Isopoda

Isopods are distinguished from other peracarid crustaceans by their dorsoventrally flattened bodies and seven pairs of walking legs of similar size and orientation (versus amphipods, which are laterally compressed with legs of differing sizes). The absence of a carapace separates them from decapods and other carapace-bearing crustaceans. Terrestrial isopods (Oniscidea) are the only fully land-adapted crustaceans with widespread distribution; they are often confused with millipedes or centipedes but possess only 14 legs (versus 30+ in myriapods) and have two pairs of antennae (visible as one prominent pair, with antennules reduced). Pillbugs (Armadillidiidae) can be distinguished from sowbugs by their ability to conglobate (roll into a complete ball).

Marine species predominantly benthic, occupying seabed habitats from intertidal zones to abyssal depths exceeding 8,000 meters; some taxa capable of limited swimming. Freshwater species inhabit lakes, rivers, and subterranean waters. Terrestrial species (woodlice) restricted to moist microhabitats: leaf litter, decaying wood, soil crevices, beneath stones and bark; some species adapted to arid environments through nocturnal behavior and burrowing. Specialized habitats include mangrove marshes, caves, and commensal/parasitic associations with other organisms.

Cosmopolitan distribution across all continents except Antarctica for terrestrial forms. Marine isopods occur in all oceans from poles to tropics. Approximately 4,500 marine species, 500 freshwater species, and 5,000 terrestrial species described. Highest diversity in temperate and tropical regions; deep-sea asellotans particularly diverse in Southern Hemisphere. Some groups show Gondwanan distribution patterns suggesting ancient vicariance.

Activity patterns vary by habitat. Marine and freshwater species active year-round. Terrestrial species in temperate zones active primarily during warmer months; some species remain active in near-freezing conditions. Desert species nocturnal, retreating to burrows during day. Intertidal species such as Ligia spp. exhibit circadian activity patterns, foraging at night and hiding in crevices during daylight.

Highly diverse feeding strategies across the order: detritivores and scavengers consuming dead plant and animal matter; grazers on algae and diatoms; filter feeders; predators; and internal or external parasites (primarily of fish). Terrestrial isopods are predominantly herbivorous-detritivorous, feeding on decaying plant material, moss, bark, algae, and fungi. Some marine wood-boring species (gribbles) consume timber and associated fungal mycelia. Parasitic forms feed on host blood, tissues, or replace host structures (e.g., Cymothoa exigua replacing fish tongues).

Development is direct (no larval stages). Eggs brooded in female marsupium until hatching as mancae—miniature adults lacking the final pair of pereopods. Mancae gradually acquire full adult characteristics through post-embryonic development. Ecdysis (molting) occurs in two stages (biphasic): posterior exoskeleton shed first, followed later by anterior exoskeleton (except Glyptonotus antarcticus, which molts in single process). Lifespan varies: some small species live months, larger species several years.

Locomotion primarily by crawling; some marine species bore into wood, sediment, or calcareous substrates. Limited swimming ability in some taxa with modified pleopods. Terrestrial species exhibit thigmotaxis (preference for contact with surfaces) and negative phototaxis. Conglobation (rolling into ball) occurs in multiple lineages as defense against predation and desiccation. Some intertidal species (Ligia spp.) can run rapidly on land. Parasitic species show host-seeking and clinging behaviors with specialized mouthparts and limbs for attachment.

Terrestrial isopods are significant decomposers, contributing to humification and nutrient cycling through mechanical breakdown and chemical processing of plant detritus, and by enhancing microbial activity. Marine detritivores and scavengers contribute to benthic nutrient cycling. Some species are ecosystem engineers through wood-boring activities. Parasitic isopods can regulate host populations and occasionally cause significant mortality in fish populations, affecting commercial fisheries.

Terrestrial isopods are common in gardens and compost systems, generally beneficial as decomposers but occasionally damaging to seedlings and tender plant parts in high densities. Some species serve as bioactive cleanup crew in reptile and amphibian vivaria. Giant isopods (Bathynomus spp.) are commercially fished as seafood in Mexico, Japan, and Vietnam. Wood-boring marine isopods (Limnoria spp.) cause significant economic damage to wooden maritime structures. Parasitic isopods can be pests in aquaculture and aquarium systems, causing fish mortality.

• AmphipodaLaterally compressed body (versus dorsoventrally flattened in isopods); legs of unequal size and function; typically more active swimmers.
• DecapodaPossess carapace covering cephalothorax; ten walking legs (five pairs); includes crabs, lobsters, shrimp—distinctly different body plan.
• Millipede (Diplopoda)Myriapod with elongated body bearing numerous leg pairs (two per segment, 30+ total); single pair of antennae; not crustaceans.
• Centipede (Chilopoda)Myriapod with elongated body, one leg pair per segment, venomous forcipules; predatory; single pair of antennae.

Isopods are often mistakenly called "insects" due to their small terrestrial forms, but they are crustaceans with 14 legs and two pairs of antennae. The name "pillbug" is sometimes applied to all terrestrial isopods, but true pillbugs (conglobating species, primarily Armadillidiidae) represent only a subset; many woodlice cannot roll into complete balls. The common name "potato bug" is ambiguous, applied to both isopods and unrelated insects (Jerusalem crickets).

• crustacean
• woodlouse
• detritivore
• decomposer
• terrestrial crustacean
• marine benthos
• parasite
• deep sea
• bioindicator
• vivarium

• BugGuide
• Wikipedia
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Bohart Museum Open House on 'Many Legged Wonders': How Many Legs Does an Isopod Have? | Bug Squad
• I see icy isopods: Pillbugs, terrestrial Isopoda — Bug of the Week
• Super Crop Challenge #8 | Beetles In The Bush
• Hades, god of the dead, and now a deep-sea isopod
• Creepy Crawly Crustaceans | Beetles In The Bush
• New giant sea bug species named after Darth Vader
• Isopoda
• Terrestrial isopods (Isopoda: Oniscidea) in the Upper Mežica valley Kopenski enakonožci (Isopoda: Oniscidea) zgornje Mežiške doline
• A Molecular Phylogeny of Cavernicolous Oniscidea (Isopoda) Reveals Multiple Origins of Troglodytic Behavior (Oniscidea, Isopoda)
• Distribution of Woodlice (Isopoda: Oniscoidea) in the Faroe Islands
• Terrestrial Isopods (Isopoda: Oniscidea) from the Slovenian Karst Kopenski raki enakonožci (Isopoda: Oniscidea) na območju Slovenskega Krasa
• Vertical distribution ofSphaeroma terebrans (isopoda) on submerged stationary structures Vertikale Verbreitung von Sphaeroma terebrans (Isopoda) an untergetauchten Hafenbefestigungsanlagen
• Figure 4: Distribution of upper and lower vertical limits in species of Isopoda.
• Faunistic survey of terrestrial isopods (Isopoda: Oniscidea) in the Boč Massif area Favnistični pregled mokric (Isopoda: Oniscidea) na območju Boškega masiva
• The complete mitogenome of Ligia barack (Crustacea, Isopoda, Ligiidae), a recently described intertidal isopod species from the remote island of Nihoa.