In the overcast days of March, a rocky shore can seem a bleak and uninviting place. In down on the shore amongst the wracks draped over the boulders, it will seem a colourless place and glimpses of colour can be seen in the sea anemones, often hidden in cracks, or in the case of the Beadlet Anemone, Actinia equina, appearing like lumps of jelly fastened to rocks with their tentacles retracted when the tide goes out.
Sea anemones are found in the sea fastened to rocks and shells, and on the shore throughout the length of the British coastline. They are not flowers but primitive animals related to the jellyfish and the tropical corals. They consist of a soft polyp with numerous stinging tentacles which are used to catch their prey. They attach to the rocks by their base and have a single opening which functions as a mouth to ingest food, and also to expel their waste products.
The stinging cells in the tentacles and other parts of the sea anemone are called cnidae. The commonest type are called nematocysts. These cells can be seen under the microscope and their mechanism is arguable the fastest action in the whole of the animal kingdom. The fluid filled capsule called the nematocyst everts and forces the filament to discharge venom into its prey of smaller organisms.
Sea anemones can remain in the same place for days, or even weeks. They tend to choose a location where the currents and tides can bring food to them as plankton or suspended organic matter.
They can move around in two ways:
They creep around on their basal disc, so slowly that the movement can only be ascertained by a change of position in aquaria or on the shore, or observed by time lapse photography.
They inflate and let the tides and currents take them into a new location. This phenomenon is seen in aquaria when conditions become unfavourable.
Anemones reproduce both sexually and asexually and the full range of methods can be found in the common British species.
There are about a dozen species found on British seashores that are reasonably common. One of the most common and widespread is the Beadlet Anemone which is found in two major colour groups of red and green, and in numerous colour varieties between the two with brown specimens and anemones with yellow stripes and dots on the column.
This specimen inhabits the shore from the mid-tide zone down to low water and in the shallow seas.
Beadlet Anemone, strawberry variant, Actinia fragacea, Photo: Andy Horton
Underwater, the tentacles expand, and its beauty can be appreciated in home aquaria where it is a very hardy exhibit. Beadlet Anemones are about 25 cm across the base.The name 'Beadlet' comes from the blue beads called acrorhagi around the fosse at the top of the column, which have stinging powers and are used in territorial battles with other anemones.
Reproduction is asexual and there is dispute on how this occurs. The adult Beadlet Anemones brood young within their body cavity which are ejected as miniature anemones with 12 or more tentacles. These may arise by parthogenesis or by budding off internally.
There is a larger variety with a column coloured like a strawberry and this is usually regarded as a separate species Actinia fragacea. Their method of reproduction has not been recorded definitively.
Snakelocks Anemones, Anemonia viridis, are found on the southern and western shores of Britain and as far south as the Mediterranean Sea. They are distinguished by long flowing tentacles and will usually be found in a bright green colour, sometimes with purple tips to the tentacles. The green colour is caused by the chlorophyll in the symbiotic algae called zooxanthellae that are found in its tissues. These algae are necessary for the long time survival of the sea anemone. When the numbers of algae diminish the anemone may appear and dull grey in colour. Almost always Snakelocks Anemones, will be found in the sunniest pools and although it is possible for them to retract their tentacles this is unlikely to happen in the wild. Snakelocks anemone reproduce by a process called longitudinal fission in which they literally tear themselves in two identical clones. Pairs can often be seen on the shore in spring. Snakelocks Anemone can capture small fish in their stinging tentacles which will feel tacky to the human touch.
Submerged with their feathery concentric circles of tentacles, Plumose Anemones, Metridium senile, are regarded by some enthusiasts as the beautiful of the anemones found in British seas. They are usually be found in white and orange varieties with the tentacles matching the colour of their column. However, when left high and dry by the receding tide their bedraggled appearance is like a deflated balloon.
Plumose are the commonest of the British sea anemones are have a distribution in all the seas and oceans of the Northern Hemisphere. They are often found attached to wharves and pontoons in harbours, but on the shore they will only be present at the extreme low water mark. Plumose Anemones have fine individual tentacles and will normally feed only on the minutest of plankton.
They reproduce both sexually, although not frequently, and by a process called basal laceration where the anemone creeps along a rock leaving fragments of its base behind which develop into miniature new anemones.
The Dahlia Anemone, Urticina felina, is the largest of the anemones that will be found on British shores, and in deep water where they are normally be found they will reach the size of a plate. They come in a variety of different hues, often with an orange column and a red and orange disc. Gravel attaches to suckers on the column and they can easily be overlooked despite their large size.
They are known to feed on large crabs, but not every crab that blunders into the anemone will be consumed. Reproduction is not recorded, but is probably sexual.
Of the other anemones the most often encountered are the species in family Sagartiidae which include three species of the genus Sagartia. Species of this genus come with the tentacles and disc in all colours and a mixture of colours except blue. They can sometimes be found scattered over the seashore like daisies on a lawn. They have 'catch tentacles' that are temporarily enlarged ordinary tentacles. They can change colour over a period of time, with the larger specimens often a buff- and dark-brown mixture.
In the south and west from the same family, the multi-tentacled Daisy Anemone, Cereus pedunculatus, can often be found embedded in limestone cracks in shore pools. Some colonies are very colourful and others are a buff-brown.
The Gem Anemone. Bunodactis verrucosa, is an attractive grey and translucent patterned anemone that is found only on wave battered shores in the south-west where its main diet consist of very small mussels. The Pimplet Anemone, Anthopleura ballii is another species coloured green with symbiotic algae. It is regularly recorded emitting sperm or eggs. This species contains beads and is unable to retract its tentacles.
Sea Anemones are classified in the phylum Cnidaria because of the stinging cells. They comprise the order Actinaria in the class of Anthozoa.