The order Ceriantharia comprises the so-called tube-dwelling anemones or cerianthids, cnidarians characterized by living inside mucous tubes reinforced with sand, shells, or detrital particles, which they secrete themselves. These animals have an elongated, worm-like body with two crowns of tentacles: an outer crown, long and extended for prey capture, and an inner crown, shorter and used to manipulate food. Unlike other anemones (Actiniaria), they lack a pedal disc to attach to the substrate, instead burying themselves in soft sediments. The scientific name comes from the Greek keras (horn) and anthos (flower), alluding to their tubular shape and the floral arrangement of their tentacles. They are distributed throughout all the world’s oceans, from shallow waters to depths exceeding 2,000 meters, with approximately 140 species currently described in this order.
Morphologically, tube-dwelling anemones exhibit the radial symmetry typical of cnidarians, with a cylindrical body ranging from 10 to 40 cm in length, though some species exceed half a meter. Their protective tube, to which they retreat quickly when threatened, is a flexible structure composed of mucopolysaccharides and specialized cells called ptychocysts. The outer tentacles, often brightly colored in shades such as purple, blue, or orange, form a conspicuous crown for prey capture, while the shorter inner tentacles are arranged around the mouth to aid ingestion. A distinguishing feature compared to true anemones (Actiniaria) is the presence of a simple digestive system with a siphonoglyph that helps maintain water circulation within the gastrovascular cavity.
Species of tube-dwelling anemones are carnivorous and opportunistic predators. They feed mainly on zooplankton, crustacean larvae, small fish, and other invertebrates captured with their stinging tentacles armed with cnidocytes. Thanks to their double crown of tentacles, they maximize prey capture: the outer tentacles detect and immobilize food, while the inner tentacles transport it to the mouth. As they do not depend on symbiosis with zooxanthellae—as many corals do—they are entirely heterotrophic and can thrive in environments with little or no light, such as deep-sea habitats.
Reproduction occurs both sexually and asexually. Most species are dioecious (separate sexes) and release gametes into the water column, where external fertilization occurs. The planktonic larvae, known as cerinula, have a prolonged development period and can disperse over long distances before settling on the substrate to construct their first tube. Asexual reproduction through fragmentation is rare, but some species can regenerate body parts after damage, giving them some resilience in unstable environments.
The sub-classification of this order includes the true tube anemones (family Cerianthidae), which comprise large and colorful species such as Ceriantheopsis americana commonly found in tropical and temperate waters. Other families include Arachnactidae and Botrucnidiferidae. The morphological and ecological diversity of tube-dwelling anemones makes them a key group in benthic ecosystems, as their tubes provide shelter for small invertebrates and contribute to the structural complexity of marine habitats.
