Crustaceans (Crustacea) are a group of arthropods that includes crabs, lobsters, shrimp, krill, crayfishes, wood lice and barnacles. There are about 42,000 species of crustaceans alive today. Crustaceans evolved in the sea and are the only group of arthropods that remains mostly aquatic to this day.
Although the vast majority of crustaceans are aquatic animals that live in either marine or freshwater habitats, there are a few terrestrial groups (such as terrestrial crabs, terrestrial hermit crabs and woodlice) and even a handful of parasitic groups (such as tongue worms, whale lice and fish lice). Most crustaceans are free-living but some (such as the barnacles) are sessile.
The crustacean body is divided into segments (or tagmata) which include a head, a thorax and an abdomen. In some crustaceans, the head and thorax are fused into a structure referred to as the cephalothorax. Crustaceans have a hard external skeleton called an exoskeleton that is rigid and must be shed and replaced repeatedly, as the crustacean grows. The cuticle (or outer layer) is ofte reinforced with calcium carbonte, providing them with added structural support (especially useful to the larger species).
Most crustaceans have five pairs of appendages on their head (these include two pairs of anteannae, a pair of mandibles, and two pairs of feeding appendages). Their compound eyes are located at the end of stalks. The thorax bears one pair of appendages called the maxillipeds followed by several pairs of pereopods and the segmented abdomen bears appendages called pleopods. The posterior end of the crustacean body is referred to as the telson. The larger species of crustaceans breath using gills. Smaller crustaceans do not require gills and gas exchange across their body surface is sufficient to support the organism.
Most species of crustaceans have separate sexes and undergo sexual reproduction, though some crustacean groups (such as barnacles) are hermaphroditic. The general life cycle of a crustacean begins with fertilized eggs that are either released directly into the water or are held in place either in a brood chamber or clustered around the legs of the parent. After hatching, crustaceans go through several larval forms before developing into the adult stage.
Crustaceans occupy a crucial place in the marine food web and are among the most abundant animals on Earth. They often occupy the tropic level between primary producers and higher-level organisms within the food web. Thus crustaceans consume organisms such as phytoplankton and, in turn, crustaceans consumed by larger organisms such as fishes. Crustaceans also serve as a food source for humans, with crabs, lobsters, shrimp and prawns being the most popular of crustacean foods.
Crustaceans range in size from the minute water fleas and copepods (barely visible to the naked eye) to the massive Japanese spider crab, whose legs span a distance of as much as 14 feet.
Crustaceans feed in a number of different ways. Some crustaceans are filter feeders that extract tiny plankton or bacteria from the water. Other crustaceans, especially the larger species, are active predators that persue and capture their prey by crushing or tearing it with powerful appendages. Some species, especially the smaller species are scavengers that graze on microorganisms, nutrients and detritis that they can scrape of of substrate surfaces.
Crustaceans are well represented in the fossil record. Early crustaceans date back to the Cambrian and are represented in the fossils recovered from the Burgess Shale Formation.
Crustaceans are divided into the following basic groups:
- Brine shrimp and relatives (Branchiopoda)
- Horseshoe shrimp (Cephalocarida)
- Barnacles and copepods (Maxillopoda)
- Crabs, lobsters, and relatives (Malacostraca)
- Ostracodeans (Ostracodea)
- Remipedians (Remipedia)