Crustacean Bluegrass Instrumentalist

Thursday, May 22, 2025

Crustacean Bluegrass Instrumentalist

Today’s special animal friend is the fiddler crab. According to Wikipedia, there are hundreds of species in the family Ocypodidae, including “ghost crabs” (subfamily Ocypodinae) and fiddler crabs (subfamilies Gelasiminae and Ucinae). Ghost crabs are nocturnal and usually light colored, while male fiddler crabs have one claw that is much bigger than the other one. The fiddler crabs found on salt marshes and other tidal habitats in coastal North Carolina are Uca (or Leptuca) pugilator, the Atlantic sand fiddler crab.

Fiddler crabs are detritovores, eating algae, bacteria, and decomposing bits of stuff that they pick out of the mud. Males can eat with only one front claw – because one is too big – while females eat with both, regular-sized front claws. Because of this, females take in food twice as fast as males do. Both sexes leave small balls of mud made from the mineral matter left after they get the organic bits out. They also make balls of mud when they excavate their burrows, which can be up to two feet deep.

A male fiddler crab’s big claw continues growing throughout his life. The claw can weigh up to 65% of his body weight. The claw is used in territorial conflicts between males. Most of their “fights” involve only rude and threatening gestures, but sometimes they will clobber one another with the claws, attempting to flip a rival onto his back and embarrass him. If a male loses his large claw in battle or to a predator, he will begin to recover after his next exoskeleton shed: his remaining small claw will begin to grow in as a big claw, while a new small claw will grow in place of the previous big claw.

Because the claw is not used for feeding, it has smooth surfaces rather than serrated edges like a blue crab’s or lobster’s claws.

The other use of the big claw is to impress females by waving it around. The male does this while bowing to the female. If she likes him, they both go into his burrow for mating. The female carries the fertilized eggs on her abdomen as they develop. Then she goes into the water, and the eggs hatch into tiny larvae called zoea. Like some insects, the larvae go through instars as they increase the size of their exoskeletons. Larger larvae are called megalopa. Finally, a molt leads to the emergence of a small but recognizable crab. The larval stage leading to crabhood can take weeks or months depending on conditions. Mating can occur every two weeks throughout the spring and summer, and a female may lay 250,000 eggs in a batch.

Fiddler crabs live up to three years. They are eaten by blue crabs and by many species of coastal birds, including rails, herons, and ibis. For defense, they run into their burrows and, if necessary, plug the opening with mud. Because the males are more visible, they are the first to hide from a predator and the last to emerge to resume feeding. Fiddler crabs also seal their burrows to protect themselves from high water and to stay warm during cold weather.

The Atlantic sand fiddler crab has not been evaluated by IUCN. They are considered common throughout their range, which includes the coastline from Cape Cod to Texas. Declining water quality and loss of marine wetlands are the main threats.