Well, looking back at the situation, both the sea emperor and sea dragons look about the same, meaning one of the two is probably a subspecies of the other.

Or at least share some sort of evolution trait.

Basically everything except for the forelimbs and size is the same.

And other, slight differences.

Fair enough

Its settled then, the crashfish is the mutant offspring of the emporer and leviathan, hence its imunity. Ill update the wiki!

Oh and the precursors never found them as they ain't a big fan of a) the shallows due to their probable fair skin (space faring and their bases have no windows for uv) and b) their distaste for small caves (Probably because they are fairly tall based on their disecting table sizes)

Solved! Thanks folks

You forgot the fact that Crashfish are essentially oceanic KAMIKAZE PILOTS THAT TRY TO USE THEIR EXPLOSIVE BODIES TO BLOW YOU UP!!!!

Wait, Floaters are also immune. I have been collecting them to BURN in my bioreactor and have yet to see one that was infected. More mutant offspring? Also has anyone seen a Bleeder with the virus? Don't the Bleeders, Floaters, and Crashfish all fall under the symbiotic/parasite or whatever it is in the PDA? Maybe parasites and such are immune to it. Doesn't explain the Sea Dragon though.

Idk, but this are all good theories.