412
12 Small Nitrogenous Compounds
excretion have been examined in more detail in
Lumbricus terrestris; the critical factor is a decline in glycogen reserves from the normal 14 mglg
to less than 4 mglg during starvation [255].
The typical excretory product of the arachnids
is guanine, and this predominates in the real spiders, mites, uropygi, solifugi and amblypygi. The
situation is more complicated in the scorpions.
Vejovis mexican us excretes only guanine, but
Lycas tricarinatus also produces some urea. The
excretions of Paruroctonus mesaensis contain
over 90 % xanthine and some hypoxanthine, and
this is the first known xanthotelic animal [278].
As expected, the crustaceans are strictly ammoniotelic; this is also true of the terrestrial isopods,
which in fact excrete gaseous ammonia. The only
myriapod investigated in any detail (CylindroiuIus longinensis) excreted 57 % ammonia, 38 %
amino acids and 5 % uric acid. The insects in general are a model for uricotelic excretion, although
systematic investigation of over 400 species
showed that the activity of uricolytic enzymes
resulted in the excretion also of allantoin,
allantoic acid and urea [51]. Excretion of allantoin in fact predominates in Dysdercus fasciatus,
and hypoxanthine and xanthine are found in addition to uric acid in some species. The butterfly
Colias croceus stores guanine in the wings, and
the tsetse fly Glossina morsitans excretes relatively large amounts of amino acids, especially
arginine and histidine. Uric acid is sometimes
stored in the specialized fat-body cells of insects;
in Periplaneta and other cockroaches the storage
is indefinite, and the uric acid is not released during the insect's lifetime [50, 51].
Echinoderms excrete large quantities of amino
acids, e.g. 24-30 % of excreted nitrogen in the
cases of Asterias and Diadema. In many echinoderms, more than 10 % of the excreted nitrogen is
in the form of urea [237]. The tunicates are
ammoniotelic. In the eggs of the ascidian Corella
willmeriana, ammonium salts at a concentration
of about 250 mmol/l serve to reduce the buoyant
density, as in the pelagic squids. Some ascidians
excrete ammonia but in addition store uric acid in
the form of concretions in the nephridia [82].
In the fish, the elasmobranchs are ureotelic, as
is the only living crossopterygian Latimeria chalumnae. The urea, however, is retained by the
kidneys and accumulates in the intra- and extracellular fluids, where it is the most important substance of osmoregulation. In the freshwater ray
Potamotrygon sp., the urea level in the plasma is
relatively low and remains almost unchanged
when the animal acclimatizes to dilute seawater.
The bony fish excrete almost exclusively ammonia, although in the case of the mudskipper Periophthalmus up to 33 % urea is excreted. The
nitrogen metabolism of the three dipnoan genera
varies greatly in parallel with their abilities to
withstand dry periods. The Australian lungfish
Neoceratodus, which never leaves the water and
whose lungs are used only for accessory air
breathing, is almost purely ammoniotelic, with no
more than 6 % of the excreted nitrogen occurring
as urea. On the other hand, the South American
lungfish Lepidosiren, which can live out of water
but does not build a cocoon like Protopterus,
excretes 38 % urea when living in water. The African lungfish Protopterus already has a urea fraction of over 50 % when in the water. During aestivation, the tissue urea concentration is as much
as 1 % of the fresh weight. Oreochromis alcalicus,
a species of fish living in an African soda lake at
pH 10, has all the enzymes of the urea cycle in its
liver; it excretes only urea, and no ammonia
[200].
The rule for the amphibians is that the larvae
and adults of persistently aquatic species are
ammoniotelic, whilst the amphibious and terrestrial species are increasingly ureotelic [269]. The
larvae of the ovoviviparous European salamander, which develop in the uterus up to the 14th
month, are ureotelic. The larvae of the frog Leptodactylus bufonis, which develop in the foam of
a mud nest, also produce mainly urea. During
dehydration or an increase in the salt concentration of the environment, the internal osmolality
of Xenopus laevis, various Rana species and Bufo
viridis rises due to increased synthesis and retention of urea. The South African clawed frog
Xenopus laevis can acclimatize to a 300-mmol/l
sodium chloride solution; this corresponds to
60 % of the seawater osmolality. Acclimatization
involves a three- to sixfold increase in activity of
glutamate dehydrogenase and the enzymes of the
urea cycle in the liver; the urea content of the
blood plasma rises from about 4 to 200 mmol/l,
and the biological half-life of the urea in the
blood extends from 2 to 42 h. Xenopus larvae can
also survive at 300 mosmol per kg water, but
there are relatively minor changes in the urea
concentration and its half-life in the plasma. Such
adaptation to increased osmolality by an increase
in urea concentration is also reported for other
anurans; for example, Rana cancrivora from the
mangrove swamps of Thailand survives 700
mosmol per kg water. Osmoregulatory changes in
urea production are observed only in spring and
summer animals of Rana pipiens; here, arginine
12 Small Nitrogenous Compounds
excretion have been examined in more detail in
Lumbricus terrestris; the critical factor is a decline in glycogen reserves from the normal 14 mglg
to less than 4 mglg during starvation [255].
The typical excretory product of the arachnids
is guanine, and this predominates in the real spiders, mites, uropygi, solifugi and amblypygi. The
situation is more complicated in the scorpions.
Vejovis mexican us excretes only guanine, but
Lycas tricarinatus also produces some urea. The
excretions of Paruroctonus mesaensis contain
over 90 % xanthine and some hypoxanthine, and
this is the first known xanthotelic animal [278].
As expected, the crustaceans are strictly ammoniotelic; this is also true of the terrestrial isopods,
which in fact excrete gaseous ammonia. The only
myriapod investigated in any detail (CylindroiuIus longinensis) excreted 57 % ammonia, 38 %
amino acids and 5 % uric acid. The insects in general are a model for uricotelic excretion, although
systematic investigation of over 400 species
showed that the activity of uricolytic enzymes
resulted in the excretion also of allantoin,
allantoic acid and urea [51]. Excretion of allantoin in fact predominates in Dysdercus fasciatus,
and hypoxanthine and xanthine are found in addition to uric acid in some species. The butterfly
Colias croceus stores guanine in the wings, and
the tsetse fly Glossina morsitans excretes relatively large amounts of amino acids, especially
arginine and histidine. Uric acid is sometimes
stored in the specialized fat-body cells of insects;
in Periplaneta and other cockroaches the storage
is indefinite, and the uric acid is not released during the insect's lifetime [50, 51].
Echinoderms excrete large quantities of amino
acids, e.g. 24-30 % of excreted nitrogen in the
cases of Asterias and Diadema. In many echinoderms, more than 10 % of the excreted nitrogen is
in the form of urea [237]. The tunicates are
ammoniotelic. In the eggs of the ascidian Corella
willmeriana, ammonium salts at a concentration
of about 250 mmol/l serve to reduce the buoyant
density, as in the pelagic squids. Some ascidians
excrete ammonia but in addition store uric acid in
the form of concretions in the nephridia [82].
In the fish, the elasmobranchs are ureotelic, as
is the only living crossopterygian Latimeria chalumnae. The urea, however, is retained by the
kidneys and accumulates in the intra- and extracellular fluids, where it is the most important substance of osmoregulation. In the freshwater ray
Potamotrygon sp., the urea level in the plasma is
relatively low and remains almost unchanged
when the animal acclimatizes to dilute seawater.
The bony fish excrete almost exclusively ammonia, although in the case of the mudskipper Periophthalmus up to 33 % urea is excreted. The
nitrogen metabolism of the three dipnoan genera
varies greatly in parallel with their abilities to
withstand dry periods. The Australian lungfish
Neoceratodus, which never leaves the water and
whose lungs are used only for accessory air
breathing, is almost purely ammoniotelic, with no
more than 6 % of the excreted nitrogen occurring
as urea. On the other hand, the South American
lungfish Lepidosiren, which can live out of water
but does not build a cocoon like Protopterus,
excretes 38 % urea when living in water. The African lungfish Protopterus already has a urea fraction of over 50 % when in the water. During aestivation, the tissue urea concentration is as much
as 1 % of the fresh weight. Oreochromis alcalicus,
a species of fish living in an African soda lake at
pH 10, has all the enzymes of the urea cycle in its
liver; it excretes only urea, and no ammonia
[200].
The rule for the amphibians is that the larvae
and adults of persistently aquatic species are
ammoniotelic, whilst the amphibious and terrestrial species are increasingly ureotelic [269]. The
larvae of the ovoviviparous European salamander, which develop in the uterus up to the 14th
month, are ureotelic. The larvae of the frog Leptodactylus bufonis, which develop in the foam of
a mud nest, also produce mainly urea. During
dehydration or an increase in the salt concentration of the environment, the internal osmolality
of Xenopus laevis, various Rana species and Bufo
viridis rises due to increased synthesis and retention of urea. The South African clawed frog
Xenopus laevis can acclimatize to a 300-mmol/l
sodium chloride solution; this corresponds to
60 % of the seawater osmolality. Acclimatization
involves a three- to sixfold increase in activity of
glutamate dehydrogenase and the enzymes of the
urea cycle in the liver; the urea content of the
blood plasma rises from about 4 to 200 mmol/l,
and the biological half-life of the urea in the
blood extends from 2 to 42 h. Xenopus larvae can
also survive at 300 mosmol per kg water, but
there are relatively minor changes in the urea
concentration and its half-life in the plasma. Such
adaptation to increased osmolality by an increase
in urea concentration is also reported for other
anurans; for example, Rana cancrivora from the
mangrove swamps of Thailand survives 700
mosmol per kg water. Osmoregulatory changes in
urea production are observed only in spring and
summer animals of Rana pipiens; here, arginine