Sea Anemones

Sea anemones are the most eye-catching creatures to be found in rock pools. Their innocuous plant like appearance hides their real nature, of formidable predator. Sea anemones make up part of the coelenterates a Latin word derived from Greek koilos meaning hollow and enteron intestine. The term coelenterate is still used but now it is more usual to call them cnidarians.

Physiology

Sea anemones have no hard parts. The body is cylinder shaped with a mouth at one end and a basal disk at the other. The basal disk acts as a sucker for attaching to solid surfaces. With sand living species the basal end does not have a sucker instead it is adapted for digging.

Each anemone is a large individual polyp with a mouth surrounded by tentacles. The number of tentacles varies depending on the species and age, but is common for them to be in multiples of six. The tentacles have both stinging and tacky cells, the former to immobilise the prey, the latter to grip it. The tentacles pass the prey into the mouth. The mouth leads into a sack like cavity where the prey is digested. After digestion the remains are expelled through the mouth.

The body is comprised of two layers the ectoderm on the outside and the endoderm on the inside. Both of these layers are separated by the flexible jelly-like mesoglea.

Sea anemones have not evolved specialised respiratory organs, instead they absorb oxygen through direct exchange between their tissue and sea water.

Nematocysts

Sea anemones ensnare their prey with tentacles covered by stinging cells called nematocysts. Each stinging cell is a liquid filled structure with a coiled filament inside it. When a prey item brushes past a nematocyst, the cnidocil senses it, by touch or chemically. The cnidocil instantaneously activates the cell which everts, the fluid becomes pressurised, discharging the coiled filament into the prey.

The cells can be stinging or “tacky” cells. The stinging cells inject poison into the prey to immobilise the prey. The “tacky” cells are barbed to grip the prey.

Reproduction

The life cycle differs between sea anemone species however there is not a medusa stage. In most species the sexes are separate but in some they are hermaphrodite. With beadlet anemones, the females draw sperm into their body, where the eggs are fertilized. Later small copies of the adult are “spewed” out of the female’s mouth. While in the case of the snake-locks anemone, reproduction is asexual, the anemone splits longitudinally into two, both being clones of the original.

Movement

Looking at sea anemones in rock pools day after day it appears that they do not move. However this is not true, they do very gradually creep about. They will under unusual conditions detach themselves and inflate their bodies to change their buoyancy. They are then carried off by the currents to settle elsewhere.

Mild-Mannered Anemones !

It has been found that sea anemones are not as easy going as they give the impression to be. They have been found to be territorial. When anemones come into contact with each other, they will commence to fight, stinging each other. The loser will slowly move away or dislodge itself from its fixing and float away.

Surviving The Tides

The intertidal zone is a severe environment open to temperature and weather extremes. However if you visit a rocky shore you will see as many anemones out of the rock pools as in them. Outside the rock pools these extremes are even greater. These anemones are beadlet anemones which just look like blobs of jelly. The beadlet anemone when uncovered by the tide, retracts its tentacles and closes the mouth, thus reducing water loss due to dehydration.

On Cornish shores over twenty different anemone species can be found. The most common species by far is the beadlet anemone Actinia equina followed by the snake-locks anemone Anemonia viridis.

Species of Sea Anemones Found In Cornwall

     Beadlet Anemone Actinia equina

  Snake-locks Anemone Anemonia viridis

      Unidentified Anemones

  Strawberry Anemone Actinia fragacea

Beadlet Anemone Actinia equina

Beadlet anemones come in differing colours, green, red, or brown. On rocky shores one will see as many out of the rock pools as in them.

When uncovered by the tides, beadlet anemones retract their tentacles and look like small beads of jelly. Those in rock pools will have their tentacles extended, displaying a ring of bright blue spots acrorhagi.

Strawberry Anemone Actinia fragacea

There is a less common strawberry form of beadlet anemone. It was once taken as a variety of the beadlet anemone. It is currently considered a different species. It is a southern species; in this part of Cornwall it is common.

It can be distinguished from the bead anemone by greenish spots on its red body. Which when its tentacles are retracted give it an appearance of a strawberry. They grow considerably larger than beadlet anemones reaching 3 inches across.

Snake-locks Anemone Anemonia viridis

The snakes-lock is a southern species but in this part of Cornwall it is second most common sea anemone.

It is a large anemone reaching over six inches across. This is one sea anemone one should be aware of, as its stings have been known to affect humans.

The snake-locks anemone cannot retract its tentacles and is only found in rock pools, as if it was exposed to the air it would be prone to dry out and die.

The snake-locks anemone comes in two forms, brown or green with purple tipped tentacles.

This green form has an unusual biology, the green pigment coming from algae zoochlorellae. This is a mutual symbiotic relationship i.e. both the anemone and the algae benefiting. The algae produces as a by product of photosynthesis, sugars and oxygen, which the sea anemone digests. The sea anemone’s waste product carbon dioxide the algae utilises for photosynthesis. Additionally the snake-lock seeks areas of good light, where the algae can efficiently photosynthesis within the protection of the stinging anemone. The brown forms are maybe those deficient in algae.

Unidentified Anemones