Six superclasses are most recognized by carcinologists in the general class of arthropods known as crustacea. These are Branchiopoda, Remipedia, Maxillapoda, Ostracoda, Cephalocarida, and Malacostraca. These classes of crustaceans account for most of the 50,000 sub-species of crustaceans known to exist. The class Malacostraca contains the more familiar crustaceans, including lobsters, crabs, krill and shrimp. These are of the Decapoda order of crustaceans, which generally translates to those having ten legs or feet, or five pairs of equal or similar shape used for walking, collecting food and defense. This class of crustaceans is further broken down into swimming decapods (shrimp and krill) and walking decapods (lobsters and crabs). Shrimp and krill live in both freshwater and marine environments, whereas lobsters (related to crayfish) are freshwater creatures only.
These arthropods are one of the most important of all classes of crustaceans because they make up the first step in the marine life cycle. Commonly referred to as copepods, these creatures are some of the smallest sea animals in existence and include zooplankton, which are the food source for many marine life forms such as whales, fish, sea birds and squid. There are ten sub-orders of copepods (literally meaning "oar feet"), most of which swim freely in oceans and seas. Others are parasitic, feeding on food debris and microscopic organisms that inhabit marine ecosystems. These crustaceans are characterized by one simple eye that only distinguishes between light and dark contrasts. They regularly shed their outer shell (exoskeleton) as do all classes of crustaceans.
Other familiar aquatic life-forms populate one or another of the classes of crustaceans, such as the Branchiopoda class. Members of this crustacean family include brine shrimp (referred to as "sea monkeys" when sold as aquatic novelties) and Daphnia, which are tiny creatures often sold as food for home aquariums. This class of crustaceans also includes sand and water fleas and woodlice. Certain species of water fleas have adapted to seawater, but most Branchiopoda crustaceans are found in fresh water. They are also one of the few species of crustaceans that can live in salt lakes. These crustaceans are characterized by flattened appendages extending from their abdominal areas, where their gills are located. These appendages also act as food filters and, in some cases, are their main means of propulsion.