428
with an excessive intake of magnesium and sulphate, which constitute roughly onetenth of the salts in sea water. Another advantage in the formation of dilute urine is
that this urine, if retained in the bladder, can serve as a water reservoir. Osmoregulation
of the crab-eating frog in sea water resembles that of elasmobranchs, except in that
there is no evidence of active tubular reabsorption of urea in the frog (Smith 1936 ).
The phenomenon of salt water resistance observed for this animal is also very
intriguing from biomaterial science point of view, especially because of the skin
properties. Recent studies on crab-eating frogs in Indonesia confi rms suggestions
that these saltwater amphibians possess special skin resistance to water loss that
allows them to reduce desiccation during the critical period of acclimation from
land to salt water aerials (Wygoda et al. 2011 ). Note that semi-aquatic species typically exhibit no skin resistance to water loss. In 2010 Wygoda and co-workers discovered crab-eating frogs on Hoga Island, Indonesia. Here, some results as follow:
“The discovery presented an ideal opportunity to investigate water conservation
in perhaps the world’s most unique amphibian. Using gravimetric wind tunnel
methods, the frog’s ability to resist water loss across the skin was tested. High resistance values are common for arboreal and desert fossorial frogs, which may have
values up to 300 s*cm
−1
. However, semi-aquatic species, like the crab-eating frog,
typically exhibit no skin resistance, so that water evaporates at rates equal to free
water surface (i.e. 0 sec/cm). Surprisingly, it was found a skin resistance value of
0.27 with a standard error of ± 0.06 s*cm
−1
. While this value seems small, the relationship between vapour density, water loss, and skin resistance means that modest
increases in resistance may dramatically reduce overall skin evaporation rates,”
( Bennett et al. 2011 ).
The adaptive advantage of evaporative water loss reduction in F. cancrivora may
be related to its amphibious behaviour and the time course of the mechanism by
which it develops the ability to survive in waters of high salinity. Crab-eating Frogs
often move between fresh water and saltwater habitats (Wells 2007 ), but their compensatory establishment of elevated plasma solute levels is not accomplished immediately upon entry into high salinity water. Indeed, Dicker and Elliott ( 1970 ) found
that freshwater-acclimated frogs placed in solutions of >270 mOsm initially lost
water osmotically that they did not recover for several days. Extrapolating results of
Dicker and Elliott ( 1970 ) to full-strength seawater (ca. 2,000 mOsm) shows that
freshwater-acclimated frogs moving into the sea would initially lose water at a rate
of about 41 mg cm
−2 h
−1 . A 50-g frog in seawater would fare somewhat better, reaching the same dehydration level in ca. 4 h. Clearly, initial ventures into the marine
environment must be short-lived for these frogs, especially for smaller juveniles,
and it is during time on land that reduced evaporative water loss would be most
benefi cial. A low-magnitude cutaneous resistance, for example, could prolong survival time on land by several hours. The same 2.35-g frog in air, with a cutaneous
resistance of 0.27 s cm
−1 and in the water-conserving posture, could increase its time
out of water by 1.5 h over a frog with no cutaneous resistance. The larger 50-g frog
under similar conditions could in-crease time on land by nearly 7 h. Reduced evaporative water loss probably continues to provide an advantage to frogs even after they
fully acclimate to marine conditions, in that they must regularly venture back onto
14 Epilogue
with an excessive intake of magnesium and sulphate, which constitute roughly onetenth of the salts in sea water. Another advantage in the formation of dilute urine is
that this urine, if retained in the bladder, can serve as a water reservoir. Osmoregulation
of the crab-eating frog in sea water resembles that of elasmobranchs, except in that
there is no evidence of active tubular reabsorption of urea in the frog (Smith 1936 ).
The phenomenon of salt water resistance observed for this animal is also very
intriguing from biomaterial science point of view, especially because of the skin
properties. Recent studies on crab-eating frogs in Indonesia confi rms suggestions
that these saltwater amphibians possess special skin resistance to water loss that
allows them to reduce desiccation during the critical period of acclimation from
land to salt water aerials (Wygoda et al. 2011 ). Note that semi-aquatic species typically exhibit no skin resistance to water loss. In 2010 Wygoda and co-workers discovered crab-eating frogs on Hoga Island, Indonesia. Here, some results as follow:
“The discovery presented an ideal opportunity to investigate water conservation
in perhaps the world’s most unique amphibian. Using gravimetric wind tunnel
methods, the frog’s ability to resist water loss across the skin was tested. High resistance values are common for arboreal and desert fossorial frogs, which may have
values up to 300 s*cm
−1
. However, semi-aquatic species, like the crab-eating frog,
typically exhibit no skin resistance, so that water evaporates at rates equal to free
water surface (i.e. 0 sec/cm). Surprisingly, it was found a skin resistance value of
0.27 with a standard error of ± 0.06 s*cm
−1
. While this value seems small, the relationship between vapour density, water loss, and skin resistance means that modest
increases in resistance may dramatically reduce overall skin evaporation rates,”
( Bennett et al. 2011 ).
The adaptive advantage of evaporative water loss reduction in F. cancrivora may
be related to its amphibious behaviour and the time course of the mechanism by
which it develops the ability to survive in waters of high salinity. Crab-eating Frogs
often move between fresh water and saltwater habitats (Wells 2007 ), but their compensatory establishment of elevated plasma solute levels is not accomplished immediately upon entry into high salinity water. Indeed, Dicker and Elliott ( 1970 ) found
that freshwater-acclimated frogs placed in solutions of >270 mOsm initially lost
water osmotically that they did not recover for several days. Extrapolating results of
Dicker and Elliott ( 1970 ) to full-strength seawater (ca. 2,000 mOsm) shows that
freshwater-acclimated frogs moving into the sea would initially lose water at a rate
of about 41 mg cm
−2 h
−1 . A 50-g frog in seawater would fare somewhat better, reaching the same dehydration level in ca. 4 h. Clearly, initial ventures into the marine
environment must be short-lived for these frogs, especially for smaller juveniles,
and it is during time on land that reduced evaporative water loss would be most
benefi cial. A low-magnitude cutaneous resistance, for example, could prolong survival time on land by several hours. The same 2.35-g frog in air, with a cutaneous
resistance of 0.27 s cm
−1 and in the water-conserving posture, could increase its time
out of water by 1.5 h over a frog with no cutaneous resistance. The larger 50-g frog
under similar conditions could in-crease time on land by nearly 7 h. Reduced evaporative water loss probably continues to provide an advantage to frogs even after they
fully acclimate to marine conditions, in that they must regularly venture back onto
14 Epilogue