Appreciating the little fiddler crab

Wild Ways

You’ve probably seen them a zillion times, at least those of you who get outside a bit and visit our salt marshes and mudflats. Fiddler crabs. These small crustaceans are highly visible critters that scurry along on the mud, darting in and out of their individual
holes and waving their one large claw at each other. Yes, fiddler crabs are common in most mudflats in the world, including ours. But did you ever really watch them? Next time you see them, linger awhile and take a look.

First, you might notice that not all of them have one large front claw. It’s only the males that have one, and its main function is to show off their strength and prowess (and thus reproductive suitability) to admiring females and to intimidate other males. Sometimes this large claw may be lost, but in the next molt it either grows back or the other smaller claw gets larger to compensate. The females are choosy, though, and prefer those males with big healthy claws making vigorous displays with them.

And, if you watch longer, you will notice that each small fiddler crab has its own hole to retreat to, a hole that can be two feet or deeper in the mud. Sometimes a female will join a male in its hole to mate and afterwards the female stays in a hole and holds the eggs on her underside until they hatch out two weeks later and the larva are released into the water.

When you see a colony of fiddler crabs, you will probably also notice little round balls of mud or sand near their holes. The larger globes are the result of the crab’s excavation and cleaning of its hole, just mud rolled in a ball. The smaller globes are a result of their feeding strategy. They sift large amounts of mud or sand and extract edible items like detritus and algae and then deposit the remainder as a little ball.

Fiddler crabs are a preferred food item for sheepshead, redfish and drum, and thus valued by fishermen as bait. How the heck do these fisherman catch the quick little critters that retreat into deep holes? I was curious to find out, so I paid a visit to the White Shell bait store on Heckscher Drive to talk with a staff person there. She showed me her wooden box full of fiddler crabs and demonstrated how they herd up into a pile when disturbed. Fishermen have learned the technique of herding them too. Those wishing to sell large amounts of crabs to bait stores harvest them in a couple of different ways that involve digging holes and sinking plastic pails in the mud. When the crabs are out foraging away from their holes, they can be herded along wooden boards toward the bucket.

Others with more patience just sink a bucket in the mud, put holes in the bottom, and put in some bait, like fish meal and scraps and make a trail of similar items leading to the bucket and eventually crabs will fall in.

I often see fisherman walking out into the marsh near the back gate of Fort Clinch State Park and I know these folks have not made the effort to dig big holes and bury buckets. Instead, they can use the herding technique with small cans in the mud, or even toss a net over them, if they can find some foraging far enough away from their holes.

The more I read about fiddler crabs for this column, the more intrigued I got. I learned that fiddler crabs are abundant and although not a species of conservation concern, they are integral to the health of salt marshes. Their excavations aerate the soil, helping the marsh grasses grow, and they provide food for those creatures like fish, birds and raccoons that are higher up in the food chain.

And, now with the attention on global warming, scientists have also marked changes in fiddler crab distributions. It used to be a well-known fact that marsh fiddler crabs are found on the East Coast only as far north as Cape Cod. Well, that is true no longer. With the warmer ocean temperatures these little crabs have extended their range up through New Hampshire.

Whenever I see fiddler crabs now, I have a new respect for them. And the next time you are near a marsh at low tide, watch the crabs and I’ll bet you’ll get interested too!

Pat Foster-Turley is a zoologist on Amelia Island. She welcomes your nature questions and observations.

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