The courtship of blue crabs

February 14, 2017

There are no valentines, flowers or chocolates, but when it comes to the courtship of blue crabs, there is a little perfume and some dancing.

When a female blue crab is ready to meet her match, she releases pheromones, which act as a silent mating call to signal possible suitors. Not just any gentleman caller will do. Dr. J. Sook Chung, an endocrinologist and an expert on crab reproduction at the University of Maryland Center for Environmental Science's Institute of Marine and Environmental Technology, said the female is looking for a big crab that would be more likely to provide her with more sperm than a smaller crab.

There's much at stake for the female in her choice. The female blue crab must molt to become soft enough to mate, and unlike a male, can only shed once in her life.

"A female molts once to become an adult. She turns into an egg-making machine," Chung said.

She can hold sperm for up to two years and produce as many as one million eggs.

But first a male crab must prove himself. He has to dance for her.

"You have to display, 'I'm big, tall, I'm strong,'" Chung said.

As part of this pre-mating ritual, he stands upright on his legs to look tall, spreads his claws (called chelae) out 180 degrees to look bigger, and fans his swimming legs to show his movement.

"It isn't about appearance, it's the movement," Chung said. 

When he grabs her, it is so abrupt and quick that it can look like an attack. "My heart freezes," Chung said of this moment. "But how he grabs her, he doesn’t hurt her. He never breaks her somehow."

He guards and protects her, until she molts in front of him.

Then he moves over her and they mate.

His hold helps him protect his mate at her most vulnerable moment. He will neglect food to stay with her until her shell hardens again in two to three days, "and he never minds," Chung said.

"Those are the things that are very interesting."

Chung is passionate about the crabs and how they reproduce. She discovered an important hormone—stored in the eyes of female blue crabs--that is responsible for forming the body parts essential for female crabs to mate and raise their young.

Going forward, she wants to study how the female crabs approach the mating process. In her lab, she studies later-stage females who, once in a tank with a male, will approach them to mate, while the male takes the lead in the wild. She also wants to learn more about the maternal care of crabs. The females will fan out their egg clutches at certain times to oxygenate all the eggs they holding.

"How does she know?"