so are all the white cave fish i've been videoing a seperate population who have lost their pigment over generations, or if you raise a fish from an egg in darkness will it never bother developing pigment?
If it's not needed it evolves out. I think it's an evolutionary advantage to lose eyes and pigment if ou don't need them as it saves energy needed to make them. Only a marginal advantage but has an effect over millennia.
There have been Mexican cave fish (Astyanax mexicanus) kept in the aquarium hobby for quite a while so they're well documented, and they develop eyes as fry, then lose them. They can also interbreed with surface species and produce fertile offspring, as the blind fish are just another morph of the same species, so they're quite interesting.
In this species yes. This probably means that the cave morph populations only washed in recently and haven?t evolved into a seperate species. There will be other fish species that are solely cave dwelling.