Crustaceans (Crustacea) form a very large group of arthropods An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the Phylum Arthropoda (from Greek ἄρθρον arthron, "joint", and ποδός podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and, usually treated as a subphylum In life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum, which includes such familiar animals as crabs True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax. Other animals, such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice, are not true crabs, lobsters Clawed lobsters comprise a family of large marine crustaceans. Lobsters are economically important as seafood, forming the basis of a global industry that nets more than US$1 billion annually, crayfish Crayfish, crawfish, or crawdads — members of the superfamilies Astacoidea and Parastacoidea — are freshwater crustaceans resembling small lobsters, to which they are related. They breathe through feather-like gills and are found in bodies of water that do not freeze to the bottom; they are also mostly found in brooks and streams where there is, shrimp Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important food source for larger animals from fish to whales, krill Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world. The common name krill comes from the Norwegian word krill meaning "young fry of fish", which is also often attributed to other species of fish and barnacles A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have two nektonic larval stages. Around 1,220 barnacle. The 50,000 described species range in size from Stygotantulus stocki Stygotantulus stocki is a species of crustacean, living as an ectoparasite on harpacticoid copepods of the families Tisbidae and Canuellidae. It is the smallest arthropod in the world, at a length of less than 0.1 millimetres . The specific epithet stocki commemorates Jan Hendrik Stock, a Dutch carcinologist at 0.1 mm (0.004 in), to the Japanese spider crab The Japanese spider crab, Macrocheira kaempferi, is a species of marine crab that lives in the waters around Japan. It has the largest leg span of any arthropod, reaching up to 3.8 metres and weighing up to 41 pounds (19 kg). It is the subject of small-scale fishery with a leg span of up to 14 ft (4.3 m) and a mass of 44 lb (20 kg). Like other arthropods An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the Phylum Arthropoda (from Greek ἄρθρον arthron, "joint", and ποδός podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and, crustaceans have an exoskeleton An exoskeleton is an external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton. In popular usage, many of the larger kinds of exoskeletons are known as "shells", which they moult Ecdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support (the exoskeleton) of the body and is inelastic, it is shed during growth and a new, larger covering is formed. The old, empty exoskeleton is called an exuvia (or "exuvium") to grow. They are distinguished from other groups of arthropods, such as insects Insects are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living, myriapods Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial . Although their name suggests they have myriad legs, myriapods range from having over 750 legs (Illacme plenipes) to having fewer than ten legs and chelicerates The subphylum Chelicerata (pronounced /kəˌlɪsəˈrɒtə/) constitutes one of the major subdivisions of the phylum (or superphylum) Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites. They originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from 445 by the possession of biramous The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter (compare greater trochanter and lesser trochanter), femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus (meaning finger), (two-parted) limbs, and by the nauplius A nauplius is the first larval stage of animals classified as crustaceans (subphylum of Arthropoda). It consists of a head and a telson. The thorax and abdomen, characteristic of adult crustaceans, have not yet developed form of the larvae.

Most crustaceans are free-living aquatic animals An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that live in them can be categorized as aquatic or, but some are terrestrial (e.g. woodlice A woodlouse is a crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs. Woodlice form the suborder Oniscidea within the order Isopoda, with over 3,000 known species), some are parasitic (e.g. fish lice Branchiura, commonly called carp lice or fish lice are a group of parasitic crustaceans of uncertain position within the Maxillopoda. Although they are thought to be primitive forms, they have no fossil record. Almost all are ectoparasites on fish, with a few exceptions living on amphibians, tongue worms The Pentastomida are a group of parasitic invertebrates commonly known as tongue worms due to the resemblance of the species of the genus Linguatula to a vertebrate tongue) and some are sessile (e.g. barnacles A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have two nektonic larval stages. Around 1,220 barnacle). The group has an extensive fossil record Fossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record. The study of fossils across geological time, how, reaching back to the Cambrian The Cambrian is the first geological period of the Paleozoic Era, lasting from 542 ± 0.3 million years ago to 488.3 ± 1.7 million years ago(ICS, 2004,; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the classical name for, and includes living fossils Living fossil is an informal term for any living species of organism which appears to be the same as a species otherwise only known from fossils and which has no close living relatives. These species have all survived major extinction events, and generally retain low taxonomic diversities. A species which successfully radiates (forming many new such as Triops cancriformis, which has existed apparently unchanged since the Triassic The Triassic is a geologic period that extended from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events. The extinction event that closed the Triassic Period has recently been more accurately period. More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimps Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important food source for larger animals from fish to whales and prawns Prawns are decapod crustaceans, belonging to the sub-order Dendrobranchiata. The term "prawn" is also used in various contexts for other animals, especially caridean shrimp. They are found worldwide and include commercially significant species, such as the whiteleg shrimp Litopenaeus vannamei, Atlantic white shrimp Penaeus setiferus,. Krill Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world. The common name krill comes from the Norwegian word krill meaning "young fry of fish", which is also often attributed to other species of fish and copepods Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic (drifting in sea waters), but more are benthic (living on the ocean floor), and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests, are not as widely fished, but may be the animals with the greatest biomass Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms, plants or animals. The mass can be expressed as the on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as carcinology (alternatively, malacostracology, crustaceology or crustalogy), and a scientist A scientist in a broad sense is one engaging in a systematic activity to acquire knowledge. In a more restricted sense, a scientist is an individual who uses the scientific method. The person may be an expert in one or more areas of science. This article focuses on the more restricted use of the word. Scientists perform research toward a more who works in carcinology is a carcinologist.

Structure

The body of a crustacean is composed of body segments, which are grouped into three regions: the cephalon or head In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth (all of which aid in various sensory functions, such as sight, hearing, smell, and taste). Some very simple animals may not have a head, but many bilaterally symmetric forms do,^[3] the thorax The thorax is a division of an animal's body that lies between the head and the abdomen,^[4] and the pleon or abdomen In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax (chest) and pelvis. The region enclosed by the abdomen is termed the abdominal cavity. In arthropods it is the most distal section of the body which lies behind the thorax or cephalothorax.^[5] The head and thorax may be fused together to form a cephalothorax The cephalothorax is an anatomical term used in arachnids and malacostracan crustaceans for the first (anterior) major body section. The remainder of the body is the abdomen (opisthosoma), which may also bear lateral appendages as well as the tail, if present. The term "prosoma" can also be applied to the head of insects, but as the two,^[6] which may be covered by a single large carapace A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.^[7] The crustacean body is protected by the hard exoskeleton An exoskeleton is an external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton. In popular usage, many of the larger kinds of exoskeletons are known as "shells", which must be moulted Ecdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support (the exoskeleton) of the body and is inelastic, it is shed during growth and a new, larger covering is formed. The old, empty exoskeleton is called an exuvia (or "exuvium") for the animal to grow. The shell around each somite can be divided into a dorsal tergum, ventral sternum The sternum is a long flat bone (or, in some instances, set of three bones) shaped like a capital 'T' located in the center of the thorax (chest). It connects to the rib bones via cartilage, forming the anterior section of the rib cage with them, and thus helps to protect the lungs, heart and major blood vessels from physical trauma and a lateral pleuron. Various parts of the exoskeleton may be fused together.^[8]:289

Each somite A somite is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage, in arthropods it is a characteristic of a hypothetical ancestor, or body segment can bear a pair of appendages An appendage in the broadest sense is an additional or subsidiary part existing on, or added to, something which can generally still function if the appendage has never existed or is later provided or grown, or will still perform a primary function if the appendage is removed: on the segments of the head, these include two pairs of antennae Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes, the mandibles The mandible or inferior maxillary bone forms the lower jaw and holds the lower teeth in place. The term "mandible" also refers to both the upper and lower sections of the beaks of birds; in this case the "lower mandible" corresponds to the mandible of humans, while the "upper mandible" is functionally equivalent to and maxillae The maxilla , also known as the mustache bone, is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mental symphysis. Sometimes (e.g. in bony fish), the maxilla is sometimes called "upper maxilla", with the mandible being the &;^[3] the thoracic segments bear legs The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter (compare greater trochanter and lesser trochanter), femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus (meaning finger),, which may be specialised as pereiopods (walking legs) and maxillipeds (feeding legs).^[4] The abdomen bears pleopods,^[5] and ends in a telson The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the uropods, the telson forms the tail fan of, which bears the anus, and is often flanked by uropods to form a tail fan.^[9] The number and variety of appendages in different crustaceans may be partly responsible for the group's success.^[10] Crustacean appendages are typically biramous, meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is uniramous.^[11] It is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. Trilobites, for instance, also possessed biramous appendages.^[12]

The main body cavity is an open circulatory system, where blood is pumped into the haemocoel by a heart located near the dorsum.^[13] The alimentary canal consists of a straight tube that often has a gizzard-like gastric mill for grinding food and a pair of digestive glands that absorb food; this structure goes in a spiral format.^[14] Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia close to the antennae, and a collection of major ganglia is found below the gut.^[15]

In many decapods, the first (and sometimes the second) pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans (such as the Christmas Island red crab) mate seasonally and return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs until they hatch into free-swimming larvae.^[16]

Ecology

The majority of crustaceans are aquatic, living in either marine or fresh water environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs, and woodlice. Marine crustaceans are as ubiquitous in the oceans as insects are on land.^[17][18] The majority of crustaceans are also motile, moving about independently, although a few taxonomic units are parasitic and live attached to their hosts (including sea lice, fish lice, whale lice, tongue worms, and Cymothoa exigua, all of which may be referred to as "crustacean lice"), and adult barnacles live a sessile life – they are attached headfirst to the substrate and cannot move independently. Some branchiurans are able to withstand rapid changes of salinity and will also switch hosts from marine to non-marine species.^[19]:672 Krill are the bottom layer and the most important part of the food chain in Antarctic animal communities.^[20]:64 Some crustaceans are significant invasive species, such as the Chinese mitten crab^[21] and the Asian shore crab.^[22]

Life cycle

Mating system

The majority of crustaceans have separate sexes, and reproduce sexually.^[23] A small number are hermaphrodites, including barnacles, remipedes,^[24] and Cephalocarida.^[25] Some may even change sex during the course of their life.^[25] Parthenogenesis is also widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male.^[23] This occurs in many brachiopods, some ostracods, some isopods, and certain "higher" crustaceans, such as the Marmorkrebs crayfish.

Eggs

In many groups of crustaceans, the fertilised eggs are simply released into the water column, while others have developed a number of mechanisms for holding on to the eggs until they are ready to hatch. Most decapods carry the eggs attached to the pleopods, while peracarids, notostracans, anostracans, and many isopods form a brood pouch from the carapace and thoracic limbs.^[23] Female Branchiura do not carry eggs in external ovisacs but attach them in rows to rocks and other objects.^[26]:788 Most leptostracans and krill carry the eggs between their thoracic limbs; some copepods carry their eggs in special thin-walled sacs, while others have them attached together in long, tangled strings.^[23]

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