Sea anemones are predatory, soft-bodied marine invertebrates belonging to the Phylum Cnidaria, which also includes jellyfish and corals. Named for their resemblance to a terrestrial flower, they are carnivores found throughout the world’s oceans. Sea anemones exist as polyps, meaning they are generally attached to a substrate rather than being free-swimming.
Physical Structure and Anatomy
The sea anemone’s body structure is a cylindrical column topped by a crown of tentacles. The column is often attached to a hard surface by an adhesive structure called the pedal disc (or basal disc). The pedal disc allows the animal to anchor itself securely, though it does not prevent movement entirely.
At the top of the column is the oral disc, which features a central, slit-shaped mouth that functions as both the entrance for food and the exit for waste. Surrounding the mouth are numerous, hollow, retractable tentacles used for feeding and defense. Sea anemones maintain their shape using a hydrostatic skeleton, which relies on the pressure of fluid within the body cavity for structural support. Lacking a centralized brain, their movements are coordinated by a primitive, decentralized nerve net distributed throughout their tissues.
Hunting, Diet, and the Power of the Sting
Sea anemones are passive but effective carnivores, waiting for prey to drift into their extended tentacles. Their diet consists primarily of small fish, crustaceans, and plankton, though some species consume terrestrial insects and spiders that fall into the water. Once a potential meal makes contact, the tentacles rapidly deploy specialized stinging structures called nematocysts.
The nematocyst is a microscopic, harpoon-like capsule coiled inside a cell called a cnidocyte. Upon being triggered by mechanical contact and chemical cues, the capsule explosively discharges its barbed thread. This micro-harpoon penetrates the prey and injects a venomous cocktail of neurotoxins, which quickly paralyzes the animal. The tentacles then guide the immobilized prey toward the oral disc for ingestion.
Habitat, Movement, and Life Cycle
Sea anemones are found across diverse marine habitats, ranging from shallow intertidal zones and coral reefs to the cold waters of the deep sea. Although often described as sessile (fixed in place), they are not completely immobile. The pedal disc allows for slow gliding movements across the substrate, typically undertaken if environmental conditions become unfavorable.
Some species can detach the pedal disc completely to be carried by the current, or they may somersault across the seabed. Reproduction occurs through both sexual and asexual methods. Sexually, they are broadcast spawners, releasing sperm and eggs into the water, resulting in free-swimming planula larvae that settle and grow into polyps. Asexually, they reproduce by fission (splitting in half) or by pedal laceration, where small pieces of the pedal disc regenerate into new, genetically identical individuals.
Famous Partnerships: Sea Anemones and Symbiosis
The most famous interspecies relationship involving the sea anemone is its mutualistic partnership with the clownfish. Clownfish are protected from the anemone’s sting by a specialized layer of mucus that prevents the nematocysts from firing. The anemone benefits because the clownfish defends its host against polyp-eating fish and provides nutrients through its nitrogen-rich waste.
This cooperative lifestyle is also seen with other marine animals, such as certain species of shrimp and crab. Symbiotic shrimp gain protection among the venomous tentacles and clean the anemone by removing parasites and dead tissue. Boxer crabs hold a small anemone in each claw, using them as defensive “boxing gloves” to deter predators. This arrangement provides the anemone with mobility and access to food scraps generated by the crab’s movement.