Amphipoda

Amphipods are distinguished from other peracaridan crustaceans by the combination of: laterally compressed body (vs. dorsoventrally flattened in Isopoda), absence of carapace, sessile eyes without stalks, presence of gnathopods, and multiple pairs of uropods. They differ from true shrimp (Decapoda) in having seven pairs of thoracic legs rather than ten, and in the structure of the uropods and telson. Hyperiidean amphipods are planktonic and often associated with gelatinous zooplankton, while gammaridean amphipods are primarily benthic. Some species require microscopic examination of appendage morphology for definitive identification.

Bodies are laterally compressed and divided into 13 segments grouped into head, thorax, and abdomen. The head is fused to the thorax and bears two pairs of antennae, one pair of sessile compound eyes, and concealed mouthparts. The thorax carries eight pairs of uniramous appendages: the first pair modified as gnathopods (unique grasping feeding legs), the next four directed forward, and the last three directed backward. Gills are located on the thoracic coxae. The abdomen comprises two sections: the pleosome with three segments bearing swimming legs, and the urosome with three pairs of uropods plus a telson—amphipods are the only malacostracans with more than one pair of uropods. Most species are small, typically under 10 mm, though deep-sea species such as Alicella gigantea reach 34 cm. Some lineages show aberrant morphology: Caprellidae (skeleton shrimp) are elongate, while Cyamidae (whale lice) are dorsoventrally flattened with large claws.

Marine environments from intertidal zones to hadal depths exceeding 10,000 m, including hydrothermal vents and cold seeps. Freshwater habitats include lakes, rivers, and groundwater systems; notable radiations occur in ancient lakes such as Baikal and Titicaca. Terrestrial representatives (family Talitridae) inhabit moist coastal and inland environments including leaf litter, decaying wood, and supralittoral wrack. Approximately 750 species are troglobitic, restricted to cave systems.

Cosmopolitan across all major aquatic and moist terrestrial ecosystems. Marine diversity centers include the Southern Ocean, deep sea, and temperate coastal waters. Freshwater diversity is concentrated in ancient lakes and groundwater systems. Terrestrial talitrids have distributions reflecting Gondwanan origins with recent colonization of Europe and North America.

Most species are detritivores or scavengers, consuming dead plant and animal material. Some are grazers of algae, omnivores, or predators of small invertebrates. Specialist feeders include: whale lice (Cyamidae) parasitic on baleen whales; hyperiideans associated with gelatinous zooplankton; and driftwood talitrids that feed on decaying wood processed by symbiotic microflora. Some species exhibit compensatory feeding, consuming higher quantities of lower-quality food when mobility is limited.

• Poralia rufescens - commensalismJuvenile Pseudocallisoma coecum and Lanceola clausi clausi associated with oral arms of this deep-sea jellyfish in the Japan Trench
• Baleen whales - parasitismCyamidae (whale lice) are obligate ectoparasites, dorsoventrally flattened with large claws for attachment; the only parasitic crustaceans that cannot swim at any life stage
• Salps, medusae, siphonophores, colonial radiolarians, ctenophores - symbiosisMany Hyperiidea are symbionts of gelatinous animals during part of their life cycle

Development is direct with no larval stages; eggs hatch into miniature juveniles. Females possess a marsupium (brood pouch) for egg incubation. Males engage in amplexus, grasping females with gnathopods and carrying them ventrally for periods of two to fifteen days until the female molts and eggs are ready for fertilization. Sexual maturity is typically reached after six molts. Some species have been observed consuming their own exuviae after molting. In some lineages, sexual dimorphism occurs with males typically larger than females (reversed in Crangonyx).

Amplexus is a distinctive precopulatory behavior where males grasp and carry females using modified gnathopods. Swimming occurs via pleopod beating with characteristic lateral tilting described as "scudding." Some species exhibit compensatory feeding, remaining sedentary on host substrates and consuming abundant low-quality food rather than actively seeking higher-quality resources. Cannibalism and intraguild predation occur in some species, with adults reducing cannibalism toward juveniles when kin recognition is likely. Activity patterns are influenced by moisture and temperature; sandhoppers show increased activity in cooler, wetter conditions.

Amphipods function as mesograzers in benthic ecosystems, controlling algal growth and contributing to nutrient cycling. They are integral components of aquatic food webs, serving as prey for fish and invertebrates while consuming detritus and small organisms. In kelp forest ecosystems, they suppress brown algal dominance when predator pressure is low. They are widely used as bioindicators of ecosystem health due to their sensitivity to environmental change and pollution. Some species facilitate decomposition of marine driftwood and wrack, accelerating nutrient return to coastal ecosystems.

Occasional nuisance incidents have been reported: in 2017, lysianassid amphipods caused severe bleeding wounds on a swimmer in Melbourne, though bites are not venomous and cause no lasting damage. Some freshwater species are used as bioindicators for water quality assessment. Talitrid sandhoppers are familiar components of beach ecosystems. The order has gained cultural recognition through the hypothesis that Phronima inspired the design of the Alien Queen in the 1986 film "Aliens."

• IsopodaBoth are peracaridan crustaceans lacking carapaces, but isopods are dorsoventrally flattened with only one pair of uropods and lack gnathopods
• Decapoda (true shrimp)Shrimp possess a carapace covering the thorax, ten thoracic legs, and a tail fan formed by uropods and telson; amphipods lack carapace, have seven thoracic legs, and multiple uropod pairs not forming a fan
• CopepodaCopepods are typically smaller, have reduced body segmentation, a single median eye in many species, and lack the distinctive gnathopods and multiple uropods of amphipods

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Amphipoda | Beetles In The Bush
• amphipods | Blog
• amphipods | Beetles In The Bush
• Tiny travellers of the animal world: Hitchhikers on marine driftwood | Blog
• The invasion route of a small crustacean | Blog
• Taxonomist Day at Pensoft: Three species in the WoRMS’ Top 10 Marine Species of 2019 described in our journals | Blog
• Crustacea: Amphipoda
• Local distributions of sandhoppers and landhoppers (Crustacea: Amphipoda: Talitridae) in the coastal zone of western Tasmania
• Figure 5: Distribution of upper and lower vertical limits in species of Amphipoda.
• Pseudocallisoma coecum (Holmes, 1908) (Amphipoda, Scopelocheiridae) and Lanceola clausi clausi Bovallius, 1885 (Amphipoda, Lanceolidae) associated with Poralia rufescens Vanhöffen, 1902 (Scyphozoa, Ulmaridae)
• A new species of Thrombasia J.L. Barnard, 1966 (Crustacea, Amphipoda, Tryphosidae) from the Clarion-Clipperton Zone, Central Pacific Ocean.