Introduction to Sea Anemones

Sea anemones are beautiful and diverse organisms that populate the world's oceans from the tropics to the poles. What follows is an illustrated glossary of the anatomy of sea anemones. Captions to images are largely taken, adapted, or translated from the original captions, when available. For a list of terms pertaining to sea anemones, please follow this link: List of terms.

Although sea anemones are among the simplest of animals, they possess one of the most complex structures in the animal kingdom: the nematocyst, or stinging capsule. Their possession of this feature places sea anemones in the phylum known as Cnidaria, in which jellyfish, corals, and hydra are also found. Other characteristics that are shared by members of this phylum are a diploblastic structure and the possession of a coelenteron. While most animals are triploblastic, meaning they possess three body layers, cnidarians possess only two: the ectoderm and the endoderm. These two layers are separated by a jelly-like and largely non-cellular layer known as the mesoglea. Cnidarians also possess a coelenteron, which is the single sack-like body cavity that performs digestive, nutrient absorption and, gas exchange functions. The single opening to this structure is the mouth, through which food, water, and gametes must pass. This opening is surrounded by numerous tentacles, which are generally studded with nematocysts and are actively used in food capture and defense.

Anemones, like all cnidarians, capture prey animals with their nematocyst-laden tentacles. The prey items, which mainly include the animals that stumble into the tentacles, are transported to the mouth by the tentacles and digested in the coelenteron. Many anemones also host symbiotic algae within their cells. These algae contribute a portion of their photosynthetic product to the anemone and may be a major source of energy for the animals.

The general structure of a sea anemone is simple. While many members of the phylum Cnidaria have a life cycle that includes both medusoid and polypoid stages, sea anemones are exclusively polypoid. The external morphology of anemones is limited to a column; an oral disk, in the center of which the mouth is located, and and on which the tentacles are located; and either a pedal disc, that affixes the anemone to the substrate, or a bulb-like physa, used by burrowing anemones to anchor in soft substrate. Internally, anemones are also simple with only a few anatomical features, largely limited to a throat-like actinopharynx and radiating mesenteries. Several kinds of muscles can be found in these tissues, and are used to contract and bend the column in various directions. These include longitudinal muscles which run perpendicular to the base and serve to contract the column vertically, and circular muscles that run around the column parallel to the base and serve to reduce the diameter of the column. These muscles are also used to expand the column and tentacles by the use of hydrostatic pressure. That is, by using the muscles to apply pressure to the water in one part of the the closed coelenteron, this water is forced into other parts which subsequently appear extended and more rigid. This has been termed the hydrostatic skeleton.

Images in the Glossary:

All of the images in the glossary have been linked by species to the Hexacorallians of the World Database. By clicking on the species name below the image, you will go to the image page for that species in the Hexacorallians database.


Longitudinal Section

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This diagrammatic longitudinal section of a generic sea anemone shows many anatomical features common to these organisms. The tentacles occupy the oral disc around the mouth. The mouth opens into the actinopharynx, a throat-like passage which leads to the coelenteron. One mesentery is shown on each side of the animal, and others radiate around the coelenteron. The gametogenic tissue is embedded within the mesenteries between the musculature and the filaments. Nematocyst-studded acontia protrude from the interior edges of these mesenteries. The sphincter muscle around the margin of the oral disk is used to contract the oral disk. The anemone affixes itself to the substrate using its pedal disc.

Transverse Section (Diagrammatic)

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adapted from Hyman, 1940


Transverse Section (Photographic)

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Cross-section of an anthozoan, the sea anemone Megalactis sp., at the level of the actinopharynx. Image copyright © 2000 Adorian Ardelean.

This version of the glossary is a continuation of a previous project started by Bryan McCloskey, many of the items in this glossary were taken or adapted from items in his earlier glossary.

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