Quote:
Originally Posted by wet
Right, I was thinking of the marine fishes not chewing much, but freshwater fishes chewing due to having more amphibious ancestors at some time, and so having been selected via survival in drier climates, where suction feeding doesn't work outside of water. Their chewing muscles and tongue are probably less well developed than terrestrial chewing animals, but much better than ocean fish.
Do these fin walkers move their fins out laterally to the sides and paddle (like oars) in very shallow water? Or is the walking limited to nonshallows?
I think all these freshwater must have had lobe fins which since disappeared, or developed a eel-like crawl, or directional flipping, in the past, and this selected for better air breathing as a result.
I guess polarized vision might also be selected for under those conditions.
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Probably the modern bony fishes and amphibians both descended from a species like the lungfish.
Image:Australian-Lungfish.jpg - Wikipedia, the free encyclopedia
The several types of lungfish have lobed fins, heavy scales, and both lungs and gills. They can survive for months out of water, encased in mud with a breathing hole. They have powerful jaws and are ambush predators, grabbing and crushing their prey of large invertebrates, small fish and amphibians. Despite their lungs, they are very negatively buoyant and spend most of their time resting on the bottom. They breed in a similar way to amphibians. The eggs are gelatinous like frogspawn, and they hatch into 'tadpoles' with external gills. The male looks after them.
Some of them may have gone on to become fully aquatic. They developed an advanced swim bladder for neutral buoyancy, and lighter scales. The gills became well-developed, so using the swim bladder as a lung became unnecessary. The fins became lighter and the long tail developed into separate dorsal, caudal and anal fins. Heavy jaws became a lighter protrusible mouth for suction feeding. These types of fish gave rise to the modern bony fishes, of which the carp, tetras, etc. are the most advanced group.
Other primitive lungfish species took a different path. They lost their internal gills, and the lungs became well-developed. The fins became stronger and could support its weight on land. The tail fin was lost, or lost its rays and became a decorative 'crest'. The scales were lost because of their weight, and were replaced with moist skin. The jaws became lighter but were still used for grabbing prey. These became the amphibians.
The fin walkers use their fins only to grip the surface they are walking on. The fins are not actually moved, they are pushed into the sand as the body is curved from side to side, like a salamander walking.
Some plecos move their pelvic fins (equivalent to hind legs) up and down alternately, like someone swimming with bi-fins. This is not used for forward movement, at least in modern species. They do this while stationary with their sucker-mouth attached to a surface. The fin movement creates a backward current of water, which is directed onto their eggs or young to provide oxygen and remove sand or mud.
