It might, but keep in mind the history of the planet. Those fish more likely came from a species that already existed in warmer waters. Over the course of time they slowly drifted in and adapted. Then they could be shut off from the outside world and still thrive due to their earlier adaption.
I wonder what the ethical implications would be if we found out for certain that Europa was a barren world with a salty ocean under the ice, and then dumped a few tons of marine life into it.
