Brachyura

Brachyura, meaning "short", and οὐρά, is an infraorder of decapod crustaceans comprising the true crabs. They typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. Their exoskeleton is often thickened and hard. They generally have five pairs of legs, and they have "pincers" or "claws" on the ends of the frontmost pair, scientifically termed the chelae. They are present in all the world's oceans, in freshwater, and on land, often hiding themselves in small crevices or burrowing into sediment. True crabs are omnivores, feeding on a variety of food, including a significant proportion of algae, as well as detritus and other invertebrates. True crabs are widely consumed by humans as food, with over 1.5 million tonnes caught annually.
True crabs first appeared in the fossil record during the Jurassic period, around 200 million years ago, achieving great diversity by the Cretaceous period; around 7,000 extant species in 96 families are known. The true crabs are the largest of the crab decapod groups with carcinised body forms. Smaller groups include the porcelain crabs, king crabs, and hairy stone crabs which have independently evolved similar forms and lifestyles.

Description

Brachyuran crabs are generally covered with a thick exoskeleton, composed primarily of highly mineralized chitin. Behind their pair of chelae are six walking legs and then two swimming legs. The crab breathes through gills on its underside; gills must be at least moist to work.
True crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to. Several other groups of crustaceans with similar appearances – such as king crabs and porcelain crabs have convergently evolved similar features to true crabs, making them good examples of carcinisation.

Environment

True crabs are found in all of the world's oceans, as well as in fresh water and on land, particularly in tropical regions. About 850 species are freshwater crabs.

Sexual differences

Brachyuran crabs often show marked sexual dimorphism. Males often have larger claws, a tendency that is particularly pronounced in the fiddler crabs of the genus Uca. In fiddler crabs, males have one greatly enlarged claw used for communication, particularly for attracting a mate. Another conspicuous difference is the form of the pleon ; in most male crabs, this is narrow and triangular in form, while females have a broader, rounded abdomen. This is because female crabs brood fertilised eggs on their pleopods.

Life cycle

True crabs attract a mate through chemical, visual, acoustic, or vibratory means. Pheromones are used by most fully aquatic species, while terrestrial and semiterrestrial crabs often use visual signals, such as fiddler crab males waving their large claws to attract females. The vast number of brachyuran crabs have internal fertilisation and mate belly-to-belly. For many aquatic species, mating takes place just after the female has moulted and is still soft. Females can store the sperm for a long time before using it to fertilise their eggs. When fertilisation has taken place, the eggs are released onto the female's abdomen, below the tail flap, secured with a sticky material. In this location, they are protected during embryonic development. Females carrying eggs are called "berried" since the eggs resemble round berries.
When development is complete, the female releases the newly hatched larvae into the water, where they are part of the plankton. The release is often timed with the tidal and light/dark diurnal cycle. The free-swimming tiny zoea larvae can float and take advantage of water currents. They have a spine, which probably reduces the rate of predation by larger animals. The zoea of most species must find food, but some crabs provide enough yolk in the eggs that the larval stages can continue to live off the yolk.
File:Xantho poressa 2009 G1.jpg|thumb|Female crab Xantho poressa at spawning time in the Black Sea, carrying eggs under her abdomen
File:Grapsus tenuicrustatus - hawaii - 2015-11-01.webm|thumb|A Grapsus tenuicrustatus climbing up a rock in Hawaii
Each species has a particular number of zoeal stages, separated by moults, before they change into a megalopa stage, which resembles an adult crab, except for having the abdomen sticking out behind. After one more moult, the crab is a juvenile, living on the bottom rather than floating in the water. This last moult, from megalopa to juvenile, is critical, and it must take place in a habitat that is suitable for the juvenile to survive.
Most terrestrial crabs must migrate down to the ocean to release their larvae; in some cases, this entails very extensive migrations. After living for a short time as larvae in the ocean, the juveniles must do this migration in reverse. In many tropical areas with land crabs, these migrations often result in considerable roadkill of migrating crabs.
Once crabs have become juveniles, they still have to keep moulting many more times to become adults. They are covered with a hard shell, which would otherwise prevent growth. The moult cycle is coordinated by hormones. When preparing for moult, the old shell is softened and partly eroded away, while the rudimentary beginnings of a new shell form under it. At the time of moulting, the crab takes in a lot of water to expand and crack open the old shell at a line of weakness along the back edge of the carapace. The crab must then extract all of itself – including its legs, mouthparts, eyestalks, and even the lining of the front and back of the digestive tract – from the old shell. This is a difficult process that takes many hours, and if a crab gets stuck, it will die. After freeing itself from the old shell, the crab is extremely soft and hides until its new shell has hardened. While the new shell is still soft, the crab can expand it to make room for future growth.

Behaviour

True crabs typically walk sideways, because of the articulation of the legs which makes a sidelong gait more efficient. Some crabs walk forward or backward, including raninids, Libinia emarginata and Mictyris platycheles. Some crabs, like the Portunidae and Matutidae, are also capable of swimming, the Portunidae especially so as their last pair of walking legs are flattened into swimming paddles.
Brachyuran crabs are mostly active animals with complex behaviour patterns such as communicating by drumming or waving their pincers. Crabs tend to be aggressive toward one another, and males often fight to gain access to females. On rocky seashores, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes. Fiddler crabs dig burrows in sand or mud, which they use for resting, hiding, and mating, and to defend against intruders.
True crabs are omnivores, feeding primarily on algae, and taking any other food, including molluscs, worms, other crustaceans, fungi, bacteria, and detritus, depending on their availability and the crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness. Some species are more specialised in their diets, based in plankton, clams or fish. Some crabs, such as members of Pinnotheridae, live on or inside animals such as bivalves, either being commensal or parasitic on their host.
True crabs work together to provide food and protection for their family, and during mating season to find a comfortable spot for the female to release her eggs.
Brachyurans are durophagous, characterised by the high closing force they can exert with their claws. The Florida stone crab generates the highest muscle force per unit area reported for animals, at 2000 kiloNewtons/m², while other animals generate 100–300 kN/m². Crabs of the family Cancridae, Romaleon antennarium, R. branneri, Metacarcinus gracilis, M. magister, Glebocarcinus oregonensis, and Cancer productus, generate closing forces that exceed the force/weight ratio threshold determined for most animals by allometric equations, of 20 body masses^1/3; this exceptional muscle strength could be due to greater resting sarcomere length than those of other animals at 10–18 micrometres, compared to 2.7 micrometres in mammals.

Evolution

The earliest unambiguous brachyuran fossils date from the Early Jurassic, with the oldest being Eocarcinus from the early Pliensbachian of Britain, which likely represents a stem-group lineage, as it lacks several key morphological features that define modern crabs. Most Jurassic crabs are only known from dorsal carapaces, making it difficult to determine their relationships. Crabs radiated in the Late Jurassic, corresponding with an increase in reef habitats, though they would decline at the end of the Jurassic as the result of the decline of reef ecosystems. Crabs increased in diversity through the Cretaceous and represented the dominant group of decapods by the end of the period.
The infraorder Brachyura belongs to the group Reptantia, which consists of the walking/crawling decapods. Brachyura is the sister clade to the infraorder Anomura, which contains the hermit crabs and relatives. The cladogram below shows Brachyura's placement within the larger order Decapoda, from analysis by Wolfe et al., 2019.
Brachyura is separated into several sections, with the basal Dromiacea diverging the earliest in the evolutionary history, around the Late Triassic or Early Jurassic. The group consisting of Raninoida and Cyclodorippoida split off next, during the Jurassic period. The remaining clade Eubrachyura then divided during the Cretaceous period into Heterotremata and Thoracotremata.
A summary of the high-level internal relationships within Brachyura can be shown in the cladogram below:
There is a no consensus on the relationships of the subsequent superfamilies and families. The proposed cladogram below is from analysis by Tsang et al, 2014: