(Pillbugs, slaters, and woodlice)
Phylum Arthropoda Subphylum Crustacea Class Malacostraca Order Isopoda
Number of families Approximately 120
Small, generally gray, usually flat, marine, freshwater, or terrestrial animals with numerous legs; some species are parasitic
Photo: Pill woodlouse (Armadillidium vulgare) rolled up in defensive mode. (Photo by Nigel Cattlin/ Holt Studios Int'l/Photo Researchers, Inc. Reproduced by permission.)
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.
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 Ba-thynomus 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 sow-bug, 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.
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.
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 Ligi-idae, 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 Epi-caridea, are parasitic, blood-sucking forms that live on or in various animals, including barnacles, crabs, and shrimps.
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 re-pugnatorial 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 conglo-bation, 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 puncta-tus, 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.
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
A coney (Cephalopholis fulva) with the parasitic isopod Anilocra lati-caudata attached to it. (Photo by Andrew Martinez/Photo Researchers, Inc. Reproduced by permission.)
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.
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 offic-inalis, which can live nine years.
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, Thermo-sphaeroma thermophilum, a member of the family Sphaero-matidae. 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.
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.
1. Common shiny woodlouse (Oniscus asellus); 2. Common rough woodlouse (Porcellio scaber); 3. Gribble (Limnoria quadripunctata); 4. Common pill woodlouse (Armadillidium vulgare); 5. Sphaeroma terebrans; 6. Lirceus fontinalis; 7. Common pygmy woodlouse (Trichoniscus pusillus); 8. Water louse (Asellus aquaticus); 9. Sand isopod (Chiridotea caeca). (Illustration by Bruce Worden)
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