c) The Kidney
The kidney structure of fishes conforms mainl y to the general tetrapod pattern.
There is usually a glomerulus across which the plasma can be filtered and the filtrate
is then exposed to the modifying influences of a renal tubule where reabsorption
and secretion of water and solutes take place. The arrangement of the different types
of cells in the renal tubules of fish, however, is not the same as in tetrapods and
a different segmental arrangement may be present. As many as four or five different
segments may be distinguished in, for instance, the goldfish and lamprey (see HICKMAN , 1965; BENTLEY and FOLLETT, 1963) but the exact role of each of these is unknown. In the absenc e of micropuncture studies we have no direct information
about the physiological role of various segments of the nephrons in fish.
The relative development of the glomeruli may vary considerably in different
fishes, and in some, like the goosefish, Lophius americanus, and the toadfish, Opsanus tau , they may be completely absent. This is relevant to the life of fish in hypoosmotic environments and has been the subject of considerable discussion, especially in relation to the origins of the different groups of fishes (see page 35). A
glomerulus appears to be particularly suited to the excretion of large volumes of
water as are accumulated by fish livin g in fresh water. Most fishe s, vviththe notable
exception of certain marine species, have well developed glomeruli which prompted
HOMER SMITH to suggest that this was additional evidence to indicate the freshwater origins of vertebrates. Glomeruli are present in the agnathans,
chondrichthyeans and most osteichthyeans. Their absence from certain marine
teleosts can be considered an adaptation to a life in hyperosmotic media where a
limited glomerular filtration may be an advantage. Nevertheless, it should be remembered that man y marine fish, including hypoosmotic teleosts, retain glomeruli
while aglomerular fishes can also survive in hypoosmotic media. The toadfish
thrives in 10% sea-water, a concentration that it normally experiences in coastal
waters, and despite its lack of glomeruli, adequately secretes the water that it accumulates (LAHLOU et al., 1969 b). Whether or not the glomerulus arose as an
adaptation to life in hypoosmotic solutions is still being debated, but when pre sent
it assists the excretion of water rather well. The vascula r supplies to the glomeruli
FW = fresh water; SW = sea-water
References: ISAWYER (1966 a); 2MAETZ (1963); 3HICKMAN (1965); 4LAHLOU
et al. (1969 b) ; 5LAHLOU (1967); 6SHARRATT et al. (1964 a); 7HOLMES and
McBEAN (1963); 8FROMM (1963); 9R. M . HOLMES in W. N. HOLMES and
McBEAN (1963); lOsee B. SCHMIDT-NIELSEN (1964); llBuRGER and HESS
(1960) ; 12H. SMITH (1936); 13BENTLEY and FOLLETT (1936); 14MoRRIS (1965).
214
The kidney structure of fishes conforms mainl y to the general tetrapod pattern.
There is usually a glomerulus across which the plasma can be filtered and the filtrate
is then exposed to the modifying influences of a renal tubule where reabsorption
and secretion of water and solutes take place. The arrangement of the different types
of cells in the renal tubules of fish, however, is not the same as in tetrapods and
a different segmental arrangement may be present. As many as four or five different
segments may be distinguished in, for instance, the goldfish and lamprey (see HICKMAN , 1965; BENTLEY and FOLLETT, 1963) but the exact role of each of these is unknown. In the absenc e of micropuncture studies we have no direct information
about the physiological role of various segments of the nephrons in fish.
The relative development of the glomeruli may vary considerably in different
fishes, and in some, like the goosefish, Lophius americanus, and the toadfish, Opsanus tau , they may be completely absent. This is relevant to the life of fish in hypoosmotic environments and has been the subject of considerable discussion, especially in relation to the origins of the different groups of fishes (see page 35). A
glomerulus appears to be particularly suited to the excretion of large volumes of
water as are accumulated by fish livin g in fresh water. Most fishe s, vviththe notable
exception of certain marine species, have well developed glomeruli which prompted
HOMER SMITH to suggest that this was additional evidence to indicate the freshwater origins of vertebrates. Glomeruli are present in the agnathans,
chondrichthyeans and most osteichthyeans. Their absence from certain marine
teleosts can be considered an adaptation to a life in hyperosmotic media where a
limited glomerular filtration may be an advantage. Nevertheless, it should be remembered that man y marine fish, including hypoosmotic teleosts, retain glomeruli
while aglomerular fishes can also survive in hypoosmotic media. The toadfish
thrives in 10% sea-water, a concentration that it normally experiences in coastal
waters, and despite its lack of glomeruli, adequately secretes the water that it accumulates (LAHLOU et al., 1969 b). Whether or not the glomerulus arose as an
adaptation to life in hypoosmotic solutions is still being debated, but when pre sent
it assists the excretion of water rather well. The vascula r supplies to the glomeruli
FW = fresh water; SW = sea-water
References: ISAWYER (1966 a); 2MAETZ (1963); 3HICKMAN (1965); 4LAHLOU
et al. (1969 b) ; 5LAHLOU (1967); 6SHARRATT et al. (1964 a); 7HOLMES and
McBEAN (1963); 8FROMM (1963); 9R. M . HOLMES in W. N. HOLMES and
McBEAN (1963); lOsee B. SCHMIDT-NIELSEN (1964); llBuRGER and HESS
(1960) ; 12H. SMITH (1936); 13BENTLEY and FOLLETT (1936); 14MoRRIS (1965).
214