Isopoda

(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

List of Species

Common pill woodlouse

Armadillidium vulgare

family

Armadillidiidae

taxonomy

Armadillidium vulgare Latreille, 1804, cosmopolitan.

other common names

French: Cloporte vulgaire; German: Kugelassel.

physical characteristics

The common pill woodlouse usually is dark gray and often has distinct rows of spots. The color sometimes varies to brown or red. The antennae but not the legs are visible from above. The oval body is approximately twice as long as it is wide and can reach a length of 0.7 in (18 mm).

distribution

Originally the Mediterranean periphery; now nearly ubiquitous in temperate climates.

habitat

The common pill woodlouse, once having the genus name Armadillo, is found in wide-ranging habitats, including forests, grasslands, and even sand dunes. This woodlouse also is common in cultivated areas and in greenhouses.

behavior

The common pill woodlouse can conglobate (roll into a ball). The woodlice move about in the open but prefer areas of high humidity and thus frequently are found under leaf litter or logs. Under normal conditions, the common pill woodlouse moves slowly through its habitat and becomes more active in dry air when it actively seeks a more humid area. Studies of daily, foraging movements reveal that individuals travel an average of 43 ft (13 m) each day in the summer but only 22 ft (6.6 m) a day in the winter. In cold months, individuals have been found as deep as 10 in (25 cm) beneath the soil surface.

feeding ecology and diet

These herbivorous and detrivorous creatures feed on tender plant shoots as well as dead and decaying plant matter. Their dietary habits shift during periods of drought, when these isopods switch from being mainly vegetarians to being scavengers.

reproductive biology

Reproduction is cyclical and triggered by environmental factors, such as rising temperatures and longer days. In addition, females experience an acceleration of ovarian maturation in the presence of males, although this effect is lessened in northern climates. Mating occurs immediately before the parturitional molt, usually from late spring to early summer, but sometimes the period extends from late winter to early fall. Females in northern populations have one brood per year, and those in

southern populations typically have two, sometimes three. Broods can include more than 100 eggs, but typically approximately one half of eggs develop into mancas. Eggs move into a brooding pouch, or marsupium. These woodlice can store sperm, which remains viable for approximately one year.

conservation status

Not listed by the IUCN.

significance to humans

Large numbers in a garden or greenhouse can take a toll on tender, young, plant shoots but normally do not pose a considerable threat.

Water louse

Asellus aquaticus

family

Asellidae

taxonomy

Asellus aquaticus Linnaeus, 1758, type locality not specified.

other common names

English: Hog slater, hoglouse; French: Aselle, cloporte d'eau; German: Wasserassel.

physical characteristics

Individuals are brownish and have long, oval, dorsoventrally flattened bodies. Some have two yellow, fluorescent stripes down their backs. Males are typically larger than females, reaching approximately 0.8 in (20 mm). Individuals that live in caves and other dark environments are unpigmented and have no visible eyes.

distribution

Europe.

habitat

A freshwater isopod, the water louse lives primarily in surface-water areas, including ponds and slow-moving creeks. Researchers have found well- and cave-dwelling populations of this species.

behavior

The male guards its intended mate for several days before fertilization by hoisting up the female from the substrate and carrying her underneath him. Unpaired males sometimes are successful in attempts to take the place of mating males by struggling with and separating united pairs.

feeding ecology and diet

The diet includes coarse, particulate, vegetative matter taken from the sediment.

reproductive biology

Water lice reproduce from early spring to mid autumn. Observations differ on whether males prefer larger females. Heftier females produce more and faster-hatching eggs but are a heavier load for males to carry during the precopulatory, mate-guarding period. Males that choose larger females may cut back on precopulatory mate guarding and carry the female nearer to her fertile period, which occurs during her final molting.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Lirceus fontinalis

family

Asellidae

taxonomy

Lirceus fontinalis Rafinesque-Schmaltz, 1820, Kentucky, United States.

other common names

None known.

physical characteristics

Lirceus fontinalis is somewhat teardrop-shaped and has a rounded head and a body that tapers posteriorly. It has two long and apparent antennae and long and noticeable legs. Males are slightly larger than females, length ranging from 0.4 to 0.6 in (10–15 mm). The length of females averages approximately 0.2–0.4 in (6–10 mm).

distribution

Eastern United States.

habitat

Streams, particularly among mats of filamentous algae, but also along bare silt, sand, and gravel substrates and in submerged leaf litter.

behavior

During courtship, males of L. fontinalis first struggle with, then dominate, and finally defend their intended partners in precopulatory mate guarding. Defense is necessary to prevent a single male from dislodging the paired male and taking his place. During precopulatory mate guarding, the male carries the female beneath him for one to three days. Females are receptive for only a short time, so the behavior appears to assure that the male is in the correct location when the female begins her maturation molt and is ready to be fertilized. Males seem to prefer females who are close to parturitional molt.

feeding ecology and diet

Lirceus fontinalis is a detritus feeder, sustaining itself on organic particulates in the water and algal mats. Predators include various species of stream-dwelling fishes, such as green sunfish (Lepomis cyanellus) and sculpins (Cottus species). Results of laboratory experiments have suggested that these isopods detect sunfish chemicals and use these cues to avoid the fish by hiding in vegetation, remaining motionless, or burying themselves in the substrate. Lirceus fontinalis also uses avoidance tactics when it detects changes in current caused by approaching swimming fishes.

reproductive biology

Males inseminate females during the maturation molt. Females do not store sperm. Females may produce 20–120 eggs, which are deposited into a ventral marsupium, or brood pouch. Larger females produce more eggs. Every 0.0007 oz (2 mg) in body weight correlates to approximately 10 eggs. After hatching, the juveniles spend approximately three weeks in the marsupium, where they proceed through several molts. Upon leaving the marsupium, the juveniles live independently.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Sand isopod

Chiridotea caeca

family

Idoteidae

taxonomy

Chiridotea caeca Say, 1818, northwest Atlantic Ocean.

other common names

None known.

physical characteristics

Adults are dorsoventrally flattened with a thorax that is almost round when viewed from above and have long, robust legs equipped with conspicuous, plumose setae. A long, pointed abdomen follows. Individuals reach approximately 0.6 in (15 mm) in length and 0.3 in (7 mm) in width.

distribution

Western Atlantic Ocean, from Nova Scotia to Florida.

habitat

This benthic, marine species populates the coarse, sandy bottom of the intertidal, sometimes subtidal, zone along the ocean shoreline.

behavior

A burrowing species of the suborder Valvifera, the sand isopod uses its hindmost pereopods to dig tunnels in the sand. If they are removed from their tunnels, perhaps by wave action, sand isopods swim—often upside down—to the substrate, where they again seek underground protection. Mating occurs while the pair is buried in the substrate.

feeding ecology and diet

Apparently carnivorous. Little is known about the diet and feeding ecology of this burrowing species. Predators include a variety of fishes, including flounders and puffers, and shore birds.

reproductive biology

Eggs, which are oval shaped, number two to three dozen, occasionally reaching six dozen. Juvenile instars approximately 0.1 in (2.5 mm) long and 0.05 in (1.25 mm) wide appear in the spring. Juveniles go through approximately six instars and mature at a length of approximately 0.4 in (10 mm). Females become ovigerous in December. Mating can take several days. Females undergo the last maturation molt while in amplexus with males. Sand isopods produce one brood per year.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Gribble

Limnoria quadripunctata

family

Limnoriidae

taxonomy

Limnoria quadripunctata Holthuis, 1949, Netherlands.

other common names

German: Bohrassel.

physical characteristics

The gribble, a 14-legged isopod, is more cigar shaped than oval and reaches a length of approximately 0.1–0.2 in (3–6 mm). Gribbles have two pairs of short antennae and are generally light grayish brown.

distribution

Northern Hemisphere.

habitat

The gribble, a coastal, marine organism, makes and inhabits tunnels and grooves that it makes at or just below the surface of water-exposed wood.

behavior

Gribbles appear able to orient themselves and swim toward wood. Experimental investigation indicates that these isopods are attracted to marine fungi and vegetation, as well as to chemicals from other members of the species on wood, and orient themselves to these markers with their antennae. One pair appears to detect chemicals and the other seemingly picks up odors. Gribbles are able to conglobate.

feeding ecology and diet

Gribbles consume wood, fungi on the wood, and waterborne detritus.

reproductive biology

Young gribbles develop in their mother's tunnel, many digging their own tunnels into the wood. Although males remain with females after mating, it is unclear whether males participate in parental care of offspring.

conservation status

Not listed by the IUCN.

significance to humans

The wood-boring activities of gribbles damage wood pilings, docks, and other wooden structures. In large infestations, wood depth can decrease approximately 0.8 in (2 cm) per year.

Common shiny woodlouse

Oniscus asellus

family

Oniscidae

taxonomy

Oniscus asellus Linnaeus, 1758, Europe.

other common names

German: Mauerassel.

physical characteristics

The common shiny woodlouse, which has a somewhat shiny dorsal surface, has the typical isopod appearance of an oval,

grayish brown body and a pair of long antennae. It reaches a length of approximately 0.6 in (16 mm).

distribution

Western and northern Europe originally; eastern Europe and North America currently.

habitat

This woodlouse lives in almost any damp, terrestrial area, preferring forests, and often living under rocks and deadfall.

behavior

The common shiny woodlouse and others of its family cannot roll up (conglobate). They shun the light and seek warm areas of high humidity. A characteristic behavior is the tendency to firmly latch onto rocks and other surfaces.

feeding ecology and diet

Eats vegetative matter, primarily lime, ash, and alder leaves. Research has indicated that growth and fecundity increase when food intake includes dicotyledonous rather than monocotyledonous leaves. Growth also improves with a copper-fortified diet.

reproductive biology

The common shiny woodlouse typically has two broods and produces 27–33 eggs. Juveniles are dull, dark gray and have a pale marking on the pleon.

conservation status

Not listed by the IUCN.

significance to humans

Although common in gardens and often in greenhouses, the common shiny woodlouse does little damage to vegetation.

Common rough woodlouse

Porcellio scaber

family

Porcellionidae

taxonomy

Porcellio scaber Latreille, 1804, western Europe.

other common names

German: Kellerassel.

physical characteristics

The common rough woodlouse has the typical oval shape of terrestrial isopods but has a rough rather than shiny dorsal surface and frequently is orange at the base of the antennae. The coloration is gray, brown, or orangish brown, often with gray blotches. This woodlouse reaches a length of approximately 0.7 in (17 mm).

distribution

Native to western Europe; north to Iceland, south to South Africa and South America.

habitat

The common rough woodlouse lives in damp, dark locations, often beneath logs or rocks, in forests or along waterways, sometimes in meadows, and often extends into cultivated areas, such as gardens and greenhouses. This woodlouse is abundant in splash zones along ocean shorelines.

behavior

Lengthening daylight is a trigger for reproduction. Mainly active at night, the common rough woodlouse cannot roll into a ball, as can many other terrestrial isopods. Surveys of this

species in Europe show that individuals often travel into trees in the summer and back to the soil in the fall. This behavior is common to many other terrestrial isopods.

feeding ecology and diet

The common rough woodlouse is herbivorous, detrivorous, and coprophagous. It seems to prefer poplar leaves and leaves with fungal growth, and it eats decaying pine needles. Results of experiments indicate that ingestion of feces is important in building up copper reserves and in invigorating gut bacteria that are important in digestion.

reproductive biology

Females have one to three broods per year. The eggs, numbering one to three dozen, are approximately 0.03 in (0.7 mm) in diameter, and the young develop in the female's marsupium. Studies of broods indicate than more than 80% have multiple paternity.

conservation status

Not listed by the IUCN.

significance to humans

The common rough woodlouse sometimes becomes a pest in gardens or greenhouses, where they feed on new plant growth.

No common name

Sphaeroma terebrans

family

Sphaeromatidae

taxonomy

Sphaeroma terebrans Bate, 1866, India.

other common names

None known.

physical characteristics

The rough-surfaced pleotelson of Sphaeroma terebrans is slightly pointed. Females are larger than males, averaging a length of 0.3–0.4 in (8–10 mm) compared with the male length of 0.26–0.33 in (6.5–8.5 mm).

distribution

Mangrove forests worldwide, including Florida and Kenya.

habitat

Sphaeroma terebrans, a wood-boring species, lives chiefly in the aerial roots of red mangroves (Rhizophora mangle), although it also inhabits fallen trees and the roots of other plants.

behavior

Juveniles live in burrows, either their own or often those of the parent. These "family burrows," which lie at the end of the mothers' burrows (fathers do not participate in parental care), also often house one to eight uninvited juveniles from the related species Sphaeroma quadridentatum. Sphaeroma quadridentatum does not burrow but seeks refuge in crevices or other small hideaways. Sphaeroma terebrans females either do not distinguish the S. quadridentatum juveniles from their own or simply tolerate them. A survey of the burrows of reproductive S. terebrans females placed the proportion of those harboring S. quadridentatum juveniles at 30% and showed that young S. terebrans receive a shorter period of parental care in the shared burrows. Parental care may include general housekeeping duties, such as removal of waste, and ventilation and excavation of burrows.

feeding ecology and diet

The diet of this apparent filter feeder likely includes water-borne planktonic algae. During high tides, water rushes over the mangrove roots and into the burrows, delivering algae to the isopods.

reproductive biology

Reproductive activity is cyclical, peaking in the fall and in the late spring to early summer. A female has one or two broods per year, which develop from eggs in the marsupium. The young emerge at the manca stage approximately 0.1 in (2.5 mm) long. Larger females have the largest broods, and typically host 5–20 juveniles in their burrows, although some females host as many as five dozen juveniles. Some juveniles do not use family burrows, instead finding and living in their own tunnels. Juveniles do not grow much while in the family burrow, remaining approximately 0.1 in (2.5 mm) long. The lifespan is approximately 10 months.

conservation status

Not listed by the IUCN.

significance to humans

S. terebrans causes considerable damage to red mangroves.

Common pygmy woodlouse

Trichoniscus pusillus

family

Trichoniscidae

taxonomy

Trichoniscus pusillus Brandt, 1833, Europe.

other common names

None known.

physical characteristics

A small, reddish brown (rarely violet) woodlouse with a shiny dorsal surface that is slightly mottled with white. The legs noticeably extend beyond the body. The common pygmy wood-louse reaches a length of only 0.2 in (5 mm). Males are slightly larger than females and have additional white markings where the genital apparatus attaches to the pleon segments.

distribution

Europe and North America.

habitat

The common pygmy woodlouse typically inhabits damp soil and leaf litter in forested areas but also is found in nearly every other temperate habitat, including grasslands and sparsely vegetated fields.

behavior

The common pygmy woodlouse does not seen to mind being close to conspecifics. Reports of several thousand per 11 ft² (1 m²) are not uncommon.

feeding ecology and diet

Diet is mainly vegetative detritus.

reproductive biology

Bisexual, parthenogenetic, and mixed populations of common pygmy woodlice exist. Parthenogenetic populations in Britain are composed almost entirely of females. Two to three broods are typical, breeding occurring between May and September. Eggs number between 4 and 17 and are approximately 0.01 in (0.3 mm) in diameter. An average female produces five to seven juveniles per brood.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

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.

Periodicals

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.

Organizations

British Myriapod and Isopod Group. E-mail: [email protected] Web site: <http://www.salticus.demon.co.uk>

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: <http://www.iucn.org/themes/ssc/pubs/sgnewsl.htm>

International Isopod Research Group. Web site: <http://www.uni-kiel.de/zoologie/institut/limnologie/IIRG.htm>

Other

"World List of Marine, Freshwater and Terrestrial Isopod Crustaceans." Department of Systematic Biology, Invertebrate Zoology, Smithsonian National Museum of Natural History. [1 Aug. 2003]. <http://www.nmnh.si.edu/iz/isopod/>

Leslie Ann Mertz, PhD