We can however consider the possibility that anything that evolved or was designed to be immortal would commensurately have either longer gestational periods, fewer offspring (or BOTH) -- or no offspring at all Even in nature in real life, we see that longer lived lifeforms have fewer kids -- case in point, humans.

Looking further on the immortality front, there's that jellyfish (T. nutricula or something) that doesn't die of old age; instead it reverts to earlier stages in its life cycle and grows up again.