Malacostraca is the second largest of the sixclasses ofpancrustaceans behindinsects, containing about 40,000 livingspecies, divided among 16orders. Its members, themalacostracans, display a great diversity of body forms and includecrabs,lobsters,spiny lobsters,crayfish,shrimp,krill,prawns,isopods,amphipods,mantis shrimp, and many other less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. They aresegmented animals, united by a commonbody plan comprising 20 body segments (rarely 21), and divided into a head, thorax, and abdomen.
The name Malacostraca is from Ancient Greekμαλακός (malakós)'soft' and ὄστρακον (óstrakon)'shell'. The word was used byAristotle, who contrasted them withoysters, in comparison with which their shells are pliable.[1]
This scientific name is amisnomer, since the shell is soft only immediately aftermoulting, and is usually hard.[3]
Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, and named after the obsolete taxonEntomostraca.[4]
Leptostraca such asNebalia bipes retain the primitive condition of having seven abdominal segments.
The class Malacostraca includes about 40,000 species,[5] and "arguably ... contains a greater diversity of body forms than any other class in the animal kingdom".[6] Its members are characterised by the presence of threetagmata (specialized groupings of multiple segments) – a five-segmented head, an eight-segmentedthorax and an abdomen with six segments and atelson, except in theLeptostraca, which retain theancestral condition of seven abdominal segments.[6] Malacostracans have abdominal appendages, a fact that differentiates them from all other major crustacean taxa exceptRemipedia.[7] Each body segment bears a pair of jointedappendages, although these may be lost secondarily.[8]
The head bears two pairs ofantennae, the first of which is oftenbiramous (branching into two parts). The second pair of antennae bearsexopods (outer branches) which are often flattened into antennal scales known asscaphocerites.[7] The mouthparts consist of pairs each ofmandibles, maxillules (second pair of mouthparts) andmaxillae. Except forfairy shrimps, malacostracans are the only extant arthropods withcompound eyes placed onmoveable stalks,[9][10] although in some taxa the eyes are unstalked, reduced or lost.[11][12]
Up to three thoracic segments may be fused with the head to form acephalothorax; the associated appendages turn forward and are modified asmaxillipeds (accessorymouthparts).[7] Acarapace may be absent, present orsecondarily lost, and may cover the head, part or all of the thorax and some of the abdomen.[6] It is variable in form and may be fused dorsally with some of the thoracic segments or occasionally be in two parts, hinged dorsally.[11] Typically, each of the thoracic appendages is biramous and the endopods are the better developed of the branches, being used for crawling or grasping. Each endopod consist of seven articulating segments; the coxa, basis, ischium, merus, carpus, propodus and dactylus. Indecapods, the claw is formed by the articulation of the dactylus against an outgrowth of the propodus. In some taxa, the exopods are lost and the appendages are uniramous.[7]
There is a clear demarcation between the thorax and the six or seven-segmented abdomen. In most taxa, each abdominal segment except the last carries a pair of biramouspleopods used for swimming, burrowing, gas exchange, creating a current or brooding eggs. The first and second abdominal pleopods may be modified in the male to formgonopods (accessory copulatory appendages).[7] The appendages of the last segment are typically flattened intouropods, which together with the terminal telson, make up the "tail fan".[12] It is the sudden flexion of this tail fan that provides the thrust for the rapid escape response of these crustaceans and the tail fan is also used in steering.[7] In Leptostraca, the appendages on the telson instead formcaudal rami (spine-like protrusions).[13]
The digestive tract is straight and the foregut consists of a short oesophagus and a two-chambered stomach, the first part of which contains a gizzard-like "gastric mill" for grinding food. The walls of this havechitinous ridges, teeth and calcareous ossicles. The fine particles and soluble material are then moved into the midgut where chemical processing and absorption takes place in one or more pairs of large digestive caeca. The hindgut is concerned with water reclamation and the formation of faeces and the anus is situated at the base of the telson.[14]
Like other crustaceans, malacostracans have anopen circulatory system in which the heart pumps blood into thehemocoel (body cavity) where it supplies the needs of the organs for oxygen and nutrients before diffusing back to the heart.[15] The typicalrespiratory pigment in malacostracans ishaemocyanin.[16] Structures that function as kidneys are located near the base of the antennae. A brain exists in the form of ganglia close to the antennae, there are ganglia in each segment and a collection of major ganglia below the oesophagus.[17] Sensory organs include compound eyes (often stalked),ocelli (simple eyes),statocysts and sensorybristles. The naupliar eye is a characteristic of thenauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness.[14]
Most species of malacostracans have distinct sexes (a phenomenon known asgonochorism), although a few species exhibithermaphroditism.[12] The female genital openings orgonopores are located on the sixth thoracic segment or its appendages, while the male gonopores are on the eighth segment or its appendages, or in a small number of species, on the seventh.[11] The naupliar larval stages are often reduced and take place before hatching, but where they occur, ametamorphosis usually occurs between the larval and the adult forms. Primitive malacostracans have a free-swimming naupliar larval stage.[11] Research suggests the common ancestor of Malacostraca had lost the free-living nauplius larval stage, but re-evolved it again throughheterochrony in Dendrobranchiata and Euphausiacea, which both have a lecithotrophic (non-feeding) nauplius stage.[19][20]
Mating behavior has been studied in the freshwater shrimpCaridina ensifera.[21] Multiple paternity, common in theMalacostrica, also occurs inC. ensifera. Reproductive success of sires was found to correlate inversely with their genetic relatedness to the mother.[21] This finding suggests that sperm competition and/or pre- and post-copulatory female choice occurs. Female choice may increase the fitness of progeny by avoidinginbreeding that can lead to expression ofhomozygous deleteriousrecessive mutations.[22]
Themonophyly of Malacostraca is widely accepted. This is supported by several common morphological traits which are present throughout the group and is confirmed by molecular studies.[23] However, a number of problems make it difficult to determine the relationships between the orders of Malacostraca. These include differences inmutation rates in differentlineages, different patterns of evolution being apparent in different sources of data, includingconvergent evolution, andlong branch attraction.[24]
There is less agreement on the status of the subclassPhyllocarida with its single extant order, Leptostraca, depending on whether foliaceous (leaf-like) limbs have a single or multiple origin. Some authors advocate placing Phyllocarida in Phyllopoda, a group used in former classification systems, which would then include branchiopods, cephalocarids and leptostracans. A molecular study by American biologists Trisha Spears and Lawrence Abele concluded that phylogenetic evidence did not support the monophyly of this grouping, and that Phyllocarida should be regarded as a subclass of Malacostraca that had diverged from the main lineage at an early date.[11][25]
The next cladogram follows a 2023 revision ofpancrustacean phylogeny based on molecular data.[26]
Leptostraca is the only extant order of Phyllocarida, the other two orders,Archaeostraca andHoplostraca being extinct. Leptostracans are thought to be the most primitive of the malacostracans and date back to the Cambrian period. They range in length from 1 to 4 cm (0.4 to 1.6 in), most being suspension feeders though some are carnivores or scavengers. They have a two part carapace which encloses the head, the whole thorax and part of the abdomen and are the only malacostracans with seven abdominal segments. Three families are known with several genera and about twenty species. They are found worldwide from the intertidal zone to the deep ocean, all but one species beingbenthic (living on the seabed).[7][11]
Stomatopoda is the only extant order ofHoplocarida, the other two orders, Aeschronectida and Archaeostomatopoda being extinct. Stomatopodans, commonly known as mantis shrimps, range in length from 5 to 36 cm (2 to 14 in) and are predators. They have adorso-ventrally flattened body and a shield-like carapace and are armed with powerful, raptorial claws normally carried in a folded position. There are about 300 species, most living in tropical and subtropical seas although some live in temperate areas. They are benthic, mostly hiding in cracks and crevices or living in burrows, some emerging to forage while others are ambush predators.[7][11]
The Eumalocostraca contains the vast majority of the approximately 40,000 living species of malacostracans and consists of three superorders,Syncarida,Peracarida andEucarida. Syncaridans are mostly small and found in freshwater and subterranean habitats. Peracaridans are characterised by having amarsupium in which they brood their young. They are found in marine, freshwater and terrestrial habitats and includeAmphipoda,Cumacea,Isopoda andMysida. Eucarida includes lobsters, crabs, shrimps,prawns and krill.[27]
The following classification of living malacostracans is based onAn Updated Classification of the Recent Crustacea (2001) by the American marine biologistsJoel W. Martin, curator of crustaceans at theNatural History Museum of Los Angeles County, and George E. Davies.[30] Extinct orders have been added to this[31][32][33] and are indicated by anobelisk (†).
^Dupuis, Claude (1974). "Pierre André Latreille (1762–1833): the foremost entomologist of his time".Annual Review of Entomology.19:1–14.doi:10.1146/annurev.en.19.010174.000245.
^abcdefghRuppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004).Invertebrate Zoology (7th ed.). Cengage Learning. pp. 625–626.ISBN978-81-315-0104-7.{{cite book}}: CS1 maint: multiple names: authors list (link)
^Cronin, Thomas W. (1986). "Optical Design and Evolutionary Adaptation in Crustacean Compound Eyes".Journal of Crustacean Biology.6 (1):1–23.doi:10.2307/1547926.JSTOR1547926.
^abcdHayward, P. J.; Isaac, M. J.; Makings, P.; Moyse, J.; Naylor, E.; Smaldon, G. (1995). "Crustaceans". In Hayward, P. J.; Ryland, John Stanley (eds.).Handbook of the Marine Fauna of North-West Europe.Oxford University Press. pp. 290–461.ISBN978-0-19-854055-7.
^abRuppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004).Invertebrate Zoology (7th ed.). Cengage Learning. pp. 610–613.ISBN978-81-315-0104-7.{{cite book}}: CS1 maint: multiple names: authors list (link)
^Camacho, A. I.; Valdecasas, A. G. (2008). "Global diversity of syncarids (Syncarida; Crustacea) in freshwater". In Balian, E. V.; Lévêque, C.; Segers, H.; Martens, K. (eds.).Freshwater Animal Diversity Assessment. Developments in Hydrobiology. Vol. 198.Springer. pp. 257–266.doi:10.1007/978-1-4020-8259-7_28.ISBN978-1-4020-8258-0.
