a number of acute osmotic problems involved in the 'awakening' process. During
their long sojourn in the burrows the lungfishes suffer a considerable increase in
the osmotic concentration in their body fluids , so that when exposed to water they
accumulate it rapidly. As their kidney function is relatively deficient at thi s time
they become swollen and oedematous in appearance. Subsequently the solutes and
water are excreted and kidney function returns to normal. If aestivating lungfish
are returned to water following hypophysectomy they die, apparently from overhydration, as the kidneys cannot adjust and overcome the 'aestivation anuria' (GoDET, 1961). SAWYER (1966 a) has tentatively suggested that the diuretic action of
vasotocin could be utilized on such occasions, but other hormones are probably
also involved.
While some fascinating information about the life of lungfishes has been disclosed the physiological information is still far from complete. The Australian and
South American lungfish, as well as the African fish should provide material for
further fruitful and enjoyable physiological and geographical excursions.
b) The Sharks and Rays (Chondrichthyes)
It may be objected that this group of fish is not really relict, especially as it contains
a large number of species with a world wide distribution in the sea and even, occasionally, in rivers and lakes . However, the Chondrichthyes diverged from the
main vertebrate stock about 300 million years ago and have evolved separately since
that time . Their osmoregulatory pattern is different from that of the bony fishes
(except probably for the coelacanth),
By retaining urea and an extra amount of sodium chloride in their body fluid s
they have, in contrast to the marine teleosts, attained a slightly hyperosmotic edge
over the sea-w ater where most of them live. Water thus tends to move into the
body by osmosis, probably mainly across the gills. The sharks and rays, unlike
th e marine tcleo sts, do not appear to drink sea-w ater (H. SMITH, 1931) but further
species should be examined for such behaviour. The ability of these fish to excrete
salt does not app ear to be as great as in the marine teleosts, so that substantial drinking would be unexpected. A steady exchange of sodium takes place between the
chondrichthyeans and the sea-water in which they live, the influx of which appears
nearly all to take place across the gills. However, this is small compared to marine
teleosts (but similar to freshwater ones!) making up less than 1% of the total body
sodium each hour, compared to as much as 60 to 70% in some bony fishes (Table
7.9). About 7 to 10% of the total salt lost is excreted in the urine and a similar
quantity is secreted from the rectal 'salt' gland . The balance appears to pass out
through the gills, but whether this involves active transport is uncertain. The
chondrichthyean gills appear to lack 'chloride secreting cells' and, probably as a
result of this observation, it has been widely assumed that active salt secretion does
not occur from their gills.
Sodium-potassium activated ATPase has been identified in the gills of a marine
chondrichthyean, the dogfish, Squalus acanthias (JAMPOL and EpSTEIN, 1970). The
concentration is low compared to marine teleosts but similar to that of freshwater
species. Its role in the dogfish is unknown but it could possibly suffice for the smal248
their long sojourn in the burrows the lungfishes suffer a considerable increase in
the osmotic concentration in their body fluids , so that when exposed to water they
accumulate it rapidly. As their kidney function is relatively deficient at thi s time
they become swollen and oedematous in appearance. Subsequently the solutes and
water are excreted and kidney function returns to normal. If aestivating lungfish
are returned to water following hypophysectomy they die, apparently from overhydration, as the kidneys cannot adjust and overcome the 'aestivation anuria' (GoDET, 1961). SAWYER (1966 a) has tentatively suggested that the diuretic action of
vasotocin could be utilized on such occasions, but other hormones are probably
also involved.
While some fascinating information about the life of lungfishes has been disclosed the physiological information is still far from complete. The Australian and
South American lungfish, as well as the African fish should provide material for
further fruitful and enjoyable physiological and geographical excursions.
b) The Sharks and Rays (Chondrichthyes)
It may be objected that this group of fish is not really relict, especially as it contains
a large number of species with a world wide distribution in the sea and even, occasionally, in rivers and lakes . However, the Chondrichthyes diverged from the
main vertebrate stock about 300 million years ago and have evolved separately since
that time . Their osmoregulatory pattern is different from that of the bony fishes
(except probably for the coelacanth),
By retaining urea and an extra amount of sodium chloride in their body fluid s
they have, in contrast to the marine teleosts, attained a slightly hyperosmotic edge
over the sea-w ater where most of them live. Water thus tends to move into the
body by osmosis, probably mainly across the gills. The sharks and rays, unlike
th e marine tcleo sts, do not appear to drink sea-w ater (H. SMITH, 1931) but further
species should be examined for such behaviour. The ability of these fish to excrete
salt does not app ear to be as great as in the marine teleosts, so that substantial drinking would be unexpected. A steady exchange of sodium takes place between the
chondrichthyeans and the sea-water in which they live, the influx of which appears
nearly all to take place across the gills. However, this is small compared to marine
teleosts (but similar to freshwater ones!) making up less than 1% of the total body
sodium each hour, compared to as much as 60 to 70% in some bony fishes (Table
7.9). About 7 to 10% of the total salt lost is excreted in the urine and a similar
quantity is secreted from the rectal 'salt' gland . The balance appears to pass out
through the gills, but whether this involves active transport is uncertain. The
chondrichthyean gills appear to lack 'chloride secreting cells' and, probably as a
result of this observation, it has been widely assumed that active salt secretion does
not occur from their gills.
Sodium-potassium activated ATPase has been identified in the gills of a marine
chondrichthyean, the dogfish, Squalus acanthias (JAMPOL and EpSTEIN, 1970). The
concentration is low compared to marine teleosts but similar to that of freshwater
species. Its role in the dogfish is unknown but it could possibly suffice for the smal248