Temporal range: Cambrian–Recent
See text for orders.
Malacostraca is the largest of the six classes of crustaceans, containing over 25,000 extant species, divided among 16 orders. Its members display a greater diversity of body forms than any other class of animals, and include crabs, lobsters, shrimp, krill, woodlice, scuds (Amphipoda), mantis shrimp and many other less familiar animals. They are abundant in all marine environments and have also colonised freshwater and terrestrial habitats. They are united by a common Bauplan, comprising 20 body segments (rarely 21), divided into a head, thorax and abdomen.
Malacostracans live in a wide range of marine and freshwater habitats, and three orders have terrestrial members: Amphipoda (Talitridae), Isopoda (Oniscidea, the woodlice) and Decapoda (terrestrial hermit crabs, crabs of the families Ocypodidae, Gecarcinidae and Grapsidae, and terrestrial crayfish). They are abundant in all marine ecosystems, and most species are scavengers, although some, such as the porcelain crabs, are filter feeders, and some, such as mantis shrimp, are carnivores.
The name Malacostraca comes from the Greek roots μαλακός (malakós, meaning "soft") and ὄστρακον (óstrakon, meaning "shell"). The name is misleading, since the shell is only soft immediately after moulting, and is usually hard. Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, and named after the obsolete taxon Entomostraca.
The Class Malacostraca includes over 25,000 species, and "arguably ... contains a greater diversity of body forms than any other class in the animal kingdom". Its members are characterised by the presence of three tagmata – a five-segmented head, an eight-segmented thorax and an abdomen with six segments, except in the Leptostraca, which retain the ancestral condition of seven abdominal segments. This arrangement is known as the "caridoid facies", a term coined by William Thomas Calman in 1909. Each body segment bears a pair of jointed appendages, although these may be lost secondarily.
The head bears two pairs of antennae, the first of which is biramous and the second uniramous, and two pairs of maxillae. There is usually a pair of stalked compound eyes, although these may be sessile, reduced or lost.
Up to eight thoracic segments may be fused with the head to form a cephalothorax, and up to three pairs of appendages may be modified as maxillipeds (accessory mouthparts). A carapace may be absent, present, or secondarily lost, and may cover from two thoracic segments to the entire thorax and some of the abdomen.
Each segment of the abdomen except the last carries a pair of pleopods. The appendages of the last segment are typically flattened into uropods, which together with the terminal telson, make up the "tail fan". In Leptostraca, these appendages instead form caudal rami.
Most malacostracans are gonochoristic (i.e., they have separate sexes), although there are a few hermaphroditic species. The female genital openings are on the sixth thoracic segment, while the male genital opening is usually on the sixth thoracic segment, but is occasionally on the seventh. Each of the thoracic appendages is biramous and also carries a gill. The larval stages are often reduced, but where they occur, there is usually a metamorphosis between the larval and the adult form.
Class Malacostraca Latreille, 1802
- Subclass Phyllocarida Packard, 1879
- Subclass Hoplocarida Calman, 1904
- Subclass Eumalacostraca Grobben, 1892
- Superorder Syncarida Packard, 1885
- Superorder Peracarida Calman, 1904
- Superorder Eucarida Calman, 1904
While the monophyly of Malacostraca as a whole is widely supported, a number of problems make it difficult to determine the relationships between the orders of Malacostraca. These include differences in rates of evolution in different lineages, different patterns of evolution being apparent in different sources of data, including convergent evolution, and long branch attraction.
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