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

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

woodlice A woodlouse (plural woodlice) is an isopod crustacean from the polyphyleticThe current consensus is that Oniscidea is actually triphyletic suborder Oniscidea within the order Isopoda. They get their name from often being found in old wood. ...

in the

British Isles The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isles, ...

. It is acknowledged to be the most abundant terrestrial

isopod Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, an ...

Britain Britain most often refers to: * The United Kingdom, a sovereign state in Europe comprising the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands * Great Britain, the largest island in the United King ...

. It is found commonly across

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

north of the

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...

, and has been introduced to

Madeira ) , anthem = ( en, "Anthem of the Autonomous Region of Madeira") , song_type = Regional anthem , image_map=EU-Portugal_with_Madeira_circled.svg , map_alt=Location of Madeira , map_caption=Location of Madeira , subdivision_type=Sovereign st ...

, the

Azores ) , motto =( en, "Rather die free than subjected in peace") , anthem= ( en, "Anthem of the Azores") , image_map=Locator_map_of_Azores_in_EU.svg , map_alt=Location of the Azores within the European Union , map_caption=Location of the Azores wi ...

and

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

Description

''T. pusillus'' may be distinguished from other British woodlice chiefly by its small size, which reaches no more than . Its body is elongate and quite rounded in

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

, and typically purplish. It may be separated from related species in North America by its eyes of three

ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...

each, rather than the single ocellus in the eyes of its relative '' Hyloniscus riparius''.

Reproduction

There are two distinct reproductive strategies within the species ''Trichoniscus pusillus''. Many populations are, like most

metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

ns, bisexual and

reproduce sexually Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

; in other cases, females reproduce

parthenogenetically Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and development ...

, creating clones of themselves. The sexually reproducing form is

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

while the parthenogenetic form is

triploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei ( eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contain ...

; 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 Cline may refer to: Science * Cline (biology), a measurable gradient in a single trait in a species across its geographical range * Cline (hydrology), a fluid layer with a property that varies * Cline (mathematics) or generalised circle, a circl ...

) and in increasingly open habitats, with no males observed in most of

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...

and

Scandinavia Scandinavia; Sámi languages: /. ( ) is a subregion#Europe, subregion in Northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. In English usage, ''Scandinavia'' most commonly refers to Denmark, ...

, but more than 15% males in the Iberian and Apennine Peninsulas. The

reproductive season Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the optimization of survival of young due to factors such as ambient temperature, food and water availability, and cha ...

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

gravid In biology and human medicine, gravidity and parity are the number of times a woman is or has been pregnant (gravidity) and carried the pregnancies to a viable gestational age (parity). These terms are usually coupled, sometimes with additional te ...

for 4–5 weeks before releasing 4–18

manca The manca (plural: ''mancae'') is the post- larval juvenile in some crustaceans. The manca stage is the defining characteristic of a clade called Mancoida which comprises all the member of the Peracarida except the Amphipoda. Mancae closely resem ...

e from the brood pouch.

Ecology

Like other woodlice, ''T. pusillus'' eats decaying plant matter of various kinds, although only

alder Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few sp ...

litter is capable of sustaining a stable reproducing population. Predators of ''T. pusillus'' include the common shrew ''

Sorex araneus The common shrew (''Sorex araneus''), also known as the Eurasian shrew, 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 ve ...

'',

lycosid Wolf spiders are members of the family Lycosidae (). They are robust and agile hunters with excellent eyesight. They live mostly in solitude, hunt alone, and do not spin webs. Some are opportunistic hunters, pouncing upon prey as they find it or ...

and dysderid spiders,

centipede Centipedes (from New Latin , "hundred", and Latin , " foot") are predatory arthropods belonging to the class Chilopoda (Ancient Greek , ''kheilos'', lip, and New Latin suffix , "foot", describing the forcipules) of the subphylum Myriapoda, an ...

s such as ''

Lithobius variegatus ''Lithobius variegatus'' is a species of centipede found in Europe, sometimes called the common banded centipede or banded centipede. ''Lithobius variegatus'' can be distinguished from its close relative ''Lithobius forficatus'' by the presence ...

'' and perhaps

carabid Ground beetles are a large, cosmopolitan family of beetles, the Carabidae, with more than 40,000 species worldwide, around 2,000 of which are found in North America and 2,700 in Europe. As of 2015, it is one of the 10 most species-rich animal fam ...

beetle Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...

s. ''Trichoniscus pusillus'' is susceptible to infection by isopod iridescence virus, or

Iridovirus ''Iridovirus'' is a genus of viruses in the family ''Iridoviridae''. Insects serve as natural hosts. Currently, only two species are placed in this genus. ''Invertebrate iridescent virus 6'' (IIV-6) was recognised as the type species until such ...

(

Iridoviridae ''Iridoviridae'' is a family of viruses with double-stranded DNA genomes. Amphibians, fish, and invertebrates such as arthropods serve as natural hosts. There are currently 22 species in this family, divided among two subfamilies and seven gener ...

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

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...

a bluish

iridescence Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...

Classification

Several former

subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed. Not all species ...

of ''T. pusillus'' are now treated as separate species. These include '' Trichoniscus alticola'', '' Trichoniscus baschierii'', '' Trichoniscus provisorius'', '' Trichoniscus noriucs'' and '' Trichoniscus pygmaeus''.

References

External links