Come July, as sunny ocean beaches are jam-packed with a sea of people shrouded in oiled skin, skimpy bathing suits, and with the scent of suntan lotion, thousands if not millions of fiddler crabs along the muddy edges of an estuary busy themselves day and night with the needs of reproduction.
It’s not an easy task. Fiddler crabs do their courting not in the water, but on flat open spaces where hungry passing predators, including raccoons, herons, egrets, and gulls pose great danger. The little crabs also have to tackle the regular rush of water from each day’s tidal cycle flowing over their underground burrows or homes where copulation takes place. With hormonal cues pushing them onward, the crabs ignore the hurdles to create another generation of fiddler crabs once more.
When not breeding, the little crabs are quick to take advantage of feeding opportunities. Using their unique mouth as a sieve, fiddler crabs will feed on small food particles of algae, bacteria and decaying marsh plants by separating this detritus from mud and sand. It’s a similar way of feeding as many worms, by ingesting sand and mud along with food and later excreting what was not consumed.
Fiddler crabs are tiny crustaceans that can grow to less than two inches long. Their body is square in shape with males having a distinctive enlarged claw that can weigh as much as the rest of the crab’s body. Females have small claws that are equal in size. Fiddler crabs have gills for breathing in the water, but also have a primitive lung that allows them to live on land.
Fiddlers are named for the appearance and behavior of the males. When male crabs wave their large powerful claw back and forth to defend territory or attract mates, the action sort of resemble people playing a fiddle.
Although there are over sixty species of fiddler crabs discovered globally, there are primarily only three species of fiddler crabs located along the mud/sand flats adjacent to the tidal waters of New York Harbor. The two most common I regularly encounter while kayaking in small waterways are the mud fiddler crabs, which favor the well-vegetated mudflats that contain dark, black mud, between the low and high tide lines. The brackish-water fiddler crab is also common. It can be found, as it name implies, in brackish waters with muddy bottoms, primarily upstream in lower salinity waters.
The sand fiddler crab I typical find living in sandy areas on small isolated islands around the harbor. The crabs can only live in sandy brackish areas where they have adapted to feed on detritus within sediment particles of a certain size. Sand fiddler crabs are lighter in color than the other two species and male crabs have a blue or purplish carapace.
During the summer all three species of fiddler crabs are busy breeding, they repeatedly mate every two weeks. Recently, US scientists have found that this breeding cycle is possibly due to moon cycles. The "super claw" of courting males has greater snapping force around new and full moons. Researchers think male crabs’ enhanced snapping performance could be driven by the abundance of females searching for a mate at new and full moons. Female numbers peak at these times because their babies would emerge at the next new or full moon when greater tidal action could transport larvae away safely to deeper waters to grow.
Reproduction begins with male crabs digging an uncluttered cylindrical burrow along the edge of an estuary. To attract a female, a male will stand by his newly built home and wave his large super-claw to attract a female’s attention as the ladies walk by.
A female will often choose a mate with the biggest claw that has a good quality snap. If a female finds a male she likes, she will look at him for some time, as the male runs toward the female and runs back to his burrow to entice her inside. He will repeat this activity several times until the female either moves on or decides to be his mate and follow him inside his burrow.
If the female follows, the male drums the edge of the burrow with his claw, and then leads the female inside, plugs the entrance with mud and returns to the female to breed.
The male will conduct this mating ritual numerous times over the summer to mate with as many females as possible. Female fiddlers breed about every month during the summer.
A female will remain in a burrow for two weeks to incubate her egg sponge before returning to the surface. When she emerges, the female will release the eggs into the water. Here they will become zooplankton and float for several weeks into deeper waters, before floating back into shallow creeks and marshes before emerging onshore as a juveniles crab. Like other crabs and crustaceans, fiddler crabs molt to grow or to regenerate lost legs and claws over several molts.
Fiddle crabs are marvelous little estuarine critters, and I’m not alone in thinking this. Joanna Burger, a distinguished professor of Biology at Rutgers University, tells us in her book, A Naturalist Along the Jersey Shore, that the burrowing activity of fiddler crabs is important for the ecological health of salt marshes. She writes:
“In a year, the population of fiddler crabs in a square yard of marsh, as many as forty or fifty crabs can turn over nearly 20 percent of the mud in the upper six inches of soil. Burrows increase the surface area of the marsh, affecting the composition and the chemistry of the salt-marsh sediments. The disappearance of fiddler crabs from the high marsh can decrease production of cordgrass by almost 50 percent, attesting to the importance of fiddler crabs to the marsh ecosystem.”
Burger goes on to tell us that fiddler crabs “aerate the soil of the marsh by creating numerous holes in the mud, they are important part of the food web, and they can serve as an early warning of environmental degradation.” Have you hugged a fiddler crab today?
Enjoy the sight of these tiny crustaceans now. Around early November, cold winds will chase all the crabs underground to retreat into their muddy burrows. Without the urge to reproduce, the fiddler crabs will take on a more relaxed lifestyle, spending the winter in a type of hibernation or deep sleep. If all goes well, come next spring with the return of warm weather, masses of fiddler crabs will awake again to start reproducing near the great tidal waters of New York Harbor as they have for countless years.
To view more pictures, video, or stories of wildlife around Sandy Hook Bay, Raritan Bay, and Lower New York Bay, please visit my nature blog, NY Harbor Nature at http://www.nyharbornature.com