by Kyle Sweet, The Sanctuary Golf Club, Florida Master Naturalist
The Mangrove Tree Crab is found in tropical and sub-tropical regions all along the coast of North, Central and South America. More specifically, its range is from Florida to Brazil along the Atlantic coast and from Nicaragua to Peru on the Pacific coast. It is commonly seen on all three types of mangroves, which are the Red mangrove, White mangrove and Black mangrove. As with all of the mangroves, this tiny crab climbs to the tree tops at high tide and makes its way to the ground level at low tide.
It feeds mostly on the leaves of the mangrove tree, primarily feeding on the epidermis ( skin ) of the leaf. Characteristic scraping marks on the leaf show where it has fed. It is an omnivore however and prefers animal matter when possible, which isn’t too surprising since the leaves of mangrove trees have little nutritional value.
The Mangrove Tree Crab can vary greatly in color and is often mostly black in color. In many cases, the dark brown or black is mottled with yellows or even greens and helps the crab blend in with its surroundings. Even with its small size, 2 cm or less in width, and its varied coloration that acts as camouflage, the Mangrove Tree Crab is preyed on by birds, mammals and other larger crabs. It can move very quickly to evade its attacker with use of sharp tips at the end of its legs, but often can find itself in the water by falling off of a branch when moving too quickly. The water isn’t s safe refuge for this small crab as predatory fish such as the Mangrove Snapper or wading birds such as the White Ibism which both find them to be a great quick meal.
Mangrove Tree Crabs have to overcome some severe odds to survive. A female crab can carry as many 35,000 fertilized eggs on their bellies until they are ready to hatch. These must be some tiny eggs! She climbs down from her tree perch and shakes the new hatchlings into the water where their chances of survival are slim due to fish and filter feeders. Less than one tenth of a percent of the larvae become juvenile crabs and less than twenty percent of those survive to adulthood. An adult Mangrove Tree Crab really deserves some admiration, maybe even in the way of a photo or two, with those odds stacked against them.
What might first look like a spider will often be identified as a tree crab when enjoying a boardwalk through the mangroves at the J.N. “ Ding” Darling National Wildlife Refuge or along any one of Sanibel’s beaches that boasts the natural vegetation throughout the dune system that make Sanibel’s beaches some of the best in the country. It may be a small creature among us but is part of the island ecosystem and a sure sign that water and those important mangroves are somewhere nearly. Enjoy the sites of the Mangrove Tree Crab around the islands, just another small part of what makes Southwest Florida great.