Decapoda

Decapods are distinguished from other malacostracan crustaceans by the presence of ten thoracic legs (five pairs). Within the order, major groups are separable by body plan: true crabs (Brachyura) have a reduced abdomen folded tightly beneath the cephalothorax; hermit crabs and allies (Anomura) typically use gastropod shells or have asymmetrical abdomens; shrimps (Caridea, Dendrobranchiata) have well-developed, often laterally compressed abdomens with tail fans; lobsters and crayfish (Astacidea) possess robust, cylindrical bodies with large, often unequal claws. Specialized forms include blanket-hermit crabs (Paguropsis, Paguropsina), which carry sea anemones on their abdomens instead of shells, and pea crabs (Pinnotheridae), tiny commensals living within mollusc shells.

Decapods occupy virtually every aquatic environment on Earth. Marine habitats include intertidal zones, coral reefs, seagrass beds, continental shelves, abyssal plains, and hadal trenches to depths exceeding 5,000 metres. Freshwater decapods inhabit rivers, streams, lakes, and subterranean waters. Terrestrial adaptations have evolved independently in multiple crab lineages, particularly in tropical regions. Some species are specialized symbionts: pea crabs live within bivalve molluscs, certain shrimps inhabit tunicates or sponges, and blanket-hermit crabs maintain obligate associations with sea anemones. Specific substrates include rocky shores, sandy beaches, muddy estuaries, hydrothermal vents, and sunken wood in the deep sea.

Cosmopolitan distribution spanning all continents and oceans. Highest diversity in tropical Indo-Pacific region. Major marine biodiversity centres include the Coral Triangle, Caribbean Sea, and western Indian Ocean. Freshwater decapods are most diverse in tropical South America, Africa, and Asia. Several groups show restricted distributions: the ghost shrimp and mud lobster infraorders Axiidea and Gebiidea are predominantly tropical and subtropical; the blanket-hermit crabs (Paguropsis, Paguropsina) are restricted to the Indo-West Pacific.

Most decapods are omnivorous scavengers, feeding on detritus, carrion, and small invertebrates. Some groups exhibit specialized feeding: filter-feeding occurs in certain deep-sea shrimps (Stylodactylidae); snail-sucking snakes of the family Dipsadidae are specialist molluscivores; pea crabs feed on food filtered by their host bivalves; cleaner shrimps remove parasites and fouling organisms from fish. The deep-sea shrimp Bathystylodactylus bathyalis has been observed forming a "filter basket" with its front legs to capture particles from water currents.

• tunicates - commensalOdontonia shrimps live inside tunicate cavities
• sea anemones - mutualisticblanket-hermit crabs (Paguropsis, Paguropsina) carry anemones on abdomen for protection
• bivalve molluscs - commensal/parasiticpea crabs (Pinnotheridae) inhabit mollusc mantles
• sponges - commensalsome shrimps and amphipods inhabit sponge cavities
• fish - mutualisticcleaner shrimps remove parasites from fish

Decapods typically exhibit complex life cycles with distinct larval stages. Most marine species release eggs that hatch into planktonic larvae (zoea), which undergo multiple moults before metamorphosing into post-larval forms (megalopa or glaucothoe) that settle to the benthos. Freshwater and terrestrial species often show abbreviated or direct development, with eggs brooded by the female until hatching as miniature adults. Some deep-sea and symbiotic species have reduced or modified larval phases.

Decapods display diverse behaviours including burrowing (ghost shrimps, mud lobsters), shell-carrying (hermit crabs), and complex social interactions. Male ghost shrimps and mud lobsters use hypertrophied chelipeds to defend galleries against intruders and compete for mates. Some species exhibit parental care, with females brooding eggs on pleopods. Cleaning behaviour, where shrimps remove parasites from fish, has evolved in multiple lineages. Symbiotic associations are common, with some species showing remarkable host specificity. The blanket-hermit crab Paguropsis typica uses specialized chelate fourth legs to pull anemone tissue over its body like a blanket.

Decapods function as key ecosystem engineers, predators, scavengers, and prey. Burrowing species (ghost shrimps, mud lobsters, crabs) bioturbate sediments, enhancing nutrient cycling and oxygen penetration. As abundant scavengers, they accelerate decomposition of organic matter. Many species serve as critical prey for fish, seabirds, and marine mammals. Cleaner shrimps contribute to fish health by removing ectoparasites. Symbiotic species may influence host population dynamics. In coral reef ecosystems, decapods contribute to nutrient recycling and serve as indicators of reef health.

Decapods are of enormous economic importance as food resources—shrimp, prawns, crabs, and lobsters support major global fisheries and aquaculture industries. Ornamental trade in colourful freshwater and marine decapods is substantial, with species like Cherax snowden collected extensively for aquarium markets. Some species are invasive, such as the amphipod Leucothoe eltoni introduced to Hawaiian waters. Decapods feature prominently in scientific research as model organisms for neurobiology, development, and ecology. Several species are threatened by overharvesting and habitat destruction, with conservation concerns particularly for narrowly endemic forms.

• Euphausiacea (krill)Similar shrimp-like appearance but possess biramous (branched) pleopods and lack the decapod characteristic of five uniramous pereiopods; typically planktonic rather than benthic
• Stomatopoda (mantis shrimps)Share general crustacean body plan but possess specialized raptorial second thoracic appendages for striking prey, not found in Decapoda; carapace covers only part of thorax

• crustacean
• marine
• freshwater
• terrestrial
• commercially important
• ecosystem engineer
• symbiotic
• deep-sea

• BugGuide
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Notice me! Neglected for over a century, Black sea spider crab re-described | Blog
• marine animals | Blog
• Five new blanket-hermit crab species described 130 years later from the Pacific | Blog
• Crustacea | Blog - Part 2
• taxonomy | Blog - Part 19
• Crustacea: Decapoda
• An Update on Reproduction in Ghost Shrimps (Decapoda: Axiidea) and Mud Lobsters (Decapoda: Gebiidea)
• Within-Habitat Spatial Patterns of Ocypode Quadrata (Fabricius) (Decapoda Brachyura)