Geoscience Reference
In-Depth Information
saltwater tolerance presented above, which suggests
that lissamphibians have been restricted to fresh-
water for less than 330 Ma). Alternatively, this
suggests that the ornithine pathway has been
retained because of selective pressures exerted by
the terrestrial environment (but this explanation
applies only to relatively terrestrial lissamphibians).
In any case, the presence of an ornithine cycle in
lissamphibians suggests either a direct passage
from the marine to the terrestrial environments, or
only a short intermediate period in which stem-
amphibians inhabited freshwater environments.
When reviewing evidence on whether air-
breathing in osteichthyans had appeared in a fresh-
water or a marine environment and whether the
conquest of land among sarcopterygians had started
in a freshwater or a marine environment, Graham
(1997) found no conclusive answers to these ques-
tions. Hypoxia is more often and more regularly a
problem in stagnant freshwater than in oceans but
sheltered bays, lagoons and even enclosed seas
can experience hypoxia. Furthermore, hypoxia is
not the only selective pressure that can favour the
appearance and maintenance of air-breathing.
Farmer (1997, p. 361) indicated that 'lungs may
have evolved in early fishes to support an active life-
style by supplying oxygen to the heart and enhan-
cing cardiac performance'. This author also
pointed to the fact that air breathing is not restricted
to (or even highly correlated with) hypoxic fresh-
water environments. Among actinopterygians
several air-breathing groups inhabit coastal areas
where this ability enables them to exploit parts of
the habitat and resources unavailable to other acti-
nopterygians (Graham 1997). Similar selective
pressures may have driven the evolution of early ste-
gocephalians, in which case there is no reason to
expect that they would have been stenohaline fresh-
water forms.
In the lissamphibians that tolerate salt- or brack-
ish water, osmotic regulation may involve the exter-
nal gills. This is suggested in Fejervarya cancrivora
(formerly known as Rana cancrivora) by the fact
that tadpoles regulate their osmotic concentration.
This varies from only 250 m-osmoles/l (milli-
osmoles per litre; this means 0.001 mole of solute
per litre) to more than 900 m-osmoles/l when con-
fronted with an increase in environmental osmotic
pressure (Gordon & Tucker 1965, p. 439, fig. 1).
On the contrary, the adults are osmoconformers
(Gordon et al. 1961). This shows that neither gills
nor impervious skin are required for amphibians to
tolerate saltwater; the skin of adult Fejervarya can-
crivora is fairly permeable (Gordon et al. 1961,
p. 663).
Study of various ontogenetic stages shows that
tadpoles of stages IV to XIX maintain an internal
osmotic concentration of about 490 m-osmoles/l
in 80% seawater. In the same environment, that con-
centration rises from stages XX to XXV (the latter is
a fully metamorphosed froglet lacking gills) to
become isosmotic with the environment at stage
XXV (Gordon & Tucker 1965, p. 441, fig. 2).
Since the gills of teleosts are known to be involved
in active salt transport, and since the loss of osmor-
egulation in F. cancrivora coincides with loss of
gills in its ontogeny, Gordon & Tucker (1965)
suggest that the gills of F. cancrivora are involved
in osmoregulation.
More recent studies show that, unsurprisingly,
kidneys are also important in osmoregulation.
They retain urea to increase osmosis in dry or
hypersaline environments, at least in Rhinella
marina (called Bufo marinus by Konno et al.
2006). Fejervarya cancrivora is probably the lis-
samphibian with highest saltwater tolerance
(Gordon 1962); Gordon et al. (1961, p. 665)
reported that tadpoles can tolerate slightly hypersa-
line concentrations (up to 39‰ salinity, a salt con-
centration about 20% higher than seawater). Thus,
it is probably among the most relevant lissamphi-
bian species to understand saltwater tolerance (or
lack thereof) in lissamphibians.
External gills are useful for osmoregulation in
amphibians, but the presence of gills does not
necessarily confer osmoregulatory abilities.
Indeed, most tadpoles and anuran larvae have
gills, but most cannot tolerate saltwater. Neverthe-
less, the presence of external gills in larvae of
temnospondyls, seymouriamorphs and at least
some amphibians (Microbrachis and possibly ade-
logyrinids and lysorophians) raises the possibility
that it conferred these taxa osmoregulatory ability.
Along with the occurrence of some body fossils,
trackways or burrows of these taxa in brackish or
saltwater environments (Schultze 1985; Laurin &
Soler-Gij
´
n 2001, 2006), this suggests that these
taxa tolerated saltwater. When they lived in the
same environments and lacked gills (which appar-
ently disappeared in ontogeny in seymouriamorphs
and probably in most temnospondyls), the adults
may have been osmoconformers if they had per-
meable skin. However, such a relatively permeable
skin (a superficial layer of lipids strongly reduces its
permeability in some species) may be an autapo-
morphy of the Lissamphibia.
The facts that the most aquatic lissamphibians
have a lower skin permeability to water than most
terrestrial lissamphibians (Yorio & Bentley 1978)
and that even desert anurans can extract moisture
from soil in their estivation burrows and secrete
cocoons only when the soil becomes especially
dry (Cartledge et al. 2006) support this suggestion;
skin permeability appears to be adaptative, rather
than disadvantageous, for lissamphibians in many
terrestrial environments. The skin of stem-tetrapods
