Sea anemones

Written by astrid on 19. March 2009. Posted in Sea, Marine animals, Nature, Naxos, Animals.

On the rocks in shallow water in the small bays near Azalás, it is easy to find some particularly beautiful and interesting representatives of marine fauna: the sea anemones. The order of sea anemones (Actiniaria), with around 1200 species, belongs with the corals to the anthozoans (class Anthozoa), which together with the hydrozoans and the jellyfish form the phylum Cnidaria.

Here you can skip the introduction and go directly to the species.

The cnidarians have a rather simple anatomy a radially symmetrical body. Unlike sponges, they have true tissues made up of interconnected cells, and specialised cell types such as muscle cells and nerve cells, as well as simple sensory organs and a digestive system. However, they still lack many of the features of the “higher” bilateral animals. Many cnidarians have two different, altering forms: a generation of sessile polyps and a generation of medusae that drift in the water (some cnidarians lack the medusa generation, while others lack the polyp generation). The body of both the polyps and the medusae essentially consists of two epithelia, the outer epidermis and the inner gastrodermis, between which lies a cell-free supporting layer with collagen fibres, the mesoglea. The cnidarians capture their food (mostly plankton and other small animals) with tentacles that are either sticky or, in most cases, have harpoon-like stinging cells (cnidocytes) with which they kill their prey.

The stinging cells of the cnidaria are extremely remarkable, highly developed cells – perhaps the most complex cells found in the animal kingdom. Depending on the type, the stinging cell contains a long, coiled stinging thread (nematocyst) and an apparatus with small stylets that are explosively ejected when the hair-like sensory apparatus of the cell is touched. The stylets pierce the body of the victim and the nematocyst is ejected at lightning speed, releasing the toxins stored inside it. The discharge of the stinging cells takes place in a few milliseconds, with extremely high acceleration and unusually high pressure. This explosive discharge is achieved by very high osmotic pressure within the dormant stinging cells. The capsule wall of the cell must be extremely tear-resistant in order to withstand the high pressure in its dormant state, which is made possible by a special type of collagen. The toxins of cnidarians are very effective; some species (certain jellyfish) cause severe pain and shock and can also be dangerous to humans. The stinging cells of some sea anemones can also penetrate human skin; the glass anemone, for example, causes a stinging sensation similar to that of a nettle.

Beadlet anemone

Glass anemone

Sea anemones are sessile (although many species can move slowly) and live mostly solitary. They do not have a medusa generation. They are attached to the substrate by a short, thick trunk (the column). They have numerous movable tentacles equipped with stinging cells, which they use to capture their prey, mainly small animals and plankton. Between the tentacles lies the oral disc with the mouth. Sea anemones can retract their tentacles to a greater or lesser extent.

In sea anemones with retracted tentacles one can see the trunk or column.

Here the oral disc with the open mouth is visible among the tnetacles.

Sea anemones on Naxos

Species on Naxos

The Photo gallery of the marine plants gives an overview over the species.

A note on identification: Some species usually can be identified easily and reliably. In many cases, however, several related species are difficult to distinguish. Also not all species are included in the field guides; even using the internet, it is difficult to find information on some species. Accordingly, some of the identifications may not be correct.

Here you can jump directly to the species (return with the back arrow or by swiping back):
Beadlet anemone, Actinia equina – Glass anemone, Aiptasia mutabilis – Snakelocks anemone, Anemonia viridis – Telmatactis forskalii – Cylinder anemone, Cerianthus membranaceus

Beadlet anemone, Actinia equina, L.

Along the coasts of Naxos several species of sea anemones can be observed. The deep red Beadlet anemone is particularly common and conspicuous; it occurs up to the waterline and can survive longer periods of falling dry by retracting its relatively short tentacles completely and inverting its body.

The Beadlet anemone has a deep red colour. It often sits directly at the waterline; when they fall dry due to a low water level, they completely retract their tentacles and then look almost like a tomato.

The tentacles of the Beadlet anemone are very short.

Here an especially beautiful specimen!

Glass anemone, Aiptasia mutabilis, Gravenhorst

The inconspicuous grey-coloured Glass anemone is more difficult to spot. Its long tentacles are only slightly retractable. They are almost translucent with a slight marbling and have fine tips. The Glass anemone has a longer trunk, which usually is mostly retracted. It also occurs in shallow water, but cannot tolerate being exposed to air. Under unfavourable conditions, like many species, it can slowly crawl away on its side.

Glass anemone

fully opened Glass anemone with visible mouth

The tentacles of the Glass anemone are marbled grey.

Snakelocks anemone, Anemonia viridis (= A. sulcata), Forsskål

The Snakelocks anemone looks similar to the glass anemone, but has more numerous, longer and more pointed tentacles. The Snakelocks anemone is very tolerant of fluctuations in salinity and of water pollution and eutrophication. As it contains symbiotic algae, it can live only in the lighter upper water layers. It cannot retract its tentacles completely. The Snakelocks anemone feeds on small crustaceans, snails, small fish, etc.

A Snakelocks anemone in a small tidal pool at our coast.

Unidentified sea anemone

At the long pebble beach in the bay of Azalás, many sea anemones grow on the rocks in the shallow water, but I am unsure of their species: they most closely resemble Snakelocks anemones, but their tentacles retract completely.

These sea anemones grow in shallow water on the rocks.

They are mostly uniformly grey…

… but in some specimen the tentacles show pink tips (the colouring comes from symbiotic algae).

The about 100 tentacles are rather short and taper continually to the end.

The oral disc is brown.

In this anemone with retracted tentacles one sees the column which ist slightly striped.

Telmatactis forskalii ?, Hemprich & Ehrenberg

When you look closely, you can sometimes find very small anemones among the algae in shallow water. I’m not sure to what species they belong, perhaps Telmatactis forskalii.

This small sea anemone could belong to the species Telmatactis forskalii.

In this specimen with retracted tentacles the column is visible.

Cylinder anemone, Cerianthus membranaceus, (Gmelin)

Closely related to the sea anemones are the tube anemones that form their own order (Ceriantharia) within the anthozoans. They look very similar to sea anemones: like them, they have a trunk (column) and a ring of tentacles around an oral disc with the mouth. Unlike sea anemones, however, they do not attach themselves to the substrate with a basal disc, but live in sandy soils and build a tube that can reach over a meter in length, formed of mucus and nettle threads, into which they instantly retreat when disturbed. Worldwide exist about 100 species of tube anemones, of which about five occur in the Mediterranean; however, they have not yet been sufficiently studied.

Around Naxos one can find the Cylinder anemone (Cerianthus membranaceus) – or a similar species: a correct identification is not possible from a photo. However, the species seems to be very rare and only found in isolated specimens. It lives on sandy soil or in seagrass meadows. The cylinder anemone has two different types of tentacles: around the oral disc sit very short inner tentacles, wihch are surrounded by several rings of very long and thin outer tentacles (up to 15 cm). The colouring of the animals is very variable, ranging from sand-coloured to yellowish to brownish or violet. The specimens we found are very dark, almost black in colour.

The Cylinder anemone lives in sand. It has very short inner and long, thin outer tentacles. We have seen only very few individuals so far.

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