Crustacean

Crustaceans are a group of mandibulate arthropods whose name comes from a Latin word meaning "those with shells" or "crusted ones." They were traditionally classified under the paraphyletic subphylum Crustacea, but it is now widely accepted that hexapods—insects and entognathans—are cladistically part of this group, together forming the monophyletic clade Pancrustacea. The three classes Cephalocarida, Branchiopoda, and Remipedia are more closely related to hexapods than to other crustaceans like oligostracans and multicrustaceans.

These animals are primarily aquatic, encompassing a wide variety of forms: decapods (such as shrimps, prawns, crabs, lobsters, and crayfish), seed shrimps, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods, and mantis shrimp. Of the roughly 67,000 described species, sizes range dramatically—from *Stygotantulus stocki* at just 0.1 mm (0.004 in) to the Japanese spider crab, whose leg span can reach 3.8 m (12.5 ft) and mass up to 20 kg (44 lb). Like other arthropods, they have an exoskeleton that must be moulted for growth. They are set apart from insects, myriapods, and chelicerates by their biramous (two-parted) limbs and by larval forms such as the nauplius stage seen in branchiopods and copepods.

Most crustaceans are free-living and aquatic, but some live on land (e.g., woodlice, sandhoppers), some are parasitic (e.g., Rhizocephala, fish lice, tongue worms), and some are sessile (e.g., barnacles). Their fossil record is extensive, stretching back to the Cambrian. Each year, over 7.9 million tons of crustaceans are harvested by fishing or farming for human consumption, mostly shrimp and prawns. Krill and copepods, though less widely fished, may be the animals with the greatest biomass on Earth and are vital to the food chain. The scientific study of crustaceans is called carcinology (also malacostracology, crustaceology, or crustalogy), and a specialist in this field is a carcinologist.

**Anatomy**

A crustacean’s body is made up of segments grouped into three regions: the cephalon (head), pereon (thorax), and pleon (abdomen). The head and thorax may fuse into a cephalothorax, often covered by a single large carapace. The hard exoskeleton protects the body and is shed during moulting. On each somite (body segment), the shell can be divided into a dorsal tergum, ventral sternum, and lateral pleuron, and various parts may fuse together. Each somite can bear a pair of appendages: on the head, these include two pairs of antennae, mandibles, and maxillae; on the thorax, legs appear, which may be specialized as pereiopods (walking legs) or maxillipeds (feeding legs). Malacostraca and Remipedia (along with hexapods) have abdominal appendages, while other crustacean classes have a limbless abdomen, except for a telson and caudal rami found in many groups. In malacostracans, the abdomen bears pleopods and ends in a telson (which holds the anus), often flanked by uropods to form a tail fan. The variety and number of appendages across different crustaceans may partly explain the group’s success.

Crustacean appendages are typically biramous (split into two parts), including the second pair of antennae; the first pair is usually uniramous, except in Malacostraca, where antennules may be biramous or even triramous. It is unclear whether the biramous condition evolved within crustaceans or if a second branch was lost in all other groups—trilobites, for instance, also had biramous appendages.

The main body cavity features an open circulatory system, with blood pumped into the haemocoel by a heart near the dorsum. Malacostraca use haemocyanin as an oxygen-carrying pigment, while copepods, ostracods, barnacles, and branchiopods use haemoglobins. The alimentary canal is a straight tube that often includes a gizzard-like "gastric mill" for grinding food and a pair of digestive glands for absorption; this structure runs in a spiral. Kidney-like structures are located near the antennae. A brain exists as ganglia near the antennae, with a collection of major ganglia below the gut.

In many decapods, the first (and sometimes second) pair of pleopods is specialized in males for sperm transfer. Many terrestrial crustaceans, like the Christmas Island red crab, mate seasonally and return to the sea to release eggs. Others, such as woodlice, lay eggs on land in damp conditions. In most decapods, females retain eggs until they hatch into free-swimming larvae.

**Ecology**

Most crustaceans live in marine or freshwater environments, but a few groups have adapted to land, including terrestrial crabs, terrestrial hermit crabs, and woodlice. Marine crustaceans are as widespread in oceans as insects are on land. Most are motile and move independently, though some taxonomic units are parasitic and remain attached to hosts (e.g., sea lice, fish lice, whale lice, tongue worms, and *Cymothoa exigua*, all sometimes called "crustacean lice"), while adult barnacles are sessile—attached headfirst to a substrate and unable to move. Some branchiurans can tolerate rapid salinity changes and switch hosts between marine and non-marine species. Krill form the bottom layer and most important part of the food chain in Antarctic animal communities. Certain crustaceans are significant invasive species, such as the Chinese mitten crab (*Eriocheir sinensis*) and the Asian shore crab (*Hemigrapsus sanguineus*). Since the opening of the Suez Canal...