Lobster

Friends
The One With The Prom Video
Episode 2.14

First gap: Ross is getting frustrated with his “supposed-to-be” relationship with Rachel

Phoebe: Hang in there, it's gonna happen.
Ross: What? Okay, now how do you know that?
Phoebe: Because she's your lobster.
Chandler: Oh, she's goin' somewhere.
Phoebe: Come on, you guys. It's a known fact that lobsters fall in love and mate for life. You know what? You can actually see old lobster couples walkin' around their tank, you know, holding claws like...

~~~

Unknown Life of Lobsters: Sex, Robots, and Beyond
Sean Markey
National Geographic News
August 24, 2004

[...] Female lobsters only mate right after they shed their shell. So by moving in with him, they're getting protection from this dominant male in their most vulnerable moment.

So they shed their shell, and this beautiful copulatory event occurs. The details are probably too racy for nationalgeographic.com.

Then they have this bond for about ten days to two weeks. She lives with him until her shell hardens up again, and then she moves out. That's it. It's done.

Except that there's a new female waiting on the doorstep. So all the females of the neighborhood create this sisterhood, and they take turns mating with that one dominant male.

The scientists I write about came up with a name for this—it's also evidenced in other kinds of creatures. It's called serial monogamy. Unfortunately, the beta males in the neighborhood don't get any action.