Well not all of the things that shoot at you are made up. For example, the Sea Dragon is based on the Leafy Sea Dragon (a relative of Sea Horses) with more dragon-like characteristics, the Jellyfish that shoot stuff at you are probably based off of some deep sea jellyfish (which don't shoot energy blasts in rl), and the wierd things that shoot spines at you are based off of sea urchins.
Those blaster crabs (and a similar creature in the Cathendral) are definetly made up as they look more like some fighter ship than anything.
Anyways, MOST things ingame are based off of real creatures.