AIR BREATHING FISH AND TERRESTRIAL LIFE
Is it possible that organisms adapted to aquatic life (such as fish) could evolve into terrestrial animals that breathe air?
If evolution is correct, a number of separate aquatic lineages could develop adaptations for terrestrial life and air breathing independently. No one set of adaptations would be absolutely required; a number of alternate forms would be adaptive. Transitional states would be possible in which some, but not all, of the adaptations required for full terrestrial life would be beneficial.
If the creationism model is correct, aquatic lineages can not evolve adaptations for terrestrial life. Transitional states are not expected, nor are multiple separate origins of comparable adaptations.
If intelligent design is correct, it is not expected that the "design" of fish would be need to be reworked for terrestrial life or air breathing. It is certainly not expected that the design of separate lineages would all need to be reworked to similar ends. If the adaptation to terrestrial life is an example of irreducible complexity, aquatic lineages should not exist which benefit from some, but not all, of the traits required for terrestrial life.
If there was only a single animal body plan which would accommodate life on land, it could be argued that variations within populations of aquatic animals might never achieve terrestrial existence without an "intelligent design". Our understanding of life's diversity has demonstrated that there is no such limit on the adaptability of aquatic organisms. A great variety of aquatic organisms have modified their originally aquatic "design" for terrestrial life. Worms first diversified in the oceans but different lineages (such as nematodes and segmented worms) have successfully invaded the land. While most snails and slugs are aquatic, some are fully terrestrial. A number of arthropod groups have adapted to terrestrial life including myriapods (centipedes and millipedes), scorpions, spiders, the ancestors of insects (such as Devonohexapodus), and many crabs. Among fish, there are a variety of lineages which can venture onto land for brief periods. These fish would undoubtedly evolve greater adaptations for terrestrial life over time were it not for the richness of modern terrestrial vertebrates that would limit their success. The rhipidistian ancestors of amphibians did not develop the only possible suite of anatomical features which would allow life on land, they simply evolved one of many possible sets of terrestrial adaptations.
Some fish perform gas exchange with air through their buccopharyngeal
apparatus (mouth and throat), an opercular structure, the swim bladder,
the intestine, pharyngeal lungs, the skin, and the gastrointestinal tract
which allow gas exchange using swallowed air. Some of these fish are obligate
air breathers and may travel for short distances over land.(Maina, 1998,
p. 220). An air-breathing species of the family Loricaridae is pictured
in the following photos.
While the fine filaments of most fish gills collapse in air, a few fish
possess modified gills which can breathe atmospheric air (Johansen, Kjell
from Hoar, 1970). Some placoderms seem to have possessed lunglike structures
(Perry, 2001). The air/swim bladder of bony fish functions as a respiratory
organ in lungfish and in basal actinopterygians, suggesting that this
was its primitive function in the ancestral bony fish. In other words,
swim bladders seem to have evolved from lungs rather than lungs from swim
bladders. Basal actinopterygians even possess blood vessels which correspond
to the pulmonary circuit of lungfish and tetrapods, although there is
no separation of this blood from that of the systemic circuit in the heart
(Johansen, Kjell from Hoar, 1970).
Why would a fish evolve lungs? Lack of oxygen in water can cause mortality in fish and still waters can have very low levels of dissolved oxygen (especially at warmer temperatures). Any fish with lungs has an advantage in these environments since it can supplement its oxygen supply from the gills. A variety of modern fish can breathe oxygen from atmospheric air (which is why a goldfish in a fishbowl doesn't need an air pump to survive).
Locomotion on land does not absolutely require new structures other
than those which are already present in aquatic organisms. Terrestrial
snails, myriapods, spiders, scorpions, and crabs are not fundamentally
different from their aquatic relatives. A number of different lineages
of fish can move on land without any obvious structures which are lacking
in their purely aquatic relatives. Stout, robust fins have evolved in
a number of fish lineages ranging from placoderms (such as Bothriolepis),
actinopterygians (such as mudskippers and many catfish), and extinct sarcopterygians.
The limb bones which land vertebrates utilize today are simply modified
versions of the bones which supported fins in purely aquatic fish. Increased
mobility of these elements (such as the evolution of jointed digits in
Sauripteris) could occur in fish which were adapted to aquatic life.