Eucarida

Distinguished from other Malacostraca by the combination of stalked eyes and a carapace fused to all thoracic segments (thoracostracan condition). Differs from Peracarida (amphipods, isopods, etc.) which lack a carapace or have it fused to fewer thoracic segments. Differs from Hoplocarida (mantis shrimp) which have raptorial second thoracopods and a carapace covering only the posterior thorax. Euphausiacea identified by exposed gills and biramous pleopods modified as swimmerets; Decapoda by five pairs of thoracopods (10 legs) and gills covered by carapace.

Body plan characterized by a carapace that is completely fused to all thoracic segments, forming a continuous thoracic shield. Compound eyes are borne on movable stalks (peduncles). Body divided into distinct cephalothorax and abdomen. Size ranges from small krill (1-6 cm) to large decapods (giant crabs exceeding 3 m leg span). Exoskeleton typically calcified, though degree varies among groups.

Exclusively marine; no freshwater or terrestrial representatives. Occupies all ocean zones from intertidal to abyssal depths. Krill (Euphausiacea) primarily pelagic, often forming dense swarms in surface waters and upwelling zones. Decapoda occupy diverse habitats including benthic (lobsters, crabs), pelagic (some shrimp), and deep-sea hydrothermal vents. Extinct Angustidontida known only from fossil marine deposits.

Global marine distribution across all oceans from polar to tropical waters. Euphausiacea concentrated in cold, nutrient-rich waters of Southern Ocean, North Pacific, and upwelling zones. Decapoda distributed in all marine environments including deepest ocean trenches. Fossil Angustidontida from Paleozoic marine deposits.

Highly variable across constituent orders. Euphausiacea: primarily phytoplankton and zooplankton filter-feeding. Decapoda: omnivorous, carnivorous, detritivorous, or herbivorous depending on group; includes scavengers, predators, and suspension feeders.

Development includes nauplius and/or metanauplius larval stages, followed by zoea and/or mysis stages in decapods, with gradual metamorphosis to juvenile and adult forms. Euphausiacea have direct development or simplified larval sequences. Reproduction typically sexual with separate sexes; fertilization internal or external depending on group.

Euphausiacea form dense aggregations (swarms) that can extend for kilometers and represent some of the largest animal biomasses on Earth. Many decapods exhibit complex behaviors including burrowing, nocturnal foraging, and territorial defense. Some species undertake diel vertical migrations.

Euphausiacea are critical primary consumers and keystone species in marine food webs, transferring energy from phytoplankton to higher trophic levels including whales, seals, fish, and seabirds. Decapoda occupy diverse trophic positions as predators, scavengers, and prey. Collectively, eucaridans are foundational to global ocean productivity and fisheries.

Decapoda support major global fisheries (shrimp, prawns, crabs, lobsters) with enormous economic value. Krill (Euphausia superba) harvested for aquaculture feed, human supplements, and bait, with Antarctic krill fishery regulated by CCAMLR. Some species are invasive pests or damage fishing gear. Extinct forms have no direct relevance.

• PeracaridaSimilar malacostracan superorder but lacks carapace or has carapace fused to fewer than all thoracic segments; includes amphipods, isopods, and mysids. Some mysids were historically grouped with euphausiaceans in 'Schizopoda'.
• HoplocaridaMalacostracan superorder containing mantis shrimp; distinguished by raptorial second thoracopods and carapace covering only posterior thorax, not all segments.

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• Wikipedia
• iNaturalist taxon
• Schram F. R. and J. C. von Vaupel Klein (eds.); Charmantier-Daures, M. and J. Forest (advisory eds.). 2010. Treatise on Zoology – Anatomy, Taxonomy, Biology. The Crustacea. Volume 9, Part A. Eucarida: Euphausiacea, Amphionidacea, and Decapoda (partim). Koninklijke Brill NV, Leiden, The Netherlands, 560 pp. ISBN-13: 978 90 04 16441 3. Hardcover: €210 or $289.