Ocracoke nature, flora & fauna

Observed on Ocracoke: The Fiddler Crab

Fidler crab IMG_0002

 

Text and Photo by Peter Vankevich

 

In the late summer and early fall of the year when walking along the marshes such as Molasses Creek or even sandy trails including in the village, you should not have any trouble seeing this month’s featured subject, a fiddler crab. The males are easily identified since they have one enlarged claw called a cheliped and a much smaller one. The females have two equal- sized claws. They  get their  name from the up and down feeding motion of the smaller claw from food to mouth that in the eyes of at least one observer appeared as if the crab was moving a bow across the strings of a fiddle (the large claw). Fiddler crabs are found throughout the world’s coasts, mangroves and salt marshes and make up about 100 species. On Ocracoke one of these is known as the sand fiddler crab (Uca pugilator) and it one you most likely will encounter away from water. This species ranges the Atlantic coast from Massachusetts to the Gulf of Mexico. Small in size, they are no longer than 2 inches across. In addition to the large claw, another interesting anatomical feature is that they have eyestalks, i.e. protrusions that extend the eye away from the body which provide them a better field of view than if the eyes were unextended. Like their close cousins, the ghost crabs, they will dig burrows along the marsh edges and use them for safety, incubation, and refuge from extreme temperatures. Omnivorous scavengers, they feed on decaying detritus, algae and fungus. They have an important niche in the ecology of the island including being a food source for the rails, herons, egrets and ibises It is also believed that they play a vital role in the preservation of wetland environments. When feeding, they will sift through the sands and aerate the substrate that in turn encourages the growth of marsh grasses. Fiddler crabs are sensitive to pollution which may drastically reduce their numbers and  contaminants  such as polychlorinated biphenyls (PCBs) and insecticide/fertilizer mixtures will concentrate in their bodies which may then be transferred to birds and fish that prey upon them. The male fiddler crab will use its large claw to defend itself, fend off rival males and during courtship it will raise it up and down to attract females. I found it fascinating that the enlarged claw may be on the left or the right side. Also if the large claw is lost, the small claw will grow into the large size and only a small claw will replace the lost one. Due in part to their small size, they do not have much commercial value though they are sold as aquarium pets and fishermen will use them as bait to catch mollusk feeding fish such as black drum, pompano, and sheepshead. When environmental conditions are right, sand fiddler crabs can be abundant. A female lays as many 2000 eggs. During the two week gestation period she will stay in her burrow usually located along the marsh edges then come out and lay her eggs in a receding tide. If you come across them, take a moment to observe their behavior, see how many have left or right large claws and the ratio of males to females. If you record these observations, I’d love to see the results. Comments or suggestions for a future column, contact me: petevankevich@gmail.com.