Some occur in brackish or fresh water, or in damp habitats on land; some occur in the ocean deeps, and others are pelagic in the open sea; still others live on or in the bodies of other organisms. The crabs are found at abyssal depths of the Ocean down to 6000 m are becoming dominant in all most all estuaries where temperature and salinity fluctuate drastically daily.
The crab cephalothorax, formed by fusion of head and thorax and covered by the carapace, is short and broad, and forms virtually the whole body. The crab abdomen (corresponding to what gourmets call the "tail" of a lobster) is reduced to a thin, flat plate, tucked forward out of sight below the cephalothorax, hence the name "Brachyura," or "short-tailed" crabs.
Most of the crabs feed on either detritus or the organic matter thereby influencing the ecological functioning of the system.
Crabs play important role in the maintenance of ecosystem and also in the marine food web demonstrating prey-predator relationship.
Their sizes also vary from very small about 2mm to huge up to 5.5m with extended appendages and 40cm across carapace width or carapace length. Among arthropods, the biggest crustaceans are the brachyuran crabs.
Brachyuran crab: The true crabs
