in
higher vertebrates some have tWO additional putative ones; the urophysis and the
corpuscles of STANNIUS (see page 78).
202
has been measured (Table 7.2). The bony fishes that live in the sea generally have
higher solute levels than those of fish in fresh water. Euryhaline fish, like the flounder, salmon and eel, have a plasma osmolarity that is about 20% greater in the sea
than in fresh water, and this is the result of an elevated sodium concentration. Such
marine fish are considerably hypoosmotic, 2 to 3-fold less, to the sea-water that
bathes them . The only exception among the Osteichthyes appears to be the relict
crossopterygian (originally a predominantly freshwater group) Latimeria whose
plasma is slightly hyperosmotic to sea-water, due mainly to a retention of urea in
the fluids. It is interesting that the closest living phyletic relatives of this fish are
the lungfishes, and one of these, Protopterus, can also tolerate large amounts of
urea in its body fluids . The marine chondrichthyean fishes, which like the
crossopterygians are thought to have originated in fresh water, also maintain their
body fluids slightly hyperosmotic to sea-water by reta ining urea as well as some
sodium (H. SMITH, 1936). Chondrichthyeans that live in fresh water, like the sawfish Pristis, have a much lower plasma concentration due to a reduction in the levels
of both urea and sodium (Table 7.2). The agnathans exhibit both types of osmotic
constitution, the lampreys are hypoosmotic to sea-water, while the hagfishes are
slightly hyperosrnotic. The latter group, however, does not retain urea for this purpose, but has high salt concentrations in its body fluids.
We can predict the osmotic stresses on the various fishes from the osmotic concentrations in their body fluids. In fresh water all fishes tend to gain water by osmosis while at the same time they may be expected to lose solutes by diffusion.
In the sea, chondrichthyeans, Latimeria and the hagfishes, all gain water by osmosis, and (except for the latter) may be expected to accumulate sodium by diffusion . The marine bony fishes-and lampreys, which are hypoosrnotic to sea-water,
lose water osmotically and gain sodium. Measurements of water and solute balance
in the different groups of fishes indicate that such osmotic changes are occurring,
but they differ widely and are adequately compensated for by physiological adjustments.
1. The Piscine Endocrines
The fishes possess tissues that are homologous to the endocrine glands of tetrapods
and that form similar biological products (see for instance: BERN and NANDI, 1964;
BERN, 1967). Whether these all have an endocrine function is uncertain, but in man y
instances this seems likely. The precise molecular structures of the products of such
tissues in fish often differ somewhat from those of the secretions from the homologous tissues in other groups of vertebrates. This is particularly apparent among
the proteinaceous hormones which may exhibit different potencies, pharmacological activities and immunological behaviour from those in Other vertebrates. An
orderly pattern in their distribution may, nevertheless, be apparent. In add ition,
while the fish seem to have all the endocrine tissues that have been identified in
higher vertebrates some have tWO additional putative ones; the urophysis and the
corpuscles of STANNIUS (see page 78).
202