When the rate constants for sodium were measured in other fishes in sea-water
they were found to vary greatly, though the movement of this ion in fresh water
is always small (Table 7.9). In sea-water, Tilapiamossambica exchange 66% of their
total sodium each hour, but in the toadfish, Opsanus tau, only 16% is moved in
this time. Marine chondrichthyeans exchange sodium far less rapidly than teleosts:
less than 1% each hour.
Differences in the total ion exchange of different fish are due to several factors,
probably the most prominent being their relative surface areas. Nevertheless other
factors also can result in differences in the rates of exchange and these include the
specific permeability of the gills and skin, the amounts of salt that may be accumulated through the gut as a result of feeding and drinking, and that which is unavoidably lost in the urine. As will be seen these latter factors are usually of minor
importance.
a) The Skin and Gills
As described in the previous section, the skin is generally considered to be relatively
impermeable to the movements of solutes, as well as water, the principal exchanges
taking place through the gills. We must, however, await precise measurements of
cutaneous permeability in different species of fish .
a) Gills in Sea-Water. The flounder, Platicbtbys flesus, in sea-water exchanges
about 2600 ,u-equiv of sodium/l 00 g body weight each hour (Table 7.9). This represents 40% of its total exchangeable sodium. About 25% of this sodium is absorbed through the gut (MOTAIS and MAETZ, 1965) so that 75% of the total sodium
accumulated by these fish takes place through the gills. In the sea perch, Serranus
scriba, 90% of the sodium taken up from the sea-water takes place through the
gills. The extrabranchial gains of sodium, which occur as a result of drinking. are
extruded through the gills, urinary losses making up less than 0.1 % of the total.
The gills of such fish in sea-water are thus the site of considerable sodium exchange,
with the outflux exceeding the influx by an amount which is about equivalent to
that gained through the gut .
KEyS (1931) used a perfused heart-gill preparation of the eel to demonstrate
an active extrusion of chloride by the branchae into the external sea-water. Isolated
gills of eels have also been shown to secrete chloride actively into the sea-water that
bathes them, and this is dependent on oxidative metabolism (BELLAMY, 1961). Such
an extrarenal mechanism for excretion of sodium chloride provided the channel that
HOMER SMITH concluded must be"present in marine teleost fish in order that they
can drink sea-water and maintain a positive water balance . Such active salt excretion
appears to be characteristic of marine teleost fish and probably the lampreys (see
MORRIS, 1960) but has not been demonstrated in chondrichthyeans. Some of the
latter, however, possesss an alternative channel for extrarenal salt excretion, the
rectal gland .
Immediately following the original demonstration, by KEYS, of chloride secretion by fish gills, a histological search was made for a structural element which
could be involved in this process. KEYS and WILLMER (1932) found some large and
prominent epithelial cells at the base of the gill leaflets in the eel (Fig. 7.3). As these
220
they were found to vary greatly, though the movement of this ion in fresh water
is always small (Table 7.9). In sea-water, Tilapiamossambica exchange 66% of their
total sodium each hour, but in the toadfish, Opsanus tau, only 16% is moved in
this time. Marine chondrichthyeans exchange sodium far less rapidly than teleosts:
less than 1% each hour.
Differences in the total ion exchange of different fish are due to several factors,
probably the most prominent being their relative surface areas. Nevertheless other
factors also can result in differences in the rates of exchange and these include the
specific permeability of the gills and skin, the amounts of salt that may be accumulated through the gut as a result of feeding and drinking, and that which is unavoidably lost in the urine. As will be seen these latter factors are usually of minor
importance.
a) The Skin and Gills
As described in the previous section, the skin is generally considered to be relatively
impermeable to the movements of solutes, as well as water, the principal exchanges
taking place through the gills. We must, however, await precise measurements of
cutaneous permeability in different species of fish .
a) Gills in Sea-Water. The flounder, Platicbtbys flesus, in sea-water exchanges
about 2600 ,u-equiv of sodium/l 00 g body weight each hour (Table 7.9). This represents 40% of its total exchangeable sodium. About 25% of this sodium is absorbed through the gut (MOTAIS and MAETZ, 1965) so that 75% of the total sodium
accumulated by these fish takes place through the gills. In the sea perch, Serranus
scriba, 90% of the sodium taken up from the sea-water takes place through the
gills. The extrabranchial gains of sodium, which occur as a result of drinking. are
extruded through the gills, urinary losses making up less than 0.1 % of the total.
The gills of such fish in sea-water are thus the site of considerable sodium exchange,
with the outflux exceeding the influx by an amount which is about equivalent to
that gained through the gut .
KEyS (1931) used a perfused heart-gill preparation of the eel to demonstrate
an active extrusion of chloride by the branchae into the external sea-water. Isolated
gills of eels have also been shown to secrete chloride actively into the sea-water that
bathes them, and this is dependent on oxidative metabolism (BELLAMY, 1961). Such
an extrarenal mechanism for excretion of sodium chloride provided the channel that
HOMER SMITH concluded must be"present in marine teleost fish in order that they
can drink sea-water and maintain a positive water balance . Such active salt excretion
appears to be characteristic of marine teleost fish and probably the lampreys (see
MORRIS, 1960) but has not been demonstrated in chondrichthyeans. Some of the
latter, however, possesss an alternative channel for extrarenal salt excretion, the
rectal gland .
Immediately following the original demonstration, by KEYS, of chloride secretion by fish gills, a histological search was made for a structural element which
could be involved in this process. KEYS and WILLMER (1932) found some large and
prominent epithelial cells at the base of the gill leaflets in the eel (Fig. 7.3). As these
220