The order Actiniaria comprises the sea anemones, cnidarians characterized by their sessile form and soft body lacking both external and internal skeletons. These marine animals attach to hard substrates, such as rocks or shells, using a pedal disc, although some species can move slowly or even live freely on sediment. Their name comes from the Greek aktis (ray), referring to their tentacles arranged radially around the mouth. They are distributed across all oceans, from tropical shallow waters to polar environments and abyssal depths, showing remarkable ecological and morphological diversity. Currently, about 1,200 species are described within this order, making it one of the most diverse groups among anthozoans.
Sea anemones have a cylindrical body composed of a fleshy column topped with a crown of stinging tentacles arranged in one or several rings. These tentacles contain cnidocytes, specialized cells capable of injecting toxins to capture prey or defend against predators. The size of sea anemones varies greatly by species, ranging from a few millimeters to over one meter in diameter, as seen in some giant Pacific species. They lack a mineralized skeleton, which provides great flexibility, and their radial symmetry facilitates prey capture from any direction. Although they often exhibit bright colors and striking patterns, these hues may depend on both their own pigments and the presence of symbiotic algae of the genus Symbiodinium within their tissues.
Species of Actiniaria are carnivorous, capturing small crustaceans, fish, zooplankton, and even other cnidarians using their stinging tentacles, which transport food to the mouth located at the center of the oral disc. Some anemones maintain symbiotic relationships with photosynthetic organisms such as dinoflagellates, from which they derive additional nutrients. Mutualistic associations are also well known, such as those formed with clownfish (Amphiprion), which find refuge among their tentacles in exchange for protection from predators and nutrient input from metabolic waste.
Reproduction can be sexual or asexual. In sexual reproduction, most species are dioecious, although hermaphroditic anemones also exist. They release gametes into the water, where external fertilization occurs, producing planktonic larvae called planulae, which later settle on the substrate and develop into adult polyps. Asexual reproduction occurs through longitudinal fission, budding, or pedal laceration, allowing some species to form large clonal colonies.
The sub-classification of sea anemones is organized into various suborders and families with distinctive ecological and morphological adaptations. These include the true anemones (suborder Enthemonae), jewel anemones (family Actiniidae). Other important groups include abyssal anemones (family Actinostolidae) adapted to deep-sea environments, and symbiotic anemones (families such as Stichodactylidae) that host fish and crustaceans in mutualistic relationships. This diversity reflects the wide morphological, functional, and adaptive plasticity of the species within this order.
