The crustacea (subphylum Mandibulata, class Crustacea) are a diverse group of animals. This class includes some of the more familiar arthropods, including barnacles, copepods, crabs, prawns, lobsters, and wood lice. More than 30,000 species have been identified, the majority of which are marine-dwelling. Terrestrial species such as woodlice and pill bugs are believed to have evolved from marine species. Most crustaceans are free-living but some species are parasitic—some even on other crustaceans. Some species are free-swimming, while others are specialized at crawling or burrowing in soft sediments.
Despite such an extraordinary diversity of species, many crustaceans have a similar structure and way of life. The distinctive head usually bears five pairs of appendages: two pairs of antennae that play a sensory role in detecting food as well as changes in humidity and temperature; a pair of mandibles that are used for grasping and tearing food; and two pairs of maxillae that are used for feeding purposes. The main part of the body is taken up with the thorax and abdomen, both of which are often covered with a toughened outer skeleton, or exoskeleton. Attached to the trunk region are a number of other appendages which vary both in number and purpose in different species. In crabs, for example, one pair of appendages may be modified for swimming, another for feeding, another for brooding eggs and yet another for catching prey.
Crustacea exhibit a wide range of feeding techniques. The simplest of these are those species that practice filter feeding such as the copepods and tiny shrimps. Feeding largely on plankton and suspended materials, the animal creates a mini water current towards the mouth by the rhythmic beating of countless number of fine setae that cover the specialized feeding limbs of these species. Food particles are collected in special filters and then transferred to the mouth. Larger species such as crabs and lobsters are active hunters of small fish and other organisms, while some species adopt a scavenging role, feeding on dead animals or plants and other waste materials.
Apart from the smaller species, which rely on gas exchange through the entire body surface, most crustaceans have special gills that serve as a means of obtaining oxygen. Simple excretory organs ensure the removal of body wastes such as ammonia and urea. Most crustaceans have a series of well-developed sensory organs that include not only eyes, but also a range of chemical and tactile receptors. All crustaceans are probably capable of detecting a light source but in some of the more developed species, definite shapes and movements may also be detected.
Breeding strategies vary considerably amongst the crustacea. Most species are dioecious (being either male or female), but some, such as the barnacles, are hermaphrodite. Fertilization is usually internal through direct copulation. The fertilized eggs then mature either in a specialized brood chamber in some part of the female's body, or attached directly to some external appendage such as a claw. Most aquatic species hatch into a free-swimming larvae that progresses through a series of body molts until finally arriving at the adult size.
See also Zooplankton.