I spend a lot of time at the beach in the summer. One of the beaches I frequent in the summer is a “bay beach” in Brigantine, NJ (South Jersey by Atlantic City). Because of the bay and the lack of a large crowd, there are a lot of things at this beach that I usually don’t see at other beaches. The craziest thing I have seen at this beach are these crabs with one huge claw. They are called fiddler crabs. I have always wondered why they had one claw and what the benefit to this one, abnormally large claw was. And does their larger claw’s size matter? So SC200, you’re about to learn!

The pink shows where fiddle crabs are found throughout the world

Fiddler crabs are small, semi-terrestrial (meaning they live mostly on land but require nearby water especially for breeding) crabs that are characterized by their extremely asymmetrical claws (for males). They are found along the eastern and southern coasts of the United States (where I see them), as well as many other coasts throughout the world (see map above). Fiddler crabs are hatched from eggs. The eggs grow from larvae, and then live as plankton through several molt stages. They then molt into immature crabs. Once the crabs move to land to continue to grow, the males start to develop a large asymmetrical claw. The females keep their two small claws, used for feeding, while the males only have one small claw for feeding.

So what is going on with these large, odd-looking claws? The claws usually weigh about 1/3 of the crab’s overall body weight. The claw needs to be light enough to wave around, but weigh enough to project injury or harm other crabs. Fiddler crabs are often found waving their large claw in order to attract females into their burrow to mate. The large claw of the male is used for mating purposes, as well as combat.

A male fiddler crab raising his large claw in hopes of attracting a female fiddler crab

Some evidence suggests that females choose the male crab with the largest claw. To ensure this, I found some studies that support it. This experiment (an experiment and not a observational study because they added “dummy crabs”) tested female fiddler crab preference based on both the size and elevation of the male claw and on male handedness. Dead, resin-coated males were used as test (dummy) objects. Females were observed to approach males with larger claws and males who raised their claw in the air. The experimenters noted that the handedness of the crab had no effect on the female responses. These results are consistent with evolution theories: females prefer to mate with crabs with bigger claw size because larger claw size correlates to better at fight, which leads to better chances of survival. Check out this video of a male fiddler crab waving his claw to attract a female. Once a female chooses a mate (based on his claw size), the female enters the male’s burrow. The female remains in the males burrow for about two weeks, while her newly inseminated eggs are developing into embryos. She will leave the burrow once she is ready for her eggs to go hatch in the water.

Two male fiddler crabs fighting

The male’s giant claw is also used as a weapon to fight other male crabs. These “fights” usually ensue from an unknown male hanging around the outside of another male’s burrow. It is believed that because the winners of the fights are usually the ones with the bigger claws, the successors of the fights pass along their genes, therefore the evolution of the big claw. Check out this (start watching at around 1:30) video of male fiddler crabs fighting.

To conclude fellow friends of SC200: A fiddler crab (male only) has a large call that is used to attract females. Females are attracted to the larger claws, as well as the male who waves his claw the highest. This is consistent with evolution and wanting to mate with a male crab that has a larger claw, because it increases his chances of survival. Males also use this large claw to fight other males. The cause of the fights is mainly to protect their burrow where they hope to breed with a female. The size of the claw also matters for fighting because the crab with the larger claw usually wins.