Crustacean
Mandibulate arthropods with biramous limbs and a nauplius larval stage.
Crustaceans are mandibulate arthropods, traditionally placed in the paraphyletic subphylum Crustacea. They are mainly aquatic and include decapods (shrimps, prawns, crabs, lobsters, crayfish), seed shrimps, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods, and mantis shrimp. It is now well accepted that hexapods (insects and entognathans) are cladistically crustaceans, with both groups combined in the monophyletic clade Pancrustacea. The group has an extensive fossil record reaching back to the Cambrian, and more than 7.9 million tons of crustaceans per year are harvested for human consumption.
- field
- Carcinology (also malacostracology, crustaceology, or crustalogy)
- known_for
- Biramous limbs, nauplius larval stage, exoskeleton moulting, and dominance in aquatic ecosystems
- size_range
- 0.1 mm (Stygotantulus stocki) to 3.8 m leg span (Japanese spider crab)
- habitat
- Mostly aquatic (marine and freshwater); some terrestrial, parasitic, or sessile
Lore & Background
Crustaceans are composed of segments grouped into three regions: cephalon (head), pereon (thorax), and pleon (abdomen). The head and thorax may fuse into a cephalothorax, often covered by a carapace. Their hard exoskeleton is moulted for growth. Appendages are typically biramous (two-parted), including the second pair of antennae, while the first pair is usually uniramous except in Malacostraca where antennules may be biramous or triramous. The body cavity has an open circulatory system with a heart near the dorsum; oxygen-carrying pigments vary by group (haemocyanin in Malacostraca, haemoglobins in copepods, ostracods, barnacles, and branchiopods). The alimentary canal is a straight tube often with a gastric mill and paired digestive glands, and kidneys are located near the antennae.
Reader's Guide
Crustaceans are ecologically vital, forming the base of many food chains, particularly in Antarctic communities where krill are the most important layer. They are ubiquitous in oceans, analogous to insects on land. Their fossil record extends to the Cambrian, indicating ancient origins. Crustaceans exhibit diverse reproductive strategies: most have separate sexes and reproduce sexually, but hermaphroditism occurs in barnacles, remipedes, and Cephalocarida, and parthenogenesis is widespread among branchiopods, some ostracods, some isopods, and certain higher crustaceans like the Marmorkrebs crayfish. Larvae typically begin as a nauplius with three pairs of appendages and a single eye, followed by stages such as zoea, mysis, or megalopa depending on the group. The scientific study of crustaceans is carcinology.
Did You Know?
- The smallest crustacean, Stygotantulus stocki, measures 0.1 mm, while the Japanese spider crab can have a leg span of up to 3.8 m.
- Crustaceans are distinguished from other arthropods by biramous limbs and larval forms such as the nauplius.
- Some crustaceans, like barnacles, are sessile as adults, while others like woodlice are terrestrial.
- Krill and copepods may be the animals with the greatest biomass on the planet.
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