Although many benthic species are microscopic, measuring less than 0.1 mm, the group of microbenthos do not include any animal species. The smaller benthic animals are in the meiobenthos group, and include species measuring from 0.1 mm to 1 mm, such as nematodes or roundworms; some species of polychaetes or bristle worms, which can also found in bigger sizes; tardigrades, also called moss piglets or water bears; and gastrotrichs, flat worms with a transparent body abundant in the ocean floors.
Macrobenthos are the majority of benthic animals, and include all species larger than 1 mm, such as sponges, corals, cnidarians, some polychaete worms, such as the lugworm (Arenicola marinasea) and the sandworm (Alitta virens); sea squirts, which live attached to rocks and feed by filtering the water; starfish; anemones; mollusks, such as octopuses and oysters; and crustaceans, such as lobsters and cumaceans, or hooded shrimps.
Animal benthos are also classified in epibenthos or endobenthos, according to where they live in the ocean floor. Epibenthos are those living on the top or just above the seafloor and represent the majority of benthic animals. They are mobile, such as starfish, or live attached to the substrate, such as sponges and corals.
Endobenthos refers to the species that live in the sea floor, often buried in the sediments. Endobenthic animals include bivalves, such as shipworms, Thyasira, and other clams, some of which are found buried 1 foot into the ocean floor; and several species of polychaetes, such as Cossura longocirrata and Heteromastus filiformis, which prefers muddy sands; and the small crustacean Byblis gaimardi.