SEA ANEMONES: CHARACTERISTICS, MORPHOLOGY, HABITAT, FEEDING - BIOLOGY - 2025

Sea anemones (Actiniaria) are an order of animals belonging to the phylum Cnidarians. Because they physically more closely resemble plants and flowers, they were previously believed to belong to the plantae kingdom. However, thanks to the action of various researchers, it can be established that they are part of the animal kingdom.

Sea anemones get their name from a flower that also bears that name. Likewise, sea anemones are found on the seabed and are an important part of coral reefs. Like other members of the cnidarian phylum, anemones have the ability to secrete certain toxins that help them capture their prey.

Sea anemones. Source: OpenAperture

This order includes approximately 1200 species of anemones, which are distributed in all the seas around the world.

Taxonomy

The taxonomic classification of anemones is as follows:

- Domain: Eukarya.

- Animalia Kingdom.

- Phylum: Cnidaria.

- Class: Anthozoa.

- Subclass: Hexacorallia.

- Order: Actiniaria.

characteristics

Amphianthus sp. Nhobgood Nick Hobgood

They are multicellular eukaryotes

Sea anemones are characterized in that the cells that make them up are eukaryotic, which means that their genetic material is delimited inside the cell nucleus.

Likewise, anemones are multicellular organisms because their cells differentiate and form specialized tissues in different very specific functions.

They are diblastic

Like all cnidarians, sea anemones are diblastic animals. This implies that during their embryonic development they only have two embryonic layers: the endoderm and the ectoderm. From both layers, the different specialized tissues that make up the anemone were developed.

They show radial symmetry

Taking into account that anemones belong to the cnidarians, the most primitive group that make up the animal kingdom, it is not surprising that they present radial symmetry.

In animals that have this type of symmetry, the parts are arranged around a central axis. This axis extends from one end where the oral orifice is to the opposite end, called aboral.

They are heterotrophs

Despite the fact that anemones resemble plants, the truth is that, as part of the animal kingdom, they are heterotrophic organisms. This means that they are not capable of synthesizing their nutrients, but they feed on other living beings or the substances that they can make.

They are sessile

The anemones are fixed in the substrate, that is, they do not have any type of mobility. The only period of their life in which they have some mobility is during their larval phase, since there they can move through the water thanks to the cilia of their larvae.

They establish relationships of mutualism with animals

Despite the fact that anemones secrete a stinging and toxic substance, they are capable of establishing mutualistic relationships with other living beings such as the hermit crab and the clown fish. Anemones are associated with these individuals and obtain certain benefits related to the availability of food. In return, the anemones provide them protection.

Some species are dioecious, others hermaphrodites

The order Actiniaria is quite broad, encompassing a large number of species. Some of these species have specimens with differentiated sexes. On the other hand, others are hermaphrodites, that is, they have both male and female gonads.

Morphology

Viewed externally, sea anemones look like flowers, with numerous petals. These are not actually petals in the strict sense of the word, but rather tentacles that the anemone uses to capture its prey.

In general, its body is made up of a foot, also known as an adhesive foot disk, a body and the tentacles that surround the central mouth. They are also made up of an outer layer, the epidermis, and an inner layer, the gastrodermis.

The two distal ends of the anemone have a particular name. The lower end is known as the pedal disc, and the upper end is called the oral disc.

Body

The body is cylindrical and is sometimes smooth. There are specimens in which the body has certain fleshy protrusions (solid papillae), adhesive papillae, indentations and some small vesicles that protrude in relief.

In the oral disc there is a fairly wide hole, a slit type, which is the mouth of the animal and which is surrounded by tentacles. The mouth opens into a cavity known as the actinopharynx, which communicates directly with a cavity that serves as both the esophagus and the pharynx (gastrovascular cavity).

Anatomy of a sea anemone. (1) tentacle. (2) Pharynx. (3) gonad. (4) Wall. (5) Complete septum. (6) cynclide. (7) Acontio. (8) Pedal disk. (9) retractor muscle. (10) incomplete septum. (11) mesenteric perforation. (12) necklace. (13) mouth. (14) Oral disc. Source: © Hans Hillewaert

Likewise, the gastrovascular cavity is divided into spaces or chambers. The structure that divides them is known as the mesentery. The mesenteries originate in the wall of the body of the animal and are directed towards the interior of it. Cells are found in the mesenteries that synthesize and secrete digestive enzymes.

When the mesentery is complete, that is, it extends from the wall of the body towards the base of the pharynx, it is called macrocnema. Whereas when the mesentery is incomplete, it is called a microcnema.

Within the mesenteries are longitudinal fibers similar to muscle. These types of fibers are also found in the tentacles and at the level of the oral disc. Similarly, inside the body you can find circular muscle fibers. Sometimes these are also found in the oral disc.

Similarly, the body has a layer of gelatinous texture called mesoglea that allows the anemone to be flexible, allowing it to withstand the strong currents of the seabed, or to retract or expand. The latter is one of the most distinctive characteristics of anemones: their ability to close and open.

Tentacles

The tentacles are extensions that are arranged in concentric rings around the oral disc. A curious fact is that generally the number of tentacles that an anemone has is a multiple of six.

It is important to mention that the tentacles have cells specialized in synthesizing and secreting toxins (actinoporins). These cells are called cnidocytes and form organelles called nematocysts.

Nervous system

The nervous system of anemones is quite rudimentary, taking into account that they are one of the most primitive members of the animal kingdom. These organisms do not have specialized receptors, except for some chemoreceptors.

Anemones have two nerve networks that unite at the level of the pharynx. One runs through the gastrodermis and the other through the epidermis.

Muscular system

Anemones do not have muscle fibers proper, but certain contractile fibers. These can be of two types: circular and longitudinal.

The circular fibers are primarily embedded in the body wall, although in some species, they are also found around the oral disc.

On the other hand, the longitudinal fibers are located in the oral disc, the tentacles and in the mesenteries.

Digestive system

Members of the order Actiniaria have an incomplete digestive system. This has a single opening, which is the mouth, through which food particles enter and waste substances are also released.

Immediately after the mouth is the actinopharynx, which occupies a reduced length of the body. This continues with the gastrovascular cavity, which is quite wide.

Here in the gastrovascular cavity, the mesenteries that divide it secrete digestive enzymes that contribute to the digestion of food or ingested prey.

Reproductive system

It is quite rudimentary, since it is found inside the mesenteries. Within these there are some tissue fragments that are identified as the gonads of the animal. It is there where the gametes are generated, which are expelled to the outside through the mouth of the anemone.

Habitat

Jose Luis Cernadas Iglesias

Anemones are mainly found at the bottom of the seas, forming part of coral reefs. In large reefs such as the Great Barrier Reef off the Australian coast there are a large number of specimens and different species of anemones.

Likewise, sometimes they are held together through their feet to objects found on the seabed, such as sunken ships. Similarly, anemones are particularly abundant in the tropical zone where the seas have slightly warmer temperatures.

In general, anemones prefer to occupy small spaces, such as crevices, where they can remain semi-hidden. Similarly, there are numerous species of sea anemones that prefer a pelagic habitat, that is, close to the surface.

Feeding

Sea anemones are carnivorous animals and predators of the smallest animals in their habitat. They feed on fish, mollusks and crustaceans. The most common way of feeding is to paralyze the prey with the help of its tentacles and the toxins that they synthesize and secrete through nematocytes.

The way in which its feeding process occurs is as follows: the prey is trapped by the tentacles and immobilized by the toxin that they secrete. Later it is attracted towards the mouth, where it passes to the gastrovascular cavity.

There it is subjected to the action of the large amount of digestive enzymes that are synthesized in the mesenteries. The digestion waste, that is, the remains that are not usable by the anemone are regurgitated and released through the mouth to the outside environment.

The favorite prey for anemones are snails and slugs, as they are very easy to capture, as well as digest.

Reproduction

In the group of sea anemones there are two types of reproduction: asexual and sexual.

Asexual reproduction

This type of reproduction can occur through several processes, including budding, laceration, and binary fission.

Gemmation

Budding is a process of asexual reproduction in which a bump begins to appear somewhere in the anemone, which is known as a gem. From it the new individual begins to develop. Once it is mature enough to fend for itself, it breaks off the parent anemone, attaches itself to the substrate, and begins to thrive.

Laceration

This is a fairly straightforward playback mechanism. It consists in that the foot of the anemone detaches a portion, from which a new individual will begin to form. Perhaps the explanation for the success of this type of reproduction is that the anemones have undifferentiated cells that have a great totipotency.

Totipotent cells have the ability to transform into any type of cell according to the needs of the organism in question. This is why, when the foot fragment is detached, the totipotent cells there are activated and they begin to differentiate and specialize in the different cell types until they form a new anemone.

Binary fission

This is a fairly routine asexual reproduction process that involves one organism dividing into two. Two individuals similar to the initial anemone will originate from each half.

Sexual reproduction

It is important to note that there are species of anemones that present separate sexes, that is, there are female individuals and others male. On the other hand, there are also species that are hermaphrodites.

Sexual reproduction occurs as follows: male individuals release sperm into the water, thus stimulating the female to release the unfertilized eggs. This expulsion is done through the mouth.

In the sea, sperm and eggs meet and fertilization occurs, with the consequent fusion of gametes.

Likewise, there are also species in which fertilization is internal, that is, it occurs within the body of the individual.

Larvae, which are free-living, begin to form and develop within the fertilized eggs. This means that they can move freely in the sea. These larvae are known as planulas. This name is due to its flat shape. They also have cilia, which help them in their movement of movement.

Subsequently, the planula larva attaches itself to the substrate and transforms into a polyp, which is one of the two morphological forms that members of the phylum cnidarians can adopt during their life cycles.

The anemone itself develops from the polyp, through the differentiation of the different tissues that make it up.

Relations with other living beings

Despite the fact that anemones are known marine predators and that their tentacles secrete a fairly powerful toxin against other animals, some of the significant relationships they establish with other living beings such as some fish and crabs are also well known.

Anemone Mutualism - Clownfish

Mutualism is a positive interspecific relationship that is established between two organisms. In this, both benefit, without either of them providing any harm to the other. In this case, both the sea anemone and the clownfish benefit from each other.

As is well known, clownfish are quite colorful, having shades ranging from dull brown to bright red. Likewise, they present white lines, which help the fish stand out on the seabed and therefore attract various predators.

Clown fish swimming between the tentacles of an anemone. Source: Baruc Acosta

However, thanks to the fact that the clownfish can live among the anemone's tentacles, it can escape the attack of its predators, since these are not immune to the toxin secreted by the anemone.

Now, the benefit that the anemone obtains from the clownfish is the following: when the fish swims between the anemone's tentacles, it is constantly producing currents of water that increase the oxygenation of the tentacles, as well as the approach of the particles of food into the mouth.

Anemone mutualism - crab

Another of the most famous mutualistic relationships of the anemone is the one it establishes with the so-called hermit crab (paguroids). This crab is characterized by making use of dead snail shells and placing its body inside them to protect itself. However, this protection is not enough, so the crab is easy prey for its predators, among which is the octopus.

In some crabs of this type, the anemones are attached to the shell. The benefit that the crab gets is that the anemone protects it from predators with its tentacles and the stinging substances they produce. On the other hand, the anemone takes advantage of the movement of the crab in order to have access to a greater variety of prey.

It is important to note that, because the anemone is a sessile organism that remains fixed to the substrate, it cannot have a very varied diet. However, anemones that attach to the shell of crabs, move with them along the seafloor and can have a more diverse diet.

Anemone toxin: actinoporins

Anemones synthesize, at the level of cnidocytes, the toxins that serve as their defense. These toxins are known by the name of actinoporins and are very toxic and stinging for those who come into contact with them.

It is important to note that this toxin is synthesized by cnidocytes and is stored in nematocysts. Inside these there is a tube that ends in a needle. It is through that needle that the toxin is inoculated into the prey.

The action exerted by actinoporins is as follows: when they come into contact with the cells of some animal tissue, several actinoporin molecules unite and manage to cross the cell membrane, forming a pore and the consequent death of that cell.

In this sense, it is correct to affirm that actinoporins have a cytolytic action on the cells they attack. Likewise, they also have hemolytic action, since they drastically and irreparably destroy red blood cells.

References

1. Carter, D. (1965). Actinias from Montemar, Valparaíso. Montemar Valparíso Biological Journal. 12 (1-3). 129-159.
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
4. Quiroz, Y. (2005). Studies of the toxins of the sea anemone Anthothoe chilensis. Universidad Mayor de San Marcos. Lima Peru.
5. Zamponi, M. (2005). Study of the sexual reproduction of sea anemones (Actiniaria) and the strategy of the poor man. National University of Mar de Plata. Argentina.
6. Zamponi, M. (2004). Sea anemones and other polyps. Chapter of the book “Life between tides and animals of the coast of Mar e Plata, Argentina.