Fiddler crab (genus Uca)

Despite my thorough and long-lasting fear of spiders, I’ve always found crabs (which to me equate to the marine version of spiders) quite pleasant. I have very fond memories of being a kid and lifting every rock we could find on the beach to try and find some crabs. Or when we were on vacation on the East Coast, we’d wade through the water with nets try to catch huge crabs that ran in and out of seaweed forests. The point is, I’m always happy to blog about crabs, so today’s post is about fiddler crabs.

Fiddler crabs make up the genus Uca, and there are over a hundred species around the world. They live all over the place, from West Africa and the West Atlantic to the Eastern and Indo-Pacific. Fiddler crabs prefer to live on beaches near waters with low salinity, and because of this are often sold as fresh water crabs in pet stores. If you get one, though, don’t keep it in fresh water, as it will die in a couple of weeks without salt water.

Fiddler crabs digs burrows in which they hide form predators. Burrows usually have entrances of 2-5cm, and their depth can be up to two feet. If threatened, the crab will block the entrance of the burrow with mud. When the crab is out of the burrow foraging, and a predator comes along, it will scurry into the nearest burrow, regardless of ownership. It reminds me a lot of the Magic School Bus game where you play a crab trying to avoid getting eaten by seagulls. It was a fantastic game. Here’s a screenshot from it:

 

The player here lost the game, as the crab got eaten by a seagull.

The player here lost the game, as the crab got eaten by a seagull.

The most notable feature of fiddler crabs is their asymmetrical claws. These occur in males, with one claw being grossly oversized. You’d think such a claw might be used to hunt for prey or protect itself, but in fact the claw is pretty useless. The male crabs use them for mating displays and to fight other males over a pretty girl crab walking by. In fact, this claw is so stupidly large that it inhibits the male crab’s feeding process.

Fiddler crabs feed on detritus and algae that they find in the sediment of the beaches they live on. The crabs use their regular sized claws to sift through the sand, finding bits of food and plants. For male fiddler crabs, however, the large claw cannot be used to sift through sand, and thus his other claw must work twice as hard, and he must eat twice as much than a female to obtain the same nutrients.

A male fiddler crab sitting outside its burrow

A male fiddler crab sitting outside its burrow

Male fiddler crabs use their claws in mating displays to attract females to breed. They dig special mating burrows and stand in front of them, waving their giant claw back and forth. If the female is attracted to this display, she’ll stop in front of the burrow to watch. This makes the male wave his claw more frantically, and then he runs back and forth from the burrow to the female. If she’s still interested, she approaches the edge of the burrow, at which point the male scuttles inside and drums his claws against the sides of the hole. The female then enters, the male plugs the entrance of the burrow for some privacy, and lots of fun is had by both crabs. After two weeks the female lays her eggs in the ocean and goes on with her life.

Despite their ridiculous size, the large claw of male fiddler crabs seems to be very important. If the large claw is lost, the other claw will grow into a new large claw, while a small claw grows after the crab’s next moult. Sometimes a crab will regrow a large, but weak claw. This doesn’t stop the male’s dominance, as the large size intimidates smaller crabs, even though their claws might be stronger. Fiddler crabs can be either left or right handed, with the big claw on the left or right side respectively. Even though their bigs claws look silly and are clearly unwieldy, it certainly makes fiddler crabs an interesting species. I like them.

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