Genesis  1:6-8

"And God said,  Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.  (Notice that the scripture uses the word "let" twice.  God isn't creating anything; he's just rearranging things.  Allowing things to happen.)

And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.  And God called the firmament Heaven.  And the evening and morning were the second day."

This seems to indicate that not only was it dark, and void, and without form--but that it was a wet murky mess.  Exactly the atmosphere for some animals to survive in, and for others to die out.

Fish would have been just fine.  Fossil history includes many many species of fish as far back as you want to go.  They don't really change all that much over the millennium.  They just keep looking like fish.  Their differences are small, different perhaps, but fish just the same.

There are many animals that survived whatever happened between Genesis 1:1 and Genesis 1:2.  We have their bones.  We have their fossils.  This is why I believe that there was a "gap."  There was a period between the dinosauric age in which water life continued, but in which most forms of land life died out.  Note that I said "most."

I think it is probable that a gap occurred between verse one and two.  But this is a just a theory--the gap theory.  Many scientists who are Christians concur.  It fits the scientific evidence of the dinosaurs.

The world is entering a new stage.  It is being prepared for new kinds of animals.