427
Aside from F. cancrivora , only two other anurans, the bufonid Pseudepidalea
viridis and the pipid Xenopus laevis , are capable of tolerating higher than brackish
salinities (Balinsky et al. 1972 ; Hillmann et al. 2009 ).
The Crab-eating Frog is broadly distributed in India and Asia including mangroves, marshes, coastal scrub, and disturbed forests where it can tolerate both
brackish and sea water. Phylogenetic analysis supported the monophyly of the
Fejervarya species, and the genome organization of F. cancrivora mitochondrial
DNA differs from that of neobatrachian frogs and typical vertebrates (Ren et al.
2009 ).
“The exceptional ability of F. cancrivora to survive in saline media is probably
related to the exceptionally high rates of urea production exhibited in this species.
Experimentation with unfed amphibians may lead to underestimation of salinity
tolerance, because these fasting animals must deaminate a signifi cant fraction of
their protein to produce and maintain high concentrations of urea,” (Schoemaker
and Nagy 1977 ). As the most salt-tolerant of all frogs (Hillmann et al. 2009 ) it has
been found under laboratory conditions to tolerate salinities of up to 29 ppt (Gordon
et al. 1961 ). Adults of this species survive in high salinity by maintaining a hyperosmotic plasma that uses the increased concentrations of sodium, chloride, and urea
(Gordon et al. 1961 ; Schmidt-Nielsen and Lee 1962 ; Gordon and Tucker 1968 ).
Thus, the plasma of Fejervarya species helps them to survive in salt aquatic niches
by topping up the ionic concentration with non-ionic solubilised urea.
Adult frogs have a different, and rather more unusual, method of osmotic regulation. Instead of being osmoregulators and maintaining an imbalance between the
osmotic concentration of their internal fl uids and that of the exterior, they are partial
osmo-conformers. Internal osmotic concentration is matched with that of the exterior, at least in a hyperosmotic medium. This is brought about not by manipulating
ion levels, but by ‘topping up’ the ionic concentration of the plasma with the nonionic solute urea CO(NH 2 ) 2 . Other amphibia show slightly raised urea levels under
conditions of water shortage, but employing high urea levels to maintain osmotic
balance with the environment is a habit shared only with elasmobranch fi sh (sharks,
skates and rays) (Gordon et al. 1961 ). Urea is the standard nitrogenous excretory
product of ordinary adult amphibia, and is normally voided at the earliest convenient opportunity. Indeed, given the solubility of urea, it is not an easy substance for
a largely aquatic animal to retain, and it is not understood how Rana cancrivora
( Fejervarya raja ) achieves this. Not only is it soluble and diffi cult to retain, urea is
toxic: the concentrations of urea that occur in the frog’s plasma at exterior salinities
of 80 % sea water, should denature enzymes and affect the binding of oxygen by
haemoglobin. Somehow R. cancrivora copes with these hazards (Hogarth 1999 ).
The crab-eating frog remains somewhat hypertonic to the external medium, even
in the most concentrated solutions that have been tested. This situation is maintained by the formation of urine which is more dilute than the plasma. Since the skin
is permeable to water, the inevitable result is an osmotic infl ow of water from the
medium, and this infl ow permits the formation of urine without any necessity for the
frog to drink the external medium. There may be several advantages in this situation
(Schmidt-Nielsen and Lee 1962 ). One is that the gastro-intestinal tract is not loaded
14 Epilogue
Aside from F. cancrivora , only two other anurans, the bufonid Pseudepidalea
viridis and the pipid Xenopus laevis , are capable of tolerating higher than brackish
salinities (Balinsky et al. 1972 ; Hillmann et al. 2009 ).
The Crab-eating Frog is broadly distributed in India and Asia including mangroves, marshes, coastal scrub, and disturbed forests where it can tolerate both
brackish and sea water. Phylogenetic analysis supported the monophyly of the
Fejervarya species, and the genome organization of F. cancrivora mitochondrial
DNA differs from that of neobatrachian frogs and typical vertebrates (Ren et al.
2009 ).
“The exceptional ability of F. cancrivora to survive in saline media is probably
related to the exceptionally high rates of urea production exhibited in this species.
Experimentation with unfed amphibians may lead to underestimation of salinity
tolerance, because these fasting animals must deaminate a signifi cant fraction of
their protein to produce and maintain high concentrations of urea,” (Schoemaker
and Nagy 1977 ). As the most salt-tolerant of all frogs (Hillmann et al. 2009 ) it has
been found under laboratory conditions to tolerate salinities of up to 29 ppt (Gordon
et al. 1961 ). Adults of this species survive in high salinity by maintaining a hyperosmotic plasma that uses the increased concentrations of sodium, chloride, and urea
(Gordon et al. 1961 ; Schmidt-Nielsen and Lee 1962 ; Gordon and Tucker 1968 ).
Thus, the plasma of Fejervarya species helps them to survive in salt aquatic niches
by topping up the ionic concentration with non-ionic solubilised urea.
Adult frogs have a different, and rather more unusual, method of osmotic regulation. Instead of being osmoregulators and maintaining an imbalance between the
osmotic concentration of their internal fl uids and that of the exterior, they are partial
osmo-conformers. Internal osmotic concentration is matched with that of the exterior, at least in a hyperosmotic medium. This is brought about not by manipulating
ion levels, but by ‘topping up’ the ionic concentration of the plasma with the nonionic solute urea CO(NH 2 ) 2 . Other amphibia show slightly raised urea levels under
conditions of water shortage, but employing high urea levels to maintain osmotic
balance with the environment is a habit shared only with elasmobranch fi sh (sharks,
skates and rays) (Gordon et al. 1961 ). Urea is the standard nitrogenous excretory
product of ordinary adult amphibia, and is normally voided at the earliest convenient opportunity. Indeed, given the solubility of urea, it is not an easy substance for
a largely aquatic animal to retain, and it is not understood how Rana cancrivora
( Fejervarya raja ) achieves this. Not only is it soluble and diffi cult to retain, urea is
toxic: the concentrations of urea that occur in the frog’s plasma at exterior salinities
of 80 % sea water, should denature enzymes and affect the binding of oxygen by
haemoglobin. Somehow R. cancrivora copes with these hazards (Hogarth 1999 ).
The crab-eating frog remains somewhat hypertonic to the external medium, even
in the most concentrated solutions that have been tested. This situation is maintained by the formation of urine which is more dilute than the plasma. Since the skin
is permeable to water, the inevitable result is an osmotic infl ow of water from the
medium, and this infl ow permits the formation of urine without any necessity for the
frog to drink the external medium. There may be several advantages in this situation
(Schmidt-Nielsen and Lee 1962 ). One is that the gastro-intestinal tract is not loaded
14 Epilogue