Crabs are found in all zones of the ocean and a few on land. They have 10 legs and a hard exoskeleton. There are about 5,000 species.
Taxonomy
Kingdom Animalia: Multicellular, heterotrophs; ~ 10 million species
Phylum Arthropoda: Segmented body with paired appendages; exoskeleton; molt; largest and most diverse animal phylum; examples—insects, crustaceans
Subphylum Crustacea: Five fused segments; five pairs of appendages and two antennae; gills; most marine with some in freshwater and few on land; 42,000 species
Class Malacostraca: head, thorax, and abdomen; examples—lobsters, shrimp, crabs, krill, amphipods, isopods; 18,000 species
Order Decapoda: five pairs of legs; crabs, shrimps, crayfishes, and lobsters
Brachyura: true crabs; Most advanced decapods; body short, wide, and flat; abdomen a flap covering reproductive organs; five pairs of walking legs with first pair being chelipeds; 4500 true crabs and 500 hermit crabs
Anatomy and Physiology
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 (see photo)—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 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 (see male/female graphic) is curved under the body with its major duty as protection of the reproductive organs. After an adult female molts, 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.
Geographic Distribution
Crabs are found in virtually all ocean (some on land) habitats. Some examples of crabs in their ecological niches follow:
- Land-based—Blue Land Crab (Florida), Coconut Crab (Pacific Ocean Islands)
- High Intertidal—Fiddler Crab
- Intertidal Zone—Mole Crab (Pacific Ocean in the sand), Pacific Rock Crab
- Coral Reefs—Decorator crab
- Continental Shelf—Blue Crab (North American Atlantic and Gulf Coast waters); Stone Crab (South Atlantic to Mexico); King Crab (Alaska); Sally Lightfoot Crab (California and Pacific Islands); Dungeness Crab (Northwest Pacific coasts)
- Benthic Zone—Japanese Spider crab (North Pacific Ocean); Masking Crab
- Hydro-Thermal Vents—Picture Search
