All about Sea Anemones

Table of Contents

Sea anemones are a group of marine animals that belong to the order Actiniaria. They have a colourful appearance and are thus named after the terrestrial plant anemones. They are soft-bodied and sedentary marine animals that are found in oceans in depths deeper than 10,000 metres.

Sea Anemone Classification

Sea Anemone Characteristics

• Sea anemones have a cylindrical body that may be thick and short or slender and long. They are a few millimetres in diameter and upto 1.5 metres long. The largest genus is Stichodactyla.
• The body is a sessile polyp that is attached to the substratum by an adhesive foot called petal disc or basal. However some species lack this adhesive foot and instead have a bulbous lower end (physa) that anchors them to the surface.
• The mouth is situated in the oral disc. It is slit-shaped and surrounded by whorls of tentacles.
• The tentacles have tapered ends with a pore. In other species they may either be club-shaped, branched or have low knobs.
• The tentacles bear cnidocytes. Cnidocytes are stinging nematocysts that help in both defence and capturing prey. The presence of cnidocytes is the characteristic feature of cnidarians.
• The nematocysts are filled with actinotoxins that get triggered by the touch of a prey and paralyse it.
• They are brightly coloured in yellow, green and blue colours.
• They seldom move, some may creep and some may show somersaulting movements.
• They do not have a solid skeleton, some species secrete a horny covering.
• They usually feed on marine animals and fishes, some may even feed on microorganisms.
• Sea anemones are preyed upon by eels, starfish, codfish and flounders.
• Sexes are separate in most of the species.

Digestive System

Sea anemones have an incomplete gut. They have a gastrovascular cavity that works as a stomach and an opening that works as both mouth and anus. The mouth is ciliated which helps in the inward movement of food particles.

Nervous System

They have a primitive nervous system that helps in maintaining the homeostasis and responds to various stimuli. They have two nerve nets that are present in epidermis and gastrodermis.

Life Cycle

Sea anemones lack the free-swimming medusa stage. The polyp itself produces both eggs and sperms which fertilise to form a planula larva. It directly develops to form polyps. They can undergo both sexual and asexual reproduction.

During sexual reproduction, the males stimulate the females to release egg and fertlisation occurs either in the gastrovacular cavity or in the water column. The male and female gametes are usually released through the mouth; however, in some species they may emerge from the body cavity.

Asexual reproduction in sea anemones is by means of budding, fragmentation, transverse and longitudinal binary fission. Longitudinal division is seen in Anthopleura, transverse division is seen in Gonactinia prolifera and fragmentation is common in Metridium dianthus.

Ecology

They do not have the capability to photosynthesise or make food for themselves. Thus sea anemones develop facultative mutualistic relationships with single celled algae. The algae reside in the tentacles and oral disc, and get exposure to sun and protection from other feeders. The algae may be zoochlorellae or zooxanthellae.

The sea anemones get oxygen and food from the algae. The algae are also saved by the stinging cells and thus have less possibility to be eaten up by herbivores.

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• Cnidaria – Meaning, Examples, Characteristics and Symmetry
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