Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. They live in all the world's oceans, in freshwater, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers on each arm. They first appeared during the Jurassic period, around 200 million years ago.
The Decapoda or decapods are an order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfish, shrimp, and prawns. Most decapods are scavengers. The order is estimated to contain nearly 15,000 extant species in around 2,700 genera, with around 3,300 fossil species. Nearly half of these species are crabs, with the shrimp and Anomura including hermit crabs, porcelain crabs, squat lobsters making up the bulk of the remainder. The earliest fossils of the group date to the Devonian.
Pleocyemata is a suborder of decapod crustaceans, erected by Martin Burkenroad in 1963. Burkenroad's classification replaced the earlier sub-orders of Natantia and Reptantia with the monophyletic groups Dendrobranchiata (prawns) and Pleocyemata. Pleocyemata contains all the members of the Reptantia, as well as the Stenopodidea, and Caridea, which contains the true shrimp.
Anomura is a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans include the word crab, all true crabs are in the sister group to the Anomura, the Brachyura.
Carcinisation is a form of convergent evolution in which non-crab crustaceans evolve a crab-like body plan. The term was introduced into evolutionary biology by L. A. Borradaile, who described it as "the many attempts of Nature to evolve a crab".
Thalassinidea is a former infraorder of decapod crustaceans that live in burrows in muddy bottoms of the world's oceans. In Australian English, the littoral thalassinidean Trypaea australiensis is referred to as the yabby, frequently used as bait for estuarine fishing; elsewhere, however, they are poorly known, and as such have few vernacular names, "mud lobster" and "ghost shrimp" counting among them. The burrows made by thalassinideans are frequently preserved, and the fossil record of thalassinideans reaches back to the late Jurassic.
Cancridae is a family of crabs. It comprises six extant genera, and ten exclusively fossil genera, in two subfamilies:
Dromiacea is a group of crabs, ranked as a section. It contains 240 extant and nearly 300 extinct species. Dromiacea is the most basal grouping of Brachyura crabs, diverging the earliest in the evolutionary history, around the Late Triassic or Early Jurassic. Below is a cladogram showing Dromiacea's placement within Brachyura:
Eubrachyura is a group of decapod crustaceans comprising the more derived crabs. It is divided into two subsections, based on the position of the genital openings in the two sexes. In the Heterotremata, the openings are on the legs in the males, but on the sternum in females, while in the Thoracotremata, the openings are on the sternum in both sexes. This contrasts with the situation in other decapods, in which the genital openings are always on the legs. Heterotremata is the larger of the two groups, containing the species-rich superfamilies Xanthoidea and Pilumnoidea and all the freshwater crabs. The eubrachyura is well known for actively and constantly building its own burrows. The fossil record of the Eubrachyura extends back to the Cretaceous; the supposed Bathonian representative of the group, Hebertides jurassica, ultimately turned out to be Cenozoic in age.
Corystidae is a family of crabs, in its own superfamily, Corystoidea. It includes what was once thought to be the oldest Eubrachyuran fossil, Hebertides jurassica, thought to be dating from the Bathonian ; the species was subsequently reinterpreted as being Cenozoic in age. Corystidae contains ten extant and five extinct species in eight genera:
Cheiragonidae is a small family of crabs, sometimes called helmet crabs, placed in its own superfamily, Cheriagonoidea. It comprises three extant species, Erimacrus isenbeckii, Telmessus acutidens and Telmessus cheiragonus, there are no yet evidences of Cheiragonidae in the fossil record. Many of these crabs were formerly treated as members of the Atelecyclidae.
Dromia personata, also known as the sponge crab or sleepy crab, is a species of crab found in the North Sea, the Mediterranean Sea, and connecting parts of the northeastern Atlantic Ocean. Like most other epibenthic crustaceans, the biomass of this species is especially dense in the Mediterranean continental shelf. It mainly resides from the lower shore to a depth of 50 meters (164 ft), often in caves. Occasionally, they are found living in depths as low as 110 meters (360 ft). They serve as prey for octopus, starfish, and other fish. Their last two pairs of legs are positioned dorsally, and are used to hold a sponge in place as camouflage.
Pseudozioidea is a superfamily of crabs, formerly treated in the Eriphioidea, Carpilioidea, Xanthoidea, Pilumnoidea and Goneplacoidea. A number of fossils from the Eocene onwards are known from the family Pseudoziidae. Eleven genera are recognised in three families:
Amarinus lacustris is a species of freshwater crab from Australia, New Zealand and nearby islands, where it lives in water of various salinities. It grows up to 10 mm (0.4 in) wide, with an H-shaped groove on its back. It is an omnivore and is eaten by crayfish and fish. It was first discovered in Lake Pupuke, near Auckland, and is the only freshwater crab in New Zealand.
Platykotta akaina is a species of decapod crustacean from the Triassic of the United Arab Emirates. It is the oldest known fossil from the infraorder Anomura, and is most closely related to Eocarcinus praecursor.
Polydectus cupulifer is a species of crab in the family Xanthidae, and the only species in the genus Polydectus. Together with the genus Lybia, it forms the subfamily Polydectinae. It is found in the Indo-Pacific, ranging from Madagascar and the Red Sea in the west to Japan, Hawaii and French Polynesia in the east. P. cupulifer is densely covered with setae (bristles), and frequently carries a sea anemone in each chela (claw).
Necrocarcinidae is an extinct family of Late Jurassic and Cretaceous crabs. It comprises 40 species grouped in seven genera.
2020 in arthropod paleontology is a list of new arthropod fossil taxa, including arachnids, crustaceans, insects, trilobites, and other arthropods that were announced or described, as well as other significant arthropod paleontological discoveries and events which occurred in 2020.
The Cretaceous crab revolution was a major diversification event of brachyuran crabs that took place during the Cretaceous Period, from 145 to 66 million years ago. Nearly 80% of modern groups of crabs originated during this event. The Cretaceous crab revolution is a smaller component of the greater Mesozoic marine revolution.
2023 in arthropod paleontology is a list of new arthropod fossil taxa, including arachnids, crustaceans, trilobites, and other arthropods that were announced or described, as well as other significant arthropod paleontological discoveries and events which occurred in 2023.
