Sea anemones are a group of water dwelling, filter feeding animals of the order
Actinaria; they are called after the anemone, a terrestrial flower. As
cnidarians, sea anemones are closely connected to corals, jellyfish,
tube-dwelling anemones and Hydra.
A sea anemone is a conventional polyp: a small sac, bound to the bottom by an
adhesive foot, with a column shaped body ending in an oral disc. The mouth is in
the middle of the oral disc, enclosed by tentacles armed with numerous
cnidocytes, which are unique cells that function as a defence and as a way to
capture prey. ///
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Cnidocytes contain cnidae, capsule-like organelles capable of everting,
giving phylum Cnidaria its name.
The cnidae that sting are called nematocysts. Each nematocyst contains a
small vesicle filled with toxins—actinoporins—an inner filament and an external
When the hair is touched, it mechanically triggers the cell
explosion, that fires a harpoon-like structure which attaches to organisms that
trigger it, and injects a dose of poison in the flesh of the aggressor or prey.
This gives the anemone its characteristic sticky feeling.
The poison is actually a mix of toxins, including neurotoxins, which serve to
paralyze and capture the prey, which is then moved by the tentacles to the
mouth/anus for digestion inside the gastro vascular cavity. Actinoporins have
been reported as highly toxic to fish and crustaceans, which may be the natural
prey of sea anemones. In addition to their role in predation, it has been
suggested that actinoporins could act, when released in water, as efficient
repellents against potential predators. Clownfish are immune to an anemone's
The internal anatomy of anemones is very simple. There is a gastro vascular
cavity (which functions as a stomach) with a single opening to the outside which
functions as both a mouth and an anus: waste and undigested matter is excreted
through the mouth/anus. A primitive nervous system, without centralization,
coordinates the processes involved in maintaining homeostasis as well as
biochemical and physical responses to various stimuli. Anemones range in size
from less than 1¼ cm (½ in) to nearly 2 m (6 ft) in diameter. They can have a
range of 10 tentacles to hundreds.
The muscles and nerves in anemones are much simpler than those of other
animals. Cells in the outer layer (epidermis) and the inner layer (gastrodermis)
have microfilaments grouped together into contractile fibers. These are not true
muscles because they are not freely suspended in the body cavity as they are in
more developed animals. Since the anemone lacks a skeleton, the contractile
cells pull against the gastrovascular cavity, which acts as a hydrostatic
skeleton. The stability for this hydrostatic skeleton is caused by the anemone
shutting its mouth closed, which keeps the gastrovascular cavity at a constant
volume, making it more rigid.
Unlike other cnidarians, anemones (and other anthozoans) entirely lack the
free-swimming medusa stage of the life cycle: the polyp produces eggs and sperm,
and the fertilized egg develops into a planula that develops directly into
A few anemones are parasitic to marine organisms. Anemones tend to stay in
the same spot until conditions become unsuitable (prolonged dryness, for
example), or a predator is attacking them. In the case of an attack, anemones
can release themselves from the substrate and swim away to a new location using
The sexes in sea anemones are separate. Both sexual and asexual reproduction
may occur. In sexual reproduction males release sperm which stimulates females
to release eggs, and fertilization occurs. The eggs or sperm are ejected through
the mouth. The fertilized egg develops into a planula, which finally settles
down somewhere and grows into a single anemone. They can also reproduce
asexually by budding, binary fission, which involves pulling apart into two
halves, and pedal laceration, in which small pieces of the pedal disc break off
and regenerate into small anemones. Laceration is a process of fragmentation of
the basal disk, or by pulling itself into two parts.
The sea anemone has a foot which in most species attaches itself to rocks or
anchors in the sand. Some species attach to kelp and others are free-swimming.
Although not plants and therefore incapable of photosynthesis themselves, many
sea anemones form an important symbiosis with certain single-celled green algae
species which reside in the animals' gastrodermal cells.
These algae may be
either zooxanthellae, zoochlorellae or both. The sea anemone benefits from the
products of the algae's photosynthesis, namely oxygen and food in the form of
glycerol, glucose and alanine; the algae in turn are assured a reliable exposure
to sunlight, which the anemones actively maintain. The preponderance of species
inhabit tropical reefs, although there are species adapted to relatively cold
waters, intertidal reefs, and sand/kelp environments.
Most sea anemones form symbiotic relationships with crabs, shrimp or with
anemone fish, also known as clownfish. In the former situation, anemones will
either attach or be attached to the shell of a hermit crab (by the crab's own
volition), providing additional protection for the crab and allowing the anemone
to eat scraps when the crab feeds. A similar relationship can be formed between
a sea anemone and a clownfish.
The clownfish presses itself into the anemone,
living comfortably within the stinging tentacles. This is possible because of a
protective mucus that covers the clownfish. It is not yet fully understood
whether the protective mucus impedes the nematocysts from penetrating the flesh
of the clownfish or if the mucus suppresses nematocysts from firing. The
clownfish benefits from this symbiotic relationship because it is protected by
the anemone and eats its food scraps. Whilst doing this the clownfish cleans the
Marine Aquarists with Reef Aquariums often seek to acquire an anemone and
clownfish for their home aquarium. The majority of anemones in the aquarium
trade have been directly removed from a coral reef. In the wild, clownfish
depend upon the anemone for survival. The removal of anemones from coral reefs
is environmentally harmful to the clownfish in that area. Once an anemone is
removed from a reef, all clownfish that once inhabited that anemone will die. By
choosing to put an anemone in their home fish tank, the marine aquarist is
directly leading to the destruction of a coral reef.
Whereby, all symbionts of
the removed anemone; such as clown fishes, anemone shrimps, and anemone crabs
will be harmed or adversely impacted. In order to fulfil the demand of marine aquarists, collectors are removing
all of the anemones in some regions of the world's oceans. Anemones reproduce
extremely slowly, and it is common for collected specimens to be well over 100
years old. Based upon normal proliferation rates, this means anemones are
unlikely to ever replenish themselves in the regions where marine aquarist
collectors have over-harvested these creatures. Due to the short lifespan and
specialized needs of anemones, they do not live long in captivity. Authorities
in the marine hobbyist trade emphasize the fact that clownfish can live in
captivity without anemones.
Most Actinaria do not form hard parts that can be recognized as fossils but a
few fossils do exist; Mackenzia, from the Middle Cambrian Burgess Shale
of Canada, is the oldest fossil identified as a sea anemone.
- Campbell N. & J.
Reece (2002). Biology, 6th ed, San Francisco: Pearson Education.
- Shimek, R. (2004), p. 83. Marine Invertebrates. T.F.H.
Publications, Inc. Neptune City, NJ.