Wow! I saw that argument spiralling out of control in my mind... but what a resolution! That kind of healthy approach typifies this site. Nice!
) I think the matter of whether fish evolved first in FW
(our present conception of FW
) is moot. When fish (i.e. in this case vertebrate sea creatures) first came into being during the Cambrian Explosion (about 530 mya), water conditions were hugely different than they are at the present. Heck, the continents as we know them didn't exist. What I am sure of is that these creatures first occured in the large oceans that surrounded pre-Pangea. Not necessarily SW
(as we know it) oceans, though...
So, I think it's not quite correct to say that marine fish have had more time to develop, because FW
fish are almost certainly off-shoots of a common ancestral group of Cambrian vertebrates. After all, FW
fish didn't spontaneously come into existence in the two environments. Morphology (body shape) is sufficiently similar (look at FW
Angelfish torsos and SW
batfish torsos) to indicate common ancestry...
And, salinity and other ion concentrations in the world's oceans have changed hugely over even the last thousands of years (deglaciation causes a freshening of the seas, glaciation an increase in salinity), so I think it's fair to say that fish have had to adapt in all environments to changing conditions quite regularly over history.
I tend to agree with TheNewGuy's statement that a fish's coloration is dependent upon the environment it lives in. Is this a cop-out, just another chicken-and-egg argument? I don't think so... The coloration of their environment, mainly the plants and other primary producers around them do vary in color depending upon the minerals and light available to them. How do I explain neon tetras??? Their brilliant blue could well imitate the color of reflected light that bounces off of the rivers they live in. This could be a poweful adaptation to protect them from wading and diving birds. Also, given the fact that light is refracted under water and shimmers, this could be an added protection from predatory fish underwater--a predator could believe it was seeing a reflection of light, not prey.
Just some ideas...