Chapter 6
The Amphibia
The Amphibia bridge the phyletic gap between the fishes and terrestrial tetrapod
vertebrates. This transition has involved considerable osmoregulatory, as well as
respiratory, change so that water and salt metabolism may be expected to be particularly varied and interesting. Both for these reasons, and the relatively free liaison
between physiologists and amphibians, the group has received considerable scientific attention.
The Amphibia arose from a crossopterygian fish in Devonian times . This transition is thought to have occurred in fresh water, where the Amphibia lived for
a considerable period of time subsequently. There is only one surviving
crossopterygian fish, the coelacanth, Latimeria chalumnae, which was not found
until 1938. This coelacanth is a marine species, several of which have been caught
off the southeast coast of Africa. The contemporary Amphibia are represented by
three orders : the Apoda, the Urodela and the Anura. The Apoda, or caecilians,
are small shy worm-like animals , of which there are about 17 genera , living exclusively in tropical equatorial areas of America, Africa and Asia . Unfortunately
very little is known about their ph ysiology and there is no information about their
osmoregulation. The Urodela, or the Caudata, contain the newts and salamanders
of which there are 43 genera, mostly confined to temperate regions in the northern
hemisphere and completely absent from Australia. The Anura, are the most numerous of the three orders, containing more than 200 genera which are widely distributed around the world. 'Nothing - neither climates, nor deserts, nor mountains
nor even narrow gaps of salt water - has prevented the spread of dominant frogs
over the world in the course of time' (DARLINGTON, 1957). Anurans are present
on all the major land masse s (except Antarctica) where they occupy habitats ranging
from freshwater ponds and streams, to tropical forests and arid deserts. A few species even venture into the cold polar regions north of the arctic circle. Many are
primarily aquatic but the y are usually more comfortable in fresh than brackish water. At least one species, Rana cancrivora , the crab-eating frog, lives in sea-water
among mangroves along coastal areas of south east Asia. DARLINGTON (1957) concludes that frogs must have crossed narrow gaps of salt water, but notes that they
usually have a low tolerance to such solutions and this presumably accounts, as
commented upon by DARWIN (1859), for their absence from most oceanic islands.
Amphibians readily lose water through their permeable skin so that desert regions
may also be expected to restrict their movements. Nevertheless, while such areas
may slow their migrations, they certainly do not stop them .
The amphibians exhibit a number of well defined physiological differences from
their piscine ancestors and reptilian descendants. Such divergences may have a considerable influence on the osmoregulation of the respective groups so that I have
attempted to summarize some of them (Table 6.1). Perhaps the major factor that
affects amphibian osmoregulation is the adoption, by most of the adult form s, of
atmospheric, instead of aquatic, respiration. The Amphibia are assisted in this process by gaseous exchanges across their skins (the plethodont salamanders have no
161
The Amphibia
The Amphibia bridge the phyletic gap between the fishes and terrestrial tetrapod
vertebrates. This transition has involved considerable osmoregulatory, as well as
respiratory, change so that water and salt metabolism may be expected to be particularly varied and interesting. Both for these reasons, and the relatively free liaison
between physiologists and amphibians, the group has received considerable scientific attention.
The Amphibia arose from a crossopterygian fish in Devonian times . This transition is thought to have occurred in fresh water, where the Amphibia lived for
a considerable period of time subsequently. There is only one surviving
crossopterygian fish, the coelacanth, Latimeria chalumnae, which was not found
until 1938. This coelacanth is a marine species, several of which have been caught
off the southeast coast of Africa. The contemporary Amphibia are represented by
three orders : the Apoda, the Urodela and the Anura. The Apoda, or caecilians,
are small shy worm-like animals , of which there are about 17 genera , living exclusively in tropical equatorial areas of America, Africa and Asia . Unfortunately
very little is known about their ph ysiology and there is no information about their
osmoregulation. The Urodela, or the Caudata, contain the newts and salamanders
of which there are 43 genera, mostly confined to temperate regions in the northern
hemisphere and completely absent from Australia. The Anura, are the most numerous of the three orders, containing more than 200 genera which are widely distributed around the world. 'Nothing - neither climates, nor deserts, nor mountains
nor even narrow gaps of salt water - has prevented the spread of dominant frogs
over the world in the course of time' (DARLINGTON, 1957). Anurans are present
on all the major land masse s (except Antarctica) where they occupy habitats ranging
from freshwater ponds and streams, to tropical forests and arid deserts. A few species even venture into the cold polar regions north of the arctic circle. Many are
primarily aquatic but the y are usually more comfortable in fresh than brackish water. At least one species, Rana cancrivora , the crab-eating frog, lives in sea-water
among mangroves along coastal areas of south east Asia. DARLINGTON (1957) concludes that frogs must have crossed narrow gaps of salt water, but notes that they
usually have a low tolerance to such solutions and this presumably accounts, as
commented upon by DARWIN (1859), for their absence from most oceanic islands.
Amphibians readily lose water through their permeable skin so that desert regions
may also be expected to restrict their movements. Nevertheless, while such areas
may slow their migrations, they certainly do not stop them .
The amphibians exhibit a number of well defined physiological differences from
their piscine ancestors and reptilian descendants. Such divergences may have a considerable influence on the osmoregulation of the respective groups so that I have
attempted to summarize some of them (Table 6.1). Perhaps the major factor that
affects amphibian osmoregulation is the adoption, by most of the adult form s, of
atmospheric, instead of aquatic, respiration. The Amphibia are assisted in this process by gaseous exchanges across their skins (the plethodont salamanders have no
161