Copepods are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic, some are benthic, several species have parasitic phases, and some continental species may live in limnoterrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests, bogs, springs, ephemeral ponds, puddles, damp moss, or water-filled recesses of plants (phytotelmata) such as bromeliads and pitcher plants. Many live underground in marine and freshwater caves, sinkholes, or stream beds. Copepods are sometimes used as biodiversity indicators.
Malacostraca is the second largest of the six classes of pancrustaceans behind insects, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, prawns, woodlice, amphipods, mantis shrimp, tongue-eating lice and many other less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. They are segmented animals, united by a common body plan comprising 20 body segments, and divided into a head, thorax, and abdomen.
Mysida is an order of small, shrimp-like crustaceans in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch or "marsupium" in females. The fact that the larvae are reared in this pouch and are not free-swimming characterises the order. The mysid's head bears a pair of stalked eyes and two pairs of antennae. The thorax consists of eight segments each bearing branching limbs, the whole concealed beneath a protective carapace and the abdomen has six segments and usually further small limbs.
Ostracods, or ostracodes, are a class of the Crustacea, sometimes known as seed shrimp. Some 33,000 species have been identified, grouped into 7 valid orders. They are small crustaceans, typically around 1 mm (0.04 in) in size, but varying from 0.2 to 30 mm in the case of the marine Gigantocypris. The largest known freshwater species is Megalocypris princeps, which reach 8 mm in length. In most cases, their bodies are flattened from side to side and protected by a bivalve-like valve or "shell" made of chitin, and often calcium carbonate. The family Entocytheridae and many planktonic forms do not have calcium carbonate. The hinge of the two valves is in the upper (dorsal) region of the body. Ostracods are grouped together based on shell and soft part morphology, and molecular studies have not unequivocally supported the group's monophyly. They have a wide range of diets, and the class includes carnivores, herbivores, scavengers and filter feeders, but most ostracods are deposit feeders.
Isopoda is an order of crustaceans. Members of this group are called isopods and include both aquatic species, and terrestrial species such as woodlice. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Females brood their young in a pouch under their thorax.
Rhizocephala are derived barnacles that are parasitic castrators. Their hosts are mostly decapod crustaceans, but include Peracarida, mantis shrimps and thoracican barnacles. Their habitats range from the deep ocean to freshwater. Together with their sister groups Thoracica and Acrothoracica, they make up the subclass Cirripedia. Their body plan is uniquely reduced in an extreme adaptation to their parasitic lifestyle, and makes their relationship to other barnacles unrecognisable in the adult form. The name Rhizocephala derives from the Ancient Greek roots ῥίζα and κεφαλή, describing the adult female, which mostly consists of a network of thread-like extensions penetrating the body of the host.
Anostraca is one of the four orders of crustaceans in the class Branchiopoda; its members are referred to as fairy shrimp. They live in vernal pools and hypersaline lakes across the world, and they have even been found in deserts, ice-covered mountain lakes, and Antarctic ice. They are usually 6–25 mm (0.24–0.98 in) long. Most species have 20 body segments, bearing 11 pairs of leaf-like phyllopodia, and the body lacks a carapace. They swim "upside-down" and feed by filtering organic particles from the water or by scraping algae from surfaces, with the exception of Branchinecta gigas, or "giant fairy shrimp", which is itself a predator of other species of anostracans. They are an important food for many birds and fish, and some are cultured and harvested for use as fish food. There are 300 species spread across 8 families.
The Gnathiidae are a family of isopod crustaceans. They occur in a wide range of depths, from the littoral zone to the deep sea. The adults are associated with sponges and may not feed. The juvenile form is known as a 'praniza', and it is a temporary parasite of marine fish. These forms are not larvae; Gnathiidae instead become parasitic during the manca stage. Mancae of the Gnathiidae closely resemble the adult form, however they lack the final pair of pereiopods.
The Pentastomida are an enigmatic group of parasitic arthropods commonly known as tongue worms due to the resemblance of the species of the genus Linguatula to a vertebrate tongue; molecular studies point to them being highly-derived crustaceans.
Ascothoracida is a small group of parasitic marine crustaceans, comprising around 100 species and divided into Dendrogastrida and Laurida. They are found throughout the world on cnidarians and echinoderms. Dendrogastrida are parasites on echinoderms, and Laurida are parasites on cnidarians, except from the species Waginella Grygier, which is also a parasite on echinoderms (crinoids). Piercing and sucking mouthparts are used for feeding, and more advanced forms also absorbs nutrients through a modified integument of the carapace. More basal forms are ectoparasitic, but most genera are meso- and endoparasitic. The sexes are separate, except from secondary hermaphroditic species of the Petrarcidae. In many species the larger female often have smaller males living inside her mantle cavity.
Dipteropeltis hirundo is a little-known species of fish louse. It is an ectoparasite of fish found in South America, including piranhas and barred sorubim.
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow. The larvae of crustaceans often bear little resemblance to the adult, and there are still cases where it is not known what larvae will grow into what adults. This is especially true of crustaceans which live as benthic adults, more-so than where the larvae are planktonic, and thereby easily caught.
Argulus is a genus of fish lice in the family Argulidae. There are about 140 accepted species in the genus Argulus. They occur in marine, brackish, and freshwater environments. They sit tightly against the host body, this minimises risk of detachment. As juveniles, these species feed on mucous and skin cells of their host. With age they become blood feeders because the parasite moves from feeding on the fins to feeding on the body of the fish, causing the feeding change. At least some species can have severe impacts on their host populations.
Aega psora is a species of isopod crustacean that parasitises a number of fish species in the North Atlantic. It is a serious ectoparasite of larger species of fish, particularly when they are injured.
Crustaceans are invertebrate animals that constitute one group of arthropods that are a part of the subphylum Crustacea, a large, diverse group of mainly aquatic arthropods including decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the hexapods emerged deep in the Crustacean group, with the completed pan-group referred to as Pancrustacea. The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans.
Argulus foliaceus, also known as the common fish louse, is a species of fish lice in the family Argulidae. It is "the most common and widespread native argulid in the Palaearctic" and "one of the most widespread crustacean ectoparasites of freshwater fish in the world", considering its distribution and range of hosts. It can cause the severe disease state argulosis in a wide variety of fish species. It is responsible for epizootic outbreaks that have led to the collapse of aquaculture operations. Fish lice are not related to lice, which are insects.
Argulus japonicus, also known as the Japanese fish louse, is a species of crustacean in the family Argulidae. This species is light brown in colour and may be between 4 and 9 mm long and 3 to 6 mm wide. It has a stumpy tail, and is shaped somewhat like a round shield. The female and male are phenotypically distinct, categorizing them as a dioecious species. Although they are not drastically different, their transparent skin highlights the minor differences such as the different copulatory accessories.
Anilocra capensis is a species of parasitic isopod in the family Cymothoidae. It is endemic to southern Africa. The species preferentially attaches itself to the hottentot seabream.
Chonopeltis is a genus of fish lice in the subclass Branchiura. These crustaceans are ectoparasitic freshwater fish host. Species of this genus are exclusively found in rivers in Africa.
The carp louse Argulus coregoni is an obligate ectoparasitic branciuran species on fish. It occurs in China and Japan, and Scandinavia. Argulus coregoni can pose a threat to fish kept in fish farms.
