Crabs are decapod crustaceans that belong to the infraorder Brachyura. Just as the term decapod suggests, they have 10 legs, and a short projected “tail” which is actually a part of the abdomen that is usually entirely hidden beneath the thorax (from Greek brachys = short; oura = tail). There are other animals also termed as “crab” such as the king crab, hermit crab, crab lice and porcelain crab that aren’t true crab species.
Numerous species of crabs live in a variety of environments, from fresh water to the oceans, with some species thriving on land. All of the world’s oceans are inhabited by species of crab, from the frozen Bering Sea to the equator. The most abundance of species is in the tropical and semi-tropical regions. Crab species are generally omnivores, eating whatever is available in their environment.
While some crab species may have specialized diets and feed only on algae or other food, most are opportunistic feeders, consuming bacteria, worms, small crustaceans, mollusks and detritus. The fact that they occupy so many niches makes them important in their respective communities, controlling certain populations of other species, feeding on detritus or being food themselves for other animals.
All crabs have their cephalothorax protected by a large and hard exoskeleton or carapace, which mainly consists of calcium carbonate. They have 10 legs just as other decapods, but only a single pair of claws. They greatly vary in size, ranging from the small pea crab that is just a few millimeters wide to the huge Japanese spider crab which may have a leg span of up to 4 meters. Due to the nature of their hard and durable carapace, many fossils of crab species are well represented, with some dating back from the Jurassic.
Many species of crab present marked sexual dimorphism, which means that males and females differentiate themselves through structure particularities. In general, males have larger claws which they use to signal to their mates in order to attract them. Some males use specialized claws to fight over territory with other males or females, while others may use them for communication purposes.
Another notable difference between males and females is that the former have narrower abdomens of triangular shape, while the females have rounder and wider abdomens. This is mainly due to the fact that female crabs carry their eggs with their pleopods after fertilization.