Honeymooning with the Fiddler Crab

Florida beaches and marshlands are home to three species of fiddler crab, the sand fiddler (Uca pugilator), the mud fiddler (Uca pugnax) and the red-jointed or brackish water fiddler (Uca minax), in the order Decopoda – meaning 10-legs. Two of these “legs” – the front ones – are adapted as claws, however. The males are easily identified by their one oversized front claw, which can weigh up to one-third of the crab’s total. Accounts differ as to the origin of the name, “fiddler.” One explanation claims the name came from the use these crabs make of their smaller front claw to eat. The motion of picking up its food, depositing it in its mouth, and placing discarded bits back on the ground resembles the bow action of a fiddler. The male’s one outsized claw is useless for that purpose; he eats (or fiddles with his food) one-handed, while the female is free to gobble her meal with both hands.

The adult fidder’s notion of “food” includes the algae, bacteria, fungus, and decaying matter in sand, which the crab picks up with the “eating claw”. Edible material is separated from sand in the mouth; the discarded “sandballs” are deposited back on the ground. Because of this diet, fiddlers are important as regulators of bacteria and fungus in beach and marsh. The tunnels they dig in the intertidal zones and mud flats provide them with shelter when the tides rise and also aerate the ground, further ensuring the health of those soils.

With such a large claw, you’d think that the male fiddler would be an aggressive fighter, but not so. That large claw is better adapted to waving than fighting. One male waves it at another potentially threatening male to intimidate it; this waving seldom progresses to the fighting stage. The larger the claw and the more vigorous the wave, the better the chance of intimidating one’s opponent. If the crabs do engage in physical combat, the vanquished crab risks losing that precious claw and having to regrow another, which never reaches the size of the first.

Size matters, not only in deterring threats, but in attracting females. A male will use his large claw to dig a spacious “honeymoon suite” for his intended; the bigger the claw, the bigger the suite. Once the suite is dug, the male will station himself at the entrance and wave that claw at passing females, signaling his vitality with his healthy waving motion. If he catches a female’s eye, he will “dance” toward her and retreat back to his suite. If she takes a few steps in his direction, he taps on the wall of his burrow, so there is no mistaking his intentions. He waves again and repeats the dance of approach, retreat, and tapping the entrance – each time enticing the female closer until she finally follows him in. He then seals the entrance with mud, and mates with her. The couple stays in their honeymoon suite for two weeks, at which time the female’s eggs have gestated. She leaves the love nest to deposit her eggs, while he prepares his “digs” for another “wave” of wooing the next “lucky lady”. The male’s courtship behavior has earned it another nickname – the “calling crab.” And while it seems that for a female, her mate’s claw size does count, it may not be that simple. Research has shown that the size of the “honeymoon suite” influences its internal temperature, and that temperature influences egg gestation. The bigger the claw, the bigger the construction, and the more suitable the temperature might be. Perhaps the male’s tapping helps the female judge the size of the accommodations. She may be judging the spaciousness of the digs as much as she’s sizing him up. With summer and the mating season over, you might be able to spot the resulting juvenile fiddlers as well as their very tired parents at the GTM Research Reserve.

Resources

Wikipedia - Fiddler Crabs

Tampa Bay Feature Creature A portrait of the Tampa Bay Estuary

Common Florida Fiddler Crabs