2015-07-16 / Nature

Fiddler Crabs Are Vital to Salt Marshes

By Jack Kelly


Green heron stalks prey in salt marsh. (Photos by Jack Kelly) Green heron stalks prey in salt marsh. (Photos by Jack Kelly) Of all of the creatures that occupy Aquidneck Island’s salt marshes, the most unique and curious may be the semi-terrestrial Atlantic marsh fiddler crab. This species is an industrious and hardworking breed of detritivore and a prime indicator of the overall health of a wetlands system. The crabs obtain nutrients by consuming detritus (decomposing plant or animal matter) and help to enrich water quality by removing excess particles.

Fiddler crabs received their name due to the extreme difference in the size of the claws of the male, with the larger claw resembling a fiddle. Known as a “secondary sexual characteristic,” the males use their larger claws, some as long as two inches, to attract a mate and defend territory. The males have a second, smaller feeding claw, and females and juveniles have two small claws for defense and foraging. Males use their claws to perform an intricate series of waving signals from the entrances of burrows dug deep in the mud and sand. At other times, the males may "arm wrestle" for the attention of the female crabs.


Two male fiddler crabs wrestle in the foreground. Two male fiddler crabs wrestle in the foreground. Members of this species vary from 2-4 inches in size with a boxed-shaped body, and eyes on the end of two long, movable eyestalks. The crabs have gills for breathing in water, but also possess a primitive lung for breathing on land. Fiddlers dig slanting burrows up to three feet long, which they plug with mud as the tide rises. This species can live out of water in damp ground, which is evidenced by their months-long winter hibernation.

Fiddlers are colonial, living in communities numbering in the hundreds or even thousands for protection against predators such as gulls, great egrets, great blue herons, green herons, raccoons and others. When a predator is sighted, an alarm call goes out and all of the crabs scamper back to their burrows. Colony areas offer an amazing sight as low tide approaches and the crabs begin to emerge from their burrows. The sight of thousands of small crabs crawling out of the mud is reminiscent of a 1950s grade B horror film.


Jack Kelly, a native Newporter, is a wildlife photographer and nature enthusiast who enjoys sharing his experiences with others. Jack Kelly, a native Newporter, is a wildlife photographer and nature enthusiast who enjoys sharing his experiences with others. There has been an explosion in fiddler crab populations across the salt marsh systems of Rhode Island in recent years and biologists and scientists are still attempting to find the cause. Dr. Marcie Cole- Ekberg, coastal ecologist for Save The Bay, explained, “The study is ongoing, but we don’t have a conclusive answer yet. We do believe it is an effect of climate change and warmer waters, but the study is not complete.”

Fiddler crabs play a vital role in the overall ecology of salt marshes through their tunneling, which aerates the mud and soil, and their foraging of detritus, which aids water quality. The crabs are also a food source for numerous species of wildlife and lend to a more stable environment.

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