Davao Doctors College, Inc., Gen. Malvar Street,
Davao City, 8000
Abstract –– 10 different species of crabs were collected around the beaches of Davao Region. Some species were similar to the others, some have the same habitat and some have characteristics unique only to them. All these species belong to the Order Malacostraca, Infraorder Brachyura which means true crabs. The specimens collected under the Infraorder Brachyura were from the genus Uca, Thalamita, Sesarma, Xantho and Ocypode with the following species name: Uca vocans, Uca herradurensis, Thalamita coeruleipes, Sesarma cinereum, Ocypode quadrata, Ocypode ceratophthalmus, Ocypode pallidula and Xantho pilipes. Keywords: crab, infraorder, Brachyura, species, order, Malacostraca, genus, Uca, Thalamita, Sesarma, Ocypode
Crabs are decapod crustaceans from the infraorder Brachyura which means very short projecting “tail”. They often reach in large sizes, the largest of which is the spider crab, with long, thin legs spanning up to 3.6 m (up to 12 ft), possessing complicated nervous systems. (Microsoft Encarta Encyclopedia, 2009). Crabs are bilaterally symmetrical (identical halves on each side of an axis) except that many species have one cheliped
larger than the other. Crabs have 5 pairs of legs —one pair of chelipeds (pincers or claws), three pairs of walking legs, and one pair of swimming legs. The claws are used for feeding, excavating burrows, defense (or aggressive behavior), and signaling (a sort of crab language fending off competing crabs for territory, keeping predators at bay, and most importantly, attracting the opposite sex). The crab’s body is protected by a rigid exoskeleton. This is a tough chitinous “skin” that completely covers all parts of the body. As the crab grows, the exoskeleton is periodically shed in a process called molting (ecdysis). The resulting molt looks like a translucent creature without a body. In a few hours, the molted crab absorbs enough water to swell its body by about ten to twenty percent and the exoskeleton hardens. The crab body then grows to fill the new exoskeleton. Much of the body is protected by the carapace, the covering of the head/thorax, and the crab can pull the legs under the carapace presenting a hard rock-like creature to a predator. Under the front of the carapace two eyes on stalks, two antennae, and a mouth are located. The mouth has several movable parts, and the chelipeds, especially the smaller one, can move food into the mouth at a surprising rate. Most crabs are omnivores (plant- and meat-eaters), some are carnivores (meat-eaters), and a few are herbivores (plant-eaters). Two gill structures are also located laterally in the body cavity under the carapace. As long as these gills can be kept wet, crabs can live out of the water; however, the gills can only process the oxygen as long as they are wet. The abdomen of crabs is curved under the body with its major duty as protection of the reproductive organs. After an adult female molt, the soft shell condition allows her to become impregnated by a male. The male does not nurture the eggs, and the male abdomen is narrowed accordingly. The soft-shelled female and the hard-shelled male sometimes remain together for protection until her carapace begins to harden. Several days later, the eggs are extruded to be stored until hatching under the widened abdomen. Depending on the species, ovigerous (egg-bearing) females carry a dozen to several hundred eggs. The eggs are kept in constant motion for oxygenation by the swimmerets until they hatch into the surrounding water. The larvae, called a zoea and megalops as they molt, develop as part of the planktonic community. When the larval crab reaches a certain point, it drops to the bottom and starts its life a bottom dweller. They are highly adapted to resist changes in the...