Isopoda

(invertebrate zoology) An order of malacostracan crustaceans characterized by a cephalon bearing one pair of maxillipeds in addition to the antennae, mandibles, and maxillae.

(Pillbugs, slaters, and woodlice)

Phylum: Arthropoda

Subphylum: Crustacea

Class: Malacostraca

Order: Isopoda

Number of families: Approximately 120

Thumbnail description
Small, generally gray, usually flat, marine, freshwater, or terrestrial animals with numerous legs; some species are parasitic

Evolution and systematics

With approximately 10,000 known species in 10 suborders, the order Isopoda falls under the class Malacostraca, subphylum Crustacea, phylum Arthropoda. Five of the predominant suborders are as follows:

• Asellota, marine and freshwater isopods
• Epicaridea, parasitic isopods that live on or in other crustaceans
• Flabellifera, marine or estuarine species, including a few parasitic taxa
• Oniscidea, mainly terrestrial isopods, including the familiar pillbugs, sowbugs, and woodlice
• Valvifera, marine species

Isopods are perhaps most intriguing as models of the evolutionary transition from marine to terrestrial habitats. The terrestrial suborder Oniscidea is believed to have arisen either from the marine suborder Flabellifera, specifically from the family Cirolanidae, or from the marine suborder Asellota. The ancestors of the oniscideans were likely similar to the genus Ligia, which inhabits the ocean shoreline and has both aquatic and terrestrial characteristics. These characteristics include a primitive water-conducting system and the ability to swim, a mode of locomotion that terrestrial isopods no longer possess.

The order Isopoda, which dates to at least 300 million years ago, once was thought to be monophyletic. Recent findings, however, suggest that the suborder Flabellifera has a separate phylogeny. Early isopods were shallow marine inhabitants, then spread to freshwater, deep marine, and terrestrial areas, where they live today.

Physical characteristics

A diverse order of crustaceans, the isopods are mainly small, at least slightly dorsoventrally flattened, gray or brown organisms with numerous legs, called pereopods. Their bodies are divided into the head, which includes fused maxillipeds that form the so-called cephalon; the leg-bearing thorax or pereon; and the abdomen, or pleon. The length of isopods typically ranges from approximately 0.2 to 0.6 in (5–15 mm), but these animals can be smaller (0.02 in [0.5 mm]) or much larger. The largest species, at 19.7 in (50 cm) long, is Bathynomus giganteus. Isopods lack the carapace of many other crustaceans, instead having a cephalic shield. All but the parasitic forms have at least 14 walking legs—two on each of the seven somites that make up the pereon. The legs usually are short, but they can be quite long and spider-like in species such as Munna armoricana, which has legs as long as or longer than the body. Unlike many other crustaceans, isopods have unstalked rather than stalked compound eyes. They typically have two, well-developed antennae equipped with stiff sensory setae. Another vestigial pair of antennae is present. In addition, the ventral plates of the thorax, specifically those of the second through fifth thoracic segments, form the brood pouch used by females to house their developing young.

Two pairs of white, oval structures are apparent on the first two abdominal segments of terrestrial isopods. Desert forms have five pairs. These structures are called pleopods and are appendages modified for respiration. Pleopods contain pseudotracheae, which trap air and give the pleopods their white appearance. In addition to using this source of oxygen, terrestrial isopods breathe by diffusion of gas directly through the cuticle.

The appearance of parasitic forms of isopods differs somewhat from that of other isopods. Females commonly have asymmetric bodies—rather like a pillbug viewed in a funhouse mirror. If they are present at all, legs often are developed only on one side of the body. The mandibles are sharp, piercing devices. The male looks more like a typical pillbug or sowbug, having a symmetrical, oval body. The male is much smaller than the female and often is found attached to her abdomen. Male and female parasitic isopods have two pairs of antennae, but both are vestigial at best.

Distribution

Worldwide. Many isopod species have extended their ranges with the help of humans. Marine species have moved across the ocean in the bilge waters of sea-faring tankers, and terrestrial forms find welcome hiding places in the dark, damp storage areas of various transportation vehicles. Armadillidium vulgare is an example of a species that has successfully invaded habitats in the New World from its original distribution around the Mediterranean Sea.

Habitat

Isopods can be found in a wide range of habitats from marine or freshwater areas to deserts but are best known from their terrestrial haunts under logs, in or beneath rotting wood, or in other damp areas. Isopods are gill breathers, and even the terrestrial species, which number approximately 4,000, need wet habitats.

Among the terrestrial isopods, rock slaters of the family Ligiidae, inhabit littoral (seashore) areas. Pillbugs are species of the families Armadillididae and Armadillidae and are typically found in grasslands and arid habitats. Sowbug is the common name given to species of the families Oniscidae and Porcellionidae. These isopods favor forests and semiarid areas, respectively. A few species live in deserts. Hemilepistus reaumuri, for example, lives in deserts of northern Africa and the Middle East.

Marine and estuarine species, which number approximately 4,500, often live in shallow coastline waters, but numerous species, particularly those in the suborder Asellota, have successfully invaded the deep sea. They frequently inhabit burrows they make in the sediment or in vegetation. The freshwater species, numbering approximately 500, similarly are sediment burrowers. A wood-boring isopod, Sphaeroma terebrans, burrows into the aerial roots of mangrove trees, which periodically flood. A few, including Ligia species, are transitional between terrestrial and marine habitats and exist in the semiterrestrial, rocky coastline along the ocean.

Numerous species, particularly those in the suborder Epicaridea, are parasitic, blood-sucking forms that live on or in various animals, including barnacles, crabs, and shrimps.

Behavior

Terrestrial isopods are most often found in dark nooks and crannies of decaying logs and underneath rocks and leaf litter. Many do, however, venture into the daylight. Pillbugs even have a positive phototactic response, particularly toward sunset. When temperatures during the day become too high, terrestrial isopods generally move quickly to underground hiding places, where higher humidity can help the animals avoid desiccation. As another defense against desiccation, when temperatures rise to 68°F–86°F (20°C–30°C), pillbugs apparently become attracted to the odors of conspecifics and group together. The bunching behavior decreases the exposed surface area of each individual. If necessary, pillbugs are able to drink water droplets by taking up water through tail projections and diverting it along lateral, exterior grooves (collectively called a water transport system) to the mouth.

By spending a good deal of time hidden in or under logs and leaf litter, terrestrial isopods gain considerable protection from many of their predators, but they have additional defenses. One is camouflage. Isopods are generally brownish-gray or gray, colors that conceal them well against the ground, a log, or a rock. Another predator deterrent comes from repugnatorial glands on the thorax. These glands release a secretion that is unpleasant to predators and is enough to ward off most attacks. The European pillbug (Armadillidium klugii) is unusual in that it has aposematic coloration that mimics the orange, hourglass marking typical of a European black widow spider (Latrodectus mactans tredecimguttatus). The spider is a venomous species that predators avoid. The copycat coloration allows these isopods to reap the rewards of the spider's warning pattern.

A strange example of another species that affects the behavior of pillbugs is an acanthocephalan worm that parasitizes these isopods. The life cycle of the worm begins when its eggs are passed in the feces of birds, specifically starlings (Sturnus vulgaris). Pillbugs eat the feces and ingest the worms. The worms hatch inside the pillbug, where they grow to approximately 0.1 in (2–3 mm) long (approximately one third of the total length of the pillbug). In addition to crowding pillbugs' internal organs and rendering females sterile, the worms alter pillbugs' behavior. Infected pillbugs move from their normal damp, dark areas to wide-open spaces. Starlings feed on pillbugs and easily find the now exposed individuals. Infected starlings are the final host for the worms, which lay eggs that are passed through feces to repeat the cycle.

Isopods as a group are perhaps best known for the ability of some to roll into a ball. With this posture, called conglobation, isopods effectively use their armor-like dorsal surface to shield the softer body parts from predators and from water loss. Not all isopods can conglobate, but the behavior is common among terrestrial species. Even some intertidal and littoral species, such as Campecopea hirsuta and Tylos, respectively, can roll up. When conglomated, many species enclose their antennae in the ball, but some, such as Armadillidium, leave the antennae outside. Despite the acrobatics involved in conglobation, many species of isopods cannot right themselves if they are turned on their backs. This is particularly true if the species is especially rounded dorsally. Sowbugs, the flatter, terrestrial isopods, cannot conglobate but can right themselves easily.

Among marine and coast-living isopods, level of activity is frequently related to the tides. As water rises and lowers along the coast, some dune-dwelling species, such as Tylos punctatus, move up or down the beach slope to station themselves just beyond the water. Other marine organisms, such as Eurydice pulchra, are inactive during neap tide but become active just after a high tide and heighten their activity approximately three or four days after a new or full moon. Swimming when the water is high helps these isopods follow the water's rise and fall up and down the beach slope.

Littoral and seashore isopods may be either diurnal or nocturnal. Ligia species are an example of the former, and Tylos of the latter.

Desert isopods eke out a living through various behaviors. Hemilepistus reaumuri, for example, retains the family unit with parents tending juveniles in the burrow throughout the summer.

Feeding ecology and diet

As a group, isopods are omnivores, eating everything from living and dead vegetation to fungi and from living and dead animals to fecal matter. The terrestrial forms, commonly called pillbugs or sowbugs, are mostly detritus feeders, scouring the forest floor for decaying organic matter. Their diet is wide ranging and may include fruits and tender shoots, dead and dying vegetative matter, fungi, and their own as well as other organisms' feces. Scientists have studied the isopod habit of eating feces, called coprophagy. Research indicates that nearly one tenth of the isopod diet may be the animals' own waste products, which are believed to replenish the digestive microorganisms the system requires and to provide some nutrition. When deprived of this dietary source, isopods grow more slowly than normal. Some species, such as those in the genus Platyarthrus, eat feces or regurgitated pellets of ants. These isopods, which are blind and white, live in ant nests.

Predators of terrestrial isopods include various spiders, such as those in the family Dysderidae, that can penetrate the isopod's hard coat. Amphibians and birds also take a toll, as do centipedes. Terrestrial isopods are particularly vulnerable to predators when they molt and temporarily lose their hard, protective covering.

Marine isopods feed primarily on algae, diatoms, and other vegetation in addition to wood and vegetative detritus. A few, such as Cirolana species, eat the decaying flesh of dead animals, especially fishes. Predators of these marine species are primarily fishes. Parasitic isopods also exist. Some, such as Lironeca and Aega, attach to fishes. Others, including Stegias clibanarii, are parasitic during only one stage of their lives. They live on yolk during the first developmental stage, go through a second, parasitic stage, and become free living in the third stage.

Reproductive biology

The sexes are typically separate. Male isopods transfer sperm through the second, or the first and second, pair of pleopods. Mating generally occurs by the male climbing onto the back of the female and bending his abdomen to her ventral gonopores for sperm transfer. The female is fertile only as she goes through maturation molt, but male-female pairs can form a day or two early. During maturation molt, females shed the posterior half of the exoskeleton and two or three days later shed the anterior half. Soon after mating, the female sheds her eggs, which range in number from half a dozen to several hundred, into a brood pouch, or marsupium. The ovaria and marsupium are attached by thin tubes. Egg size varies between species and among females of the same species. Generally, larger females have larger eggs. In general, young develop in the brood pouch for the next 8–12 weeks. One or two broods per year are common, and females of many species can store sperm for as long as a year. Juveniles that leave the brood pouch are called mancas. Mancas are almost identical to adults but are missing the last pair of thoracic legs. Among burrow-dwelling species, mancas may find and live in tunnels individually or remain with the mother in a family burrow. In the burrows, the juveniles harden and darken through successive molts.

A few isopods, including the parasitic Lironeca and Aega species, are protandric hermaphrodites that switch from male to female as they develop. Pseudione and other parasitic forms, however, have separate sexes.

Isopods typically live one or two years, but some survive five years. The longest-lived species known is Armadillo officinalis, which can live nine years.

Conservation status

The IUCN lists 39 species of isopods as threatened. They include 22 species categorized as Vulnerable, seven as Critically Endangered, nine as Endangered, and one as Extinct in the Wild. The extinct species is the Socorro isopod, Thermosphaeroma thermophilum, a member of the family Sphaeromatidae. Found only in Sedillo Spring, Socorro County, New Mexico, the population became extinct in 1988 when a valve control system failed and cut off flow to the area. The problem has since been repaired, and previously obtained individuals have been bred in captivity and reintroduced to the spring. Additional captive populations are held by three organizations, including the Santa Fe, New Mexico, Department of Game and Fish.

Significance to humans

The feeding and burrowing behavior of some marine and estuarine isopods, such as Sphaeroma and Limnoria, can cause considerable damage to wooden pilings, docks, and other underwater structures. Terrestrial isopods are generally harmless, although large numbers can cause vegetation damage, particularly in gardens and greenhouses.

Species accounts

Resources

Alikhan, M. A., ed. Crustacean Issues 9: Terrestrial Isopod Biology. Rotterdam, The Netherlands: A. A. Balkema, 1995.

Raham, R. G. "Pill Bug Strategies." In Dinosaurs in the Garden: An Evolutionary Guide to Backyard Biology. Medford, NJ: Plexus Publishing, 1988.

Warburg, M. R. Evolutionary Biology of Land Isopods. Berlin: Springer-Verlag, 1993.

Brooks, R. A. "Colonization of a Dynamic Substrate: Factors Influencing Recruitment of the Wood-Boring Isopod, Sphaeroma terebrans onto Red Mangrove (Rhizophora mangle) ."Oecologia 127 (2001): 522–532.

Hassall, M., and S. P. Rushton. "The Role of Coprophagy in the Feeding Strategies of Terestrial Isopods." Oecologia 53 (1982): 374–381.

Holomuzki, J. R., and T. M. Short. "Habitat Use and Fish Avoidance Behaviors by the Stream-Dwelling Isopod Lirceus fontinalis." Oikos 52 (1988): 79–86

McDermott, J. J. "Biology of Chiridotea caeca (Say, 1818) (Isopoda: Idoteidae) in the Surf Zone of Exposed Sandy Beaches along the Coast of Southern New Jersey, U.S.A." Ophelia 55 (2001): 123–135.

Sparkes, T. C., D. P. Keogh, and R. A. Pary. "Energetic Costs of Mate Guarding Behavior in Male Stream-Dwelling Isopods." Oecologia 106 (1996): 166–171.

Thiel, M. "Reproductive Biology of a Wood-Boring Isopod, Sphaeroma terebrans, with Extended Parental Care." Marine Biology 135 (1999): 321–333.

Zimmer, M., S. Geisler, S. Walter, and H. Brendelberger. "Fluorescence in Asellus aquaticus (Isopoda: Asellota): A First Approach." Evolutionary Ecology Research 4 (2002): 181–187.

British Myriapod and Isopod Group. E-mail: steve.gregory@northmoortrust.co.uk Web site:

Inland Water Crustacean Specialist Group. Denton Belk, 840 E. Mulberry Ave., San Antonio, TX 78212-3194 USA. Phone: (210) 732-8809. Fax: (210) 732-3943. Web site:

International Isopod Research Group. Web site:

"World List of Marine, Freshwater and Terrestrial Isopod Crustaceans." Department of Systematic Biology, Invertebrate Zoology, Smithsonian National Museum of Natural History. [1 Aug. 2003].

[Article by: Leslie Ann Mertz, PhD]

An order of malacostracan crustaceans characterized by a cephalon, or head, bearing one pair of maxillipeds in addition to the antennae, mandibles, and maxillae. A carapace is lacking. The peraeon, or thorax, consists of seven distinct somites each bearing a pair of peraeopods, the legs. The pleon, or abdomen, has six somites. The first five pairs of pleonal appendages are foliaceous. The last pair is modified into hardened appendages called uropods and the last somite is fused with the telson into a pleotelson.

The most familiar isopods are the terrestrial sow bugs or pill bugs. Many of the animals roll up into a compact ball when disturbed. Land isopods are usually found in moist environments, under decaying leaves and wood, and under rocks. The destructive marine wood-boring isopod Limnoria, the gribble, causes extensive damage to wharf piling in the United States, and one species is reported to attack treated timbers.

About 3000 species of isopods are known today, but it may be estimated that only one-half of the existing species have been described. See also Crustacea.

The noun has one meaning:

Meaning #1: woodlice
  Synonym: order Isopoda

Isopoda Fossil range: 300–0 Ma PreЄ Є O S D C P T J K Pg N
Eurydice pulchra (Cirolanidae), an intertidal European isopod
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea Class: Malacostraca Subclass: Eumalacostraca Superorder: Peracarida Order: Isopoda Latreille, 1817
Suborders
Anthuridea Asellota Calabozoida Epicaridea Flabellifera Microcerberidea Oniscidea Phreatoicidea Valvifera

Isopods are an order of peracarid crustaceans, including familiar animals such as woodlice and pill bugs. The name Isopoda derives from the Greek iso meaning "same" and pod meaning "foot".^[1] The fossil record of isopods dates back to the Carboniferous period (in the US Pennsylvanian epoch), at least 300 million years ago.^[2][3]

Contents 1 Description 2 Ecology 3 Diversity and classification 4 Development 5 References 6 See also 7 External links

Description

The woodlouse Oniscus asellus from the side

Isopods are relatively small crustaceans with seven pairs of legs of similar size and form, ranging in size from 300 micrometres (0.012 in) to nearly 50 centimetres (20 in) in the case of Bathynomus giganteus.^[1] They are typically flattened dorso-ventrally, although many species deviate from this plan, particularly those from the deep sea or from ground water.^[1] Isopods lack an obvious carapace, which is reduced to a "cephalic shield" covering only the head.^[4] Gas exchange is carried out by specialised gill-like pleopods towards the rear of the animal's body. In terrestrial isopods, these are often adapted into structures which resemble lungs, and these "lungs" are readily visible on the underside of a woodlouse.^[1] Eyes, when present, are always sessile, never on stalks.^[4] They share with the Tanaidacea the fusion of the last abdominal body segment with the telson, forming a "pleotelson",^[4] and the first body segment of the thorax is fused to the head. The pereiopods are uniramous, but the pleopods are biramous.^[4]

Ecology

Anilocra (Cymothoidae) parasitising Spicara maena, Italy

Around 4,500 species of isopods are found in marine environments, mostly on the sea floor.^[2] Some 500 species are found in fresh water; the remaining 5,000 species are the woodlice in the suborder Oniscidea, which are thus by far the most successful group of terrestrial crustaceans.^[2] In the deep sea, members of the suborder Asellota predominate, to the near exclusion of all other isopods, having undergone a large adaptive radiation in that environment.^[2]

A number of isopod groups have evolved a parasitic lifestyle. The suborder Epicaridea is exclusively parasitic, while the polyphyletic suborder Flabellifera is partly parasitic. Cymothoa exigua, for example, is a parasite of the spotted rose snapper fish Lutjanus guttatus in the Gulf of California; it eats the tongue of the fish, and takes its place, in the only known instance of a parasite functionally replacing a host structure.^[5]

Diversity and classification

Isopods belong to the larger group Peracarida, which are united by the presence of a special brood pouch for brooding eggs. There are around 11,437 described species of isopod^[1] classified into ten suborders.^[6]

Development

Isopod larvae hatch as mancae, which resemble adults except for the lack of the last pair of pereiopods (thoracic legs). The lack of a swimming phase in the life cycle is a limiting factor in isopod dispersal, and may be responsible for the high levels of endemism in the order ^[2]. As adults, isopods differ from other crustaceans in that they replace their exoskeleton (in the process called ecdysis) in two phases; this is known as "biphasic moulting".^[1]

References

Wikimedia Commons has media related to: Isopoda

Wikispecies has information related to: Isopoda

1. ^ ^{a b c d e f} M. Schotte, C. B. Boyko, N. L. Bruce, J. Markham, G. C. B. Poore, S. Taiti & G. D. F. Wilson. "World List of Marine, Freshwater and Terrestrial Isopod Crustaceans". World Register of Marine Species. http://www.marinespecies.org/isopoda/index.php. Retrieved 2009-02-19. 
2. ^ ^{a b c d e} Richard Brusca (1997-08-06). "Isopoda". Tree of Life Web Project. http://tolweb.org/Isopoda/6320/1997.08.06. 
3. ^ Frederick R. Schram (1970). "Isopod from the Pennsylvanian of Illinois". Science 169 (3948): 854-855. doi:10.1126/science.169.3948.854. 
4. ^ ^{a b c d} S. J. Keable, G. C. B. Poore & G. D. F. Wilson (2002-10-02). "Australian Isopoda: Families". Australian Museum. http://www.crustacea.net/crustace/isopoda/index.htm. 
5. ^ R. C. Brusca & M. R. Gilligan (1983). "Tongue replacement in a marine fish (Lutjanus guttatus) by a parasitic isopod (Crustacea: Isopoda)". Copeia 3: 813–816. doi:10.2307/1444352. 
6. ^ Joel W. Martin and George E. Davis (2001). An Updated Classification of the Recent Crustacea. Natural History Museum of Los Angeles County. 

See also

• Cymothoa exigua (Tongue eating louse)
• Paracerceis sculpta

External links

• "Rare tongue-eating parasite found" BBC website 9 September 2009. Retrieved 14 September 2009. (image Retrieved 15 September 2009)

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)