Crustaceans are a large subphylum in the phylum Arthropoda. Some of the most common representatives of Crustacea are crabs, shrimps, lobsters, barnacles and crayfish. There are over 67,000 described species around the world.
Most members of Crustacea are aquatic, and many species live in the marine environment. There are also terrestrial forms that have adapted to living on land. Just as insects are abundant on land, so are crustaceans in the ocean. Most of these animal species are motile and many travel in large colonies. However, there are species that live attached to a host as parasites, or they live a sessile life, such as in the case of barnacles that attach themselves headfirst unto a substrate.
As for the depth at which these animals live, they inhabit marine environments from intertidal zones to abyssal depths. Krill are the bottom of the food chain and they play a highly important role in many communities, including extreme environments such as the Antarctic Ocean. There are also some invasive species of Crustacea, the most notable ones being the Asian shore crab Hemigrapsus sanguineus and the Chinese mitten crab Eriocheir sinensis.
All crustaceans have their body composed of body segments, just as other arthropods. These body segments have been grouped into three categories: the head, thorax and abdomen. In some species, the head and thorax are fused together and may be protected by a single carapace, while in other species, the hard exoskeleton that protects their body may be formed from numerous plates or segments. As the animals grow, they molt their exoskeleton and form a larger one. Each body segment may have a pair of appendages, such as the antennae, mandibles and maxillae, legs that are either specialized for walking or for feeding.
Most species of the Crustacea have separate sexes, and thus reproduce sexually. There are some species that are hermaphrodites, the most notable group being barnacles. In some species, individuals may change their sex at a certain stage of life, while in the case of others, reproduction could be accomplished through parthenogenesis, where a female may reproduce asexually without having the eggs fertilized by a male.
Most species release their eggs into the water column, while in other species, individuals may have specialized appendages or mechanisms to hold unto the eggs and take care of them until they hatch. Among these, some species may carry them with thoracic limbs, other species have external egg sacks, while some crustaceans may attach the eggs unto surfaces such as rocks.