Table 7.12 Comparison of the composition of the rectal gland secretion of Squalus
acanthlus with that of its plasma, urine and sea-water
Osmolarity
(m-osmole/I)
Sodium
Potassium
Chloride
(m-equiv /I)
Sea-water
Rectal gland secretion
Plasma
Urine
1000-1100
1018
1018
780
470
540
289
339
12
7
5
2
550
533
246
203
From BURGER and HESS (1960)
ler need for sodium excretion experienced by these fish. The evidence for active
sodium excretion from the gills of marine chondrichthyeans is equivocal.
The sodium and chloride concentrations in the urine of sharks are slightly
higher than those in the plasma, while the rectal gland can secrete fluid in wh ich
these ions are present at even higher levels than in sea-water (Table 7.12). The rectal
gland is characteristically chondrichthyean and is a small gland, which, even in marine sharks weighing more than 10 kg, weighs only 1 to 3 grams (BONTING, 1966).
Its duct opens into the rectal region of the gut. Histologically it consists of masses
of tubules like in the avian salt gland (DOYLE, 1962). These tubules may be arranged
either radially and open to central ducts or longitudinally in lobules. The rectal
gland contains high concentrations of Na-K activated ATPase (BONTING, 1966;
jAMPOL and EpSTEIN, 1970). BURGER and HESS (1960) showed that in the spiny
dogfish, Squalus acanthias, this gland secretes a fluid that is isoosmotic with the
body fluids and contains sodium chloride at about twice the concentration seen
in the plasma (Table 7.12) . The total amounts of sodium chloride secreted from
the salt gland are similar to those that appear in the urine; in the spiny dogfish it
was found to be twice as great (BURGER, 1962), while in Scyliorhinus cuniculus it
was half as great (MAET? and LAHLOU, 1966) and in the lip shark, Hemiscyllium
plagiosum, it may be greater or less (CHAN, PHILLIPS, and CHESTER JONES, 1967
b). The rectal gland appears to be able to direct the outflux of about 5 to 10%
of the total sodium accumulated by the fish. Removal or incapacitation of this gland
does not have a very dramatic effect on the levels of sodium and chloride in the
plasma; they tend to rise, but only slightly (BURGER, 1962; 1965; CHAN et al., 1967
b). Renal salt excretion increases in such circumstances. This results from an increase in the urine volume rather than a change in concentration so that such a process is more wasteful of water. BURGER has suggested that on these occasions the
ac~umulation of salt is decreased due to a reduced branchial influx that may be
controlled by 'humors not yet defined'. In the presence of additional salt loads the
absence of the salt gland may be felt more severely while in normal circumstances
it leaves the kidney free to carry out other tasks, such as the excretion of divalent
IOns.
Little is known about the control of secretion from the salt gland. In the spiny
dogfish the injection of 1 M sodium chloride solutions initiates secretion (BURGER,
1962; 1965). Immersion in dilute solutions, which have a hydrating effect and dilute
249
acanthlus with that of its plasma, urine and sea-water
Osmolarity
(m-osmole/I)
Sodium
Potassium
Chloride
(m-equiv /I)
Sea-water
Rectal gland secretion
Plasma
Urine
1000-1100
1018
1018
780
470
540
289
339
12
7
5
2
550
533
246
203
From BURGER and HESS (1960)
ler need for sodium excretion experienced by these fish. The evidence for active
sodium excretion from the gills of marine chondrichthyeans is equivocal.
The sodium and chloride concentrations in the urine of sharks are slightly
higher than those in the plasma, while the rectal gland can secrete fluid in wh ich
these ions are present at even higher levels than in sea-water (Table 7.12). The rectal
gland is characteristically chondrichthyean and is a small gland, which, even in marine sharks weighing more than 10 kg, weighs only 1 to 3 grams (BONTING, 1966).
Its duct opens into the rectal region of the gut. Histologically it consists of masses
of tubules like in the avian salt gland (DOYLE, 1962). These tubules may be arranged
either radially and open to central ducts or longitudinally in lobules. The rectal
gland contains high concentrations of Na-K activated ATPase (BONTING, 1966;
jAMPOL and EpSTEIN, 1970). BURGER and HESS (1960) showed that in the spiny
dogfish, Squalus acanthias, this gland secretes a fluid that is isoosmotic with the
body fluids and contains sodium chloride at about twice the concentration seen
in the plasma (Table 7.12) . The total amounts of sodium chloride secreted from
the salt gland are similar to those that appear in the urine; in the spiny dogfish it
was found to be twice as great (BURGER, 1962), while in Scyliorhinus cuniculus it
was half as great (MAET? and LAHLOU, 1966) and in the lip shark, Hemiscyllium
plagiosum, it may be greater or less (CHAN, PHILLIPS, and CHESTER JONES, 1967
b). The rectal gland appears to be able to direct the outflux of about 5 to 10%
of the total sodium accumulated by the fish. Removal or incapacitation of this gland
does not have a very dramatic effect on the levels of sodium and chloride in the
plasma; they tend to rise, but only slightly (BURGER, 1962; 1965; CHAN et al., 1967
b). Renal salt excretion increases in such circumstances. This results from an increase in the urine volume rather than a change in concentration so that such a process is more wasteful of water. BURGER has suggested that on these occasions the
ac~umulation of salt is decreased due to a reduced branchial influx that may be
controlled by 'humors not yet defined'. In the presence of additional salt loads the
absence of the salt gland may be felt more severely while in normal circumstances
it leaves the kidney free to carry out other tasks, such as the excretion of divalent
IOns.
Little is known about the control of secretion from the salt gland. In the spiny
dogfish the injection of 1 M sodium chloride solutions initiates secretion (BURGER,
1962; 1965). Immersion in dilute solutions, which have a hydrating effect and dilute
249