Malacostraca

Latreille, 1802

malacostracans

Class Guides

Eucarida(Decapods and Krill)
Peracarida(Amphipods, Isopods, and Allies)
Stomatopoda(Mantis Shrimps)

Malacostraca is the largest class of crustaceans, containing approximately 40,000 living divided among 16 orders. Members display exceptional morphological diversity and include crabs, lobsters, crayfish, shrimp, krill, prawns, isopods, amphipods, mantis shrimp, and numerous other forms. The class is united by a distinctive body plan of 20 body segments (rarely 21) arranged into three tagmata: a five-segmented , eight-segmented , and six-segmented with terminal (seven segments in Leptostraca). Malacostracans are abundant across all marine environments and have successfully colonized freshwater and terrestrial .

Pilumnidae by (c) Javier, some rights reserved (CC BY), uploaded by Javier. Used under a CC-BY license.

Cancer irroratus by (c) Leïla Brunner, some rights reserved (CC BY), uploaded by Leïla Brunner. Used under a CC-BY license.

Cancer borealis by (c) Ken-ichi Ueda, some rights reserved (CC BY). Used under a CC-BY license.

Pronunciation

How to pronounce Malacostraca: /ˌmæləkəˈstreɪkə/

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Images

Habitat

Marine environments across all depths and latitudes; freshwater including rivers, lakes, and subterranean groundwater; terrestrial habitats including coastal zones, forests, and deserts. Three orders contain terrestrial members: Amphipoda (Talitridae), Isopoda (Oniscidea, the woodlice), and Decapoda (terrestrial hermit crabs, certain crab , and terrestrial crayfish).

Distribution

Global distribution across all continents and oceans. Marine occur from intertidal zones to abyssal depths. Freshwater representatives found on all continents except Antarctica. Terrestrial species primarily restricted to coastal and humid regions, with some extending into arid environments.

Life Cycle

Most exhibit gonochorism (distinct sexes), with female on the sixth thoracic segment and male gonopores on the eighth (rarely seventh). Naupliar larval stages are often reduced and occur before hatching; where present, typically occurs between larval and forms. Primitive malacostracans retain a free-swimming naupliar stage. Some lineages (Dendrobranchiata and Euphausiacea) have re-evolved a lecithotrophic (non-feeding) nauplius stage through heterochrony.

Ecological Role

Malacostracans occupy diverse trophic positions including scavengers, , herbivores, and . Ponto-Caspian peracarids function as significant in European river systems, causing retreat of native and altering structure.

Human Relevance

Economically vital as food sources (crabs, lobsters, shrimp, prawns). Some serve as wood-destroying pests. Research organisms for developmental and neurobiology.

Misconceptions

The name Malacostraca (Greek: 'soft shell') is a misnomer coined by Aristotle and applied by Latreille in 1802; the shell is soft only immediately after molting and is typically hard. The class is sometimes contrasted with 'Entomostraca,' an obsolete term for all non-malacostracan crustaceans.