Xiphactinus audax was a carnivorous fish that swam the oceans during the Late Cretaceous Period, between 112 and 66 million years ago. It could grow up to nearly seventeen feet in length, so you wouldn’t want to bump into it if you were swimming in the Cretaceous seas (that is, if one of the myriad other big and fierce monsters lurking in there didn’t get you first).
The bioluminescent spots I have given this specimen are a speculative addition on my part.