I wonder if their long lifespan is an adaptation to take advantage of some extremely rare event that benefits their reproduction.

For example, perhaps their eggs are usually eaten by a kind of crab. But once every 50 years there is a a massive hurricane that decimates the crab population. In the following spawning season, nearly all of the eggs hatch. The 150 year lifespan allows them to participate in 2-3 of these rare spawning seasons.

Since the event is rare, scientists get few chances to notice it.