Sea anemones and corals are distinct marine animals, though often mistaken for one another due to their close biological relationship. Both invertebrates populate diverse ocean environments. They are classified into different groups within the same broad phylum, possessing unique characteristics that define their individual forms and ecological roles.
Shared Characteristics
Sea anemones and corals share fundamental biological features, primarily due to their classification within the phylum Cnidaria. This phylum also includes jellyfish and hydras, all characterized by specialized stinging cells. Both exhibit a polyp body plan: a cylindrical column topped by an oral disc with a central mouth surrounded by tentacles. This structure gives them radial symmetry, meaning their body parts are arranged around a central axis.
A defining trait for both groups is the presence of nematocysts, microscopic stinging cells on their tentacles. These cells are used for defense and to capture prey, paralyzing small fish or invertebrates. Once prey is immobilized, tentacles guide it to the mouth. Many species also host symbiotic algae, called zooxanthellae, within their tissues, which provide additional nutrients through photosynthesis. Both organisms are sessile, meaning they remain attached to a substrate.
Key Distinctions
A primary difference between anemones and corals lies in their skeletal structure. Hard corals secrete a rigid external skeleton composed of calcium carbonate, specifically aragonite. This stony framework provides structural support and protection, forming the complex, three-dimensional structures known as coral reefs. Soft corals, conversely, possess internal, flexible skeletons made of protein and small calcareous structures called sclerites, allowing them to sway with currents.
Sea anemones typically do not produce a hard external skeleton. Their soft, fleshy bodies are supported by hydrostatic pressure, functioning as a hydrostatic skeleton.
Another distinction is their growth pattern. Most corals are colonial organisms, forming large interconnected groups of genetically identical polyps that share resources. While some coral species can be solitary, the vast majority contribute to colonial structures. Anemones, however, are predominantly solitary animals, living as individual polyps.
These structural and lifestyle differences also influence their ecological roles. Hard corals are primary reef builders, creating diverse habitats that support marine life. Their collective skeletal growth forms the foundation of entire reef ecosystems. Sea anemones, while important predators and providers of shelter for certain species like clownfish, do not contribute to large-scale structural reef formation.
Their Place in the Animal Kingdom
Both sea anemones and corals belong to the phylum Cnidaria. Within Cnidaria, they are further classified into the class Anthozoa, often referred to as “flower animals.” Anthozoans lack a medusa (jellyfish) stage, existing solely as polyps throughout their adult lives.
The class Anthozoa is subdivided into two main subclasses: Hexacorallia and Octocorallia. Sea anemones, along with stony corals and black corals, are grouped under Hexacorallia. Members of this subclass typically exhibit six-fold symmetry in their internal structures and tentacles. The other subclass, Octocorallia, includes soft corals, sea pens, and sea fans, characterized by eight tentacles and eight-fold symmetry. These distinctions highlight their shared ancestry and differing forms and functions in marine ecosystems.