contemporary species of some of the main orders within these groups of fish may
today live solely in fresh water (the lungfishes and bichir) or the sea (the coelacanth), in the course of geological time some representatives of all such evolutionary divisions have occupied both habitats (see DARLINGTON, 1957). The dominant
teleosts are thought to have originated in the sea, but some subsequently mo ved
into fresh water and a number of these have even returned to the sea. DARLINGTON
(1957) gives a fascinating description of the dispersal of freshwater fishes throughout the world, and this has often involved several successive migrations between
the sea and fresh water. The freshwater catfishes of Australia, for instance, moved
into the antipodean rivers from the sea, but their immediate marine ancestors previously originated from freshwater ancestors that probably lived in Asia. When one
considers the complexities of such transformations, it is not surprising to find considerable diversity in the detailed pattern of osmoregulation of fishes, though the
basjc blueprint remains remarkably uniform.
Most contemporary species.. of fish have only a limited ability to move between
fresh water and salt solutions like sea-water. Such osmotically conservative fishes
are 'stenohaline' while others which can adapt more readily to either type of solution are 'euryhaline'. As emphasized by FONTAINE and KOCH (1950), such a classification is not rigid , some fish being 'more or less' stenohaline while others are
'more or less' euryhaline. Thus the freshwater goldfish and the marine perch can
both live in salt solutions about one-third the concentration of the sea (LAHLOU,
HENDERSON, and SAWYER, 1969a ; MOTAI S, GAR CIA ROM EU, and MAETZ, 1966).
Some euryhaline fishes can withstand direct transfer from fresh water to sea-water,
while others such as the lamprey once having migrated into rivers , will die if the y
are then replaced in the sea (MORRIS, 1960). Euryhaline fishe s are often mi gratory
breeders, some like lampreys and salmon mo ve from the sea into rivers where they
produce their eggs, while others, like the eel, return to the sea to breed. Many euryhaline fishes occupy estuaries and certain inland lakes and streams where the salinity ma y vary considerably during different seasons. The osmotic adjustments,
which must be made by such fishes, would seem to offer a situation for endocrine
coordination. In natural conditions adaptation to solutions of differing osmotic
concentration probably can take place gradually, as when such water is evaporated
from an inland lake or fish migrate through estuarine areas between rivers and the
sea (FONTAINE and KOCH, 1950). Such situations would seem to be particularly
suitable to the relatively long term actions that are characteristic of many hormones.
Fish can live in solutions with a wide range of osmotic concentration. As described above, fish that normally live in the sea or fresh water can often tolerate
solutions of intermediate concentration. Others can live in waters with a solute
concentration considerably higher than that of sea-water; Tilapia mossambica may
survive in a salinity of 6.9% , while the cyprinodont Cyprinodon variegatus has
been found in saline waters with a concentration of 14.2% (see PARRY, 1966). Such
osmotic tolerance presumably constitutes a rather special physiological adaptability about which we have little knowledge.
Fishes contain an amount of water that is equivalent to 70 to 75% of their body
weight which is similar to the water content of most other vertebrates (Table 1.2).
The distribution of this water in the body varies somewhat in different species (THORSON, 1961 and Table 1.2). The plasma volume of agnathans and
200
today live solely in fresh water (the lungfishes and bichir) or the sea (the coelacanth), in the course of geological time some representatives of all such evolutionary divisions have occupied both habitats (see DARLINGTON, 1957). The dominant
teleosts are thought to have originated in the sea, but some subsequently mo ved
into fresh water and a number of these have even returned to the sea. DARLINGTON
(1957) gives a fascinating description of the dispersal of freshwater fishes throughout the world, and this has often involved several successive migrations between
the sea and fresh water. The freshwater catfishes of Australia, for instance, moved
into the antipodean rivers from the sea, but their immediate marine ancestors previously originated from freshwater ancestors that probably lived in Asia. When one
considers the complexities of such transformations, it is not surprising to find considerable diversity in the detailed pattern of osmoregulation of fishes, though the
basjc blueprint remains remarkably uniform.
Most contemporary species.. of fish have only a limited ability to move between
fresh water and salt solutions like sea-water. Such osmotically conservative fishes
are 'stenohaline' while others which can adapt more readily to either type of solution are 'euryhaline'. As emphasized by FONTAINE and KOCH (1950), such a classification is not rigid , some fish being 'more or less' stenohaline while others are
'more or less' euryhaline. Thus the freshwater goldfish and the marine perch can
both live in salt solutions about one-third the concentration of the sea (LAHLOU,
HENDERSON, and SAWYER, 1969a ; MOTAI S, GAR CIA ROM EU, and MAETZ, 1966).
Some euryhaline fishes can withstand direct transfer from fresh water to sea-water,
while others such as the lamprey once having migrated into rivers , will die if the y
are then replaced in the sea (MORRIS, 1960). Euryhaline fishe s are often mi gratory
breeders, some like lampreys and salmon mo ve from the sea into rivers where they
produce their eggs, while others, like the eel, return to the sea to breed. Many euryhaline fishes occupy estuaries and certain inland lakes and streams where the salinity ma y vary considerably during different seasons. The osmotic adjustments,
which must be made by such fishes, would seem to offer a situation for endocrine
coordination. In natural conditions adaptation to solutions of differing osmotic
concentration probably can take place gradually, as when such water is evaporated
from an inland lake or fish migrate through estuarine areas between rivers and the
sea (FONTAINE and KOCH, 1950). Such situations would seem to be particularly
suitable to the relatively long term actions that are characteristic of many hormones.
Fish can live in solutions with a wide range of osmotic concentration. As described above, fish that normally live in the sea or fresh water can often tolerate
solutions of intermediate concentration. Others can live in waters with a solute
concentration considerably higher than that of sea-water; Tilapia mossambica may
survive in a salinity of 6.9% , while the cyprinodont Cyprinodon variegatus has
been found in saline waters with a concentration of 14.2% (see PARRY, 1966). Such
osmotic tolerance presumably constitutes a rather special physiological adaptability about which we have little knowledge.
Fishes contain an amount of water that is equivalent to 70 to 75% of their body
weight which is similar to the water content of most other vertebrates (Table 1.2).
The distribution of this water in the body varies somewhat in different species (THORSON, 1961 and Table 1.2). The plasma volume of agnathans and
200