- [Narrator] Nassau groupers are solitary most of the year.

Reproduction brings them together.

Some will travel hundreds of miles.

They arrive in December and January when Caribbean waters are coolest.

- [Expert] They start communicating to each other.

They have different signals, different languages, or different words or vowels for the different behaviors that they conduct.

Only when they aggregate to spawn.

- [Narrator] They also communicate with stylized movements and shifting patterns on their skin.

- They look cool already.

They got their stripes and everything, but when it's getting close to spawning, they're like trying on different outfits.

Some turn white, some turn black.

- [Narrator] More groupers arrive.

The chorus swells in volume.

(groupers droning) Some wait on the sea floor, conserving their energy.

- [Expert] They need to synchronize to one or two nights.

Everybody has to have their eggs and their sperm ready for that moment.

They are communicating.

Are we ready to spawn?

Do you think we can spawn now?

- [Narrator] As their numbers grow, they rise from the sea floor each evening as if rehearsing.

As the full moon approaches, the performances become more sophisticated.

- Where the real fun is, is in the courtship prior to spawning.

- [Expert 2] Each has their own suite of behaviors and sounds that all come together into this courtship dance.

- [Narrator] If they are ready, they change into matching costumes.

- As you get closer to spawning, they all choose this tuxedo it looks like to me, black, black, black with a bar over the eye and a white belly.

We call it bi-color phase.

It's like a vaudeville act.

Males fighting over females, making sounds at each other.

- [Narrator] Males wrestle around the females, the noise builds, but night after night, they hold back.

Then one night, the temperature is right.

The moonlight is right.

Most importantly, there's a critical mass of fish.

The chorus reaches a crescendo.

(groupers droning) Then, silence.

Spawning has begun.