Crustaceans form an extensive subphylum with more than 67,000 known species, within the phylum of arthropods. They are mostly aquatic, and inhabit a multitude of ecosystems, whether saltwater, brackish or fresh water, and almost all existing depths. If we consider the diversity of species of Crustaceans, as well as the multitude of habitats in which we can find them, we are undoubtedly faced with one of the most biologically successful groups of animals.
As a subphylum, they are characterized by having an articulated exoskeleton formed mainly of chitin, as well as having two pairs of antennae. It is characteristic of arthropods, and therefore also of Crustaceans, that in order to grow they need to go molting their exoskeleton, so that on many occasions we will be able to see the molts of their exoskeletons. Its body is made up of a variable number of segments (methameres), and can be divided into three large regions, the head (cephalon), the thorax (pereon) and the abdomen (pleon). The size of the Crustaceans is very variable, ranging from 0.1 mm (Stygotantulus stocki) to 4 m (13 ft) for the Japanese spider crab (Macrocheira kaempferi).
We find species of free aquatic life (with the exception of a few species of terrestrial life), parasite species and even species of sessile Crustaceans such as barnacles. They are a group of great importance for the ecosystems. On the one hand, there are species that are part of zooplankton and that are species with one of the largest animal biomasses on the planet, such as krill and copepods. Other species are scavengers and process other dead animals. Crustaceans are not only important from the point of view of feeding a multitude of marine species, but they are also very important for human consumption, generating more than 10 million tons of Crustaceans (mostly shrimp and crabs) for human consumption annually.