Trichoniscus pusillus, sometimes called the common pygmy woodlouse, is one of the five most common species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of woodlice

Woodlouse

A woodlouse is a crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs...

 in the British Isles

British Isles

The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...

. It is acknowledged to be the most abundant terrestrial isopod in Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

. It is found commonly across Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 north of the Alps

Alps

The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....

, and has been introduced

Introduced species

An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...

 to Madeira

Madeira

Madeira is a Portuguese archipelago that lies between and , just under 400 km north of Tenerife, Canary Islands, in the north Atlantic Ocean and an outermost region of the European Union...

, the Azores

Azores

The Archipelago of the Azores is composed of nine volcanic islands situated in the middle of the North Atlantic Ocean, and is located about west from Lisbon and about east from the east coast of North America. The islands, and their economic exclusion zone, form the Autonomous Region of the...

 and North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

.

Description

T. pusillus may be distinguished from other British woodlice chiefly by its small size, which reaches no more than 5 millimetre (0.196850393700787 in). Its body is elongate and quite rounded in cross section

Cross section (geometry)

In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...

, and typically purplish. It may be separated from related species in North America by its eyes of three ocelli each, rather than the single ocellus in the eyes of its relative Hyloniscus riparius

Hyloniscus riparius

Hyloniscus riparius is a species of woodlouse found in Central and Eastern Europe and subsequently introduced to North America. It is strongly associated with flood plains and can tolerate periods of up to eight weeks submerged under water. In North America, it was first found at St. John's,...

.

Reproduction

There are two distinct reproductive strategies within the species Trichoniscus pusillus. Many populations are, like most metazoans, bisexual and reproduce sexually

Sexual reproduction

Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

; in other cases, females reproduce parthenogenetically

Parthenogenesis

Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...

, creating clones of themselves. The sexually reproducing form is diploid while the parthenogenetic form is triploid; since parthenogenesis always produces females, males are always diploid and can only be produced by sexual reproduction.

The frequency of males in the population decreases from south to north (a latitudinal cline) and in increasingly open habitats, with no males observed in most of Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 and Scandinavia

Scandinavia

Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...

, but more than 15% males in the Iberian

Iberian Peninsula

The Iberian Peninsula , sometimes called Iberia, is located in the extreme southwest of Europe and includes the modern-day sovereign states of Spain, Portugal and Andorra, as well as the British Overseas Territory of Gibraltar...

 and Apennine Peninsulas.

The reproductive season

Breeding season

The breeding season is the most suitable season, usually with favourable conditions and abundant food and water, for breeding among some wild animals and birds . Species with a breeding season have naturally evolved to have sexual intercourse during a certain time of year in order to achieve the...

 lasts from March to September, and one to three breeding waves may be observed. Females are gravid

Gravidity

In medicine, gravidity refers to the number of times a woman has been pregnant, regardless of whether the pregnancies were interrupted or resulted in a live birth....

 for 4–5 weeks before releasing 4–18 mancae from the brood pouch

Brood pouch (Peracarida)

The marsupium or brood pouch, is a characteristic feature of Peracarida, including the orders Amphipoda, Isopoda and Cumacea. It is an egg chamber formed by oostegites, which are appendices which are attached to the coxae of the first pereiopods...

.

Ecology

Like other woodlice, T. pusillus eats decaying plant matter of various kinds, although only alder

Alder

Alder is the common name of a genus of flowering plants belonging to the birch family . The genus comprises about 30 species of monoecious trees and shrubs, few reaching large size, distributed throughout the North Temperate Zone and in the Americas along the Andes southwards to...

 litter is capable of sustaining a stable reproducing population.

Predators of T. pusillus include the common shrew Sorex araneus

Common Shrew

The Common Shrew or Eurasian Shrew, Sorex araneus, is the most common shrew, and one of the most common mammals, throughout Northern Europe, including Great Britain, but excluding Ireland. It is long and weighs , and has velvety dark brown fur with a pale underside. Juvenile shrews have lighter...

, lycosid

Wolf spider

Wolf spiders are members of the family Lycosidae, from the Ancient Greek word "" meaning "wolf". They are robust and agile hunters with good eyesight. They live mostly solitary lives and hunt alone. Some are opportunistic hunters pouncing upon prey as they find it or even chasing it over short...

 and dysderid

Dysderidae

The family Dysderidae are araneomorph spiders found primarily in Eurasia, although extending into North Africa, with very few species occurring in South America, and one introduced into many regions of the world.Dysderids have six eyes, and are haplogyne, i.e...

 spider

Spider

Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s, centipede

Centipede

Centipedes are arthropods belonging to the class Chilopoda of the subphylum Myriapoda. They are elongated metameric animals with one pair of legs per body segment. Despite the name, centipedes can have a varying number of legs from under 20 to over 300. Centipedes have an odd number of pairs of...

s such as Lithobius variegatus

Lithobius variegatus

Lithobius variegatus is a species of centipede found in Europe, sometimes called the common banded centipede or banded centipede....

and perhaps carabid beetle

Beetle

Coleoptera is an order of insects commonly called beetles. The word "coleoptera" is from the Greek , koleos, "sheath"; and , pteron, "wing", thus "sheathed wing". Coleoptera contains more species than any other order, constituting almost 25% of all known life-forms...

s.

Trichoniscus pusillus is susceptible to infection by "Isopod Iridescence Virus", or Iridovirus (Iridoviridae

Iridoviridae

Iridoviridae are a family of viruses with double stranded DNA genomes. The name is derived from Iris the Greek goddess of the rainbow. This name was chosen because of the "rainbow like" iridescence observed in heavily infected insects and pelleted samples of invertebrate iridoviruses.-Taxonomy:The...

). This is first apparent as a blue sheen on the unpigmented underside of the animals, but soon spreads to give the entire exoskeleton

Exoskeleton

An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

 and bluish iridescence

Iridescence

Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...

.

Classification

Several former subspecies

Subspecies

Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...

 of T. pusillus are now treated as separate species. These include Trichoniscus alticola, Trichoniscus baschierii, Trichoniscus provisorius, Trichoniscus noriucs and Trichoniscus pygmaeus.