Anomura

Anomurans are distinguished from true crabs (Brachyura) by the presence of only eight visible walking legs—the fifth pair of pereiopods is reduced and hidden within the gill chamber for gill cleaning. True crabs typically display all ten legs externally. Hermit crabs possess asymmetrical, soft abdomens adapted to occupy empty gastropod shells; king crabs, porcelain crabs, and hairy stone crabs exhibit carcinisation with broad, flattened carapaces and tails folded beneath. Mole crabs (Hippidae) have specialized bodies adapted for burrowing in sandy beaches. Squat lobsters (Galatheidae) have compressed bodies and long antennae.

Predominantly marine, occupying diverse benthic environments from intertidal zones to deep-sea habitats exceeding 1000 m depth. Specific habitats vary by family: hermit crabs occur in rocky shores, coral reefs, and soft sediments; king crabs inhabit cold, deep waters; porcelain crabs live under rocks and in crevices; squat lobsters associate with crinoids and other benthic structures; mole crabs burrow in sandy beaches; blanket-hermit crabs occur at 30–1125 m depth in the Indo-West Pacific.

Global marine distribution from polar to tropical waters. Documented from the Ryukyu Islands, Japan; Bermuda; Indo-West Pacific; and worldwide oceans. The oldest known fossil, Platykotta akaina, dates to the Late Triassic (Norian–Rhaetian) of the United Arab Emirates.

• gastropod shells - habitathermit crabs occupy empty shells for protection
• sea anemones - mutualismblanket-hermit crabs (Paguropsis, Paguropsina) use specialized chelipeds to stretch anemone tissue over their bodies for protection; anemone species identity currently unknown
• comatulid crinoids - commensalismsquat lobsters of genus Galathea associated with crinoids in Ryukyu Islands, Japan

Blanket-hermit crabs exhibit specialized behavior of pulling sea anemone tissue back and forth to cover their soft bodies and heads when threatened, analogous to hiding under a blanket. Mole crabs burrow in sandy beach sediments. Squat lobsters display cryptic behavior associated with crinoid hosts.

Occupies diverse ecological roles including scavengers, detritivores, and members of benthic communities. Some species serve as ecosystem engineers through shell use and burrowing activities. Associations with cnidarians and echinoderms contribute to community structure in marine benthic habitats.

King crabs (Lithodidae) support significant commercial fisheries. Hermit crabs are common in aquarium trade. Some species serve as indicators of marine ecosystem health. The HMS Challenger Expedition (1874) collected specimens that led to discovery of blanket-hermit crabs, highlighting historical scientific importance.

• Brachyura (true crabs)True crabs display all ten pereiopods externally and lack the concealed fifth pair characteristic of Anomura; together these groups form the clade Meiura
• Macrura (lobsters, crayfish)Macrura possess long, well-developed abdomens with uropods and telson forming a tail fan, lacking the reduced or asymmetrical abdomen of anomurans

• carcinisation
• convergent evolution
• decapod
• marine
• benthic
• symbiosis
• shell use
• deep sea
• commercial fishery

• BugGuide
• Wikipedia
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Five new blanket-hermit crab species described 130 years later from the Pacific | Blog
• Biology of Anomura II
• Decapod Crustacea of Bermuda. Their distribution, variations, and habits. I, Brachyura and Anomura
• Squat lobsters of the genus Galathea (Decapoda: Anomura: Galatheidae) associated with comatulid crinoids from the Ryukyu Islands, Japan
• Ecological Interactions on Sandy Beach Ecosystems: A Global Synthesis of Mole Crabs and New Insights into Emerita brasiliensis and Emerita rathbunae (Crustacea, Decapoda, Anomura, Hippidae).