sea anemones text index | photo index
Phylum Cnidaria > Class Anthozoa > Subclass Zoantharia/Hexacorallia > Order Actiniaria
Sea anemones
Order Actiniaria
updated Mar 13
if you learn only 3 things about them ...
A sea anemone is an animal and not a plant. Don't step on it.
Some sea anemones can sting. Don't touch them.
Sea anemones can be found in all kinds of places. Look for them.

Where seen? A wide variety of sea anemones can be seen on all our shores. Tiny sea anemones often wedge in crevices on rocky shores. Sandy shores teem with sea anemones; some small and well camouflaged or hidden in the sand. Seagrass meadows often have carpet anemones that may be bigger than your face! Large colourful sea anemones are also common in coral rubble areas. Sea anemones may also settle on shells occupied by hermit crabs or even living snails.

What are sea anemones? Sea anemones are to Phylum Cnidaria, which includes jellyfishes and corals. Sea anemones belong to the Class Anthozoa that includes Peacock anemones and hard corals. There are about 1,350 known species of sea anemones. They are found from deep to shallow waters, attached to hard surfaces or burrowing into the ground.

Features: Most are small, from 1.5-10cm long and 1-5cm in diameter. But some can be 50cm in diameter or more! Unlike hard corals which are colonies of small polyps connected to one another that produce a hard skeleton, sea anemones are larger solitary polyps that don't produce a hard skeleton. Like jellyfish and other cnidarians, sea anemones have tentacles with stingers. What we first notice of the sea anemone is a broad, flat disk. This is called the oral disk because that is where the mouth is, at the centre of disk. The mouth is usually a slit. Sea anemones don't have teeth.

The upper side of the oral disk is usually covered with lots of hollow tentacles. The tentacles are armed with stingers that are used for feeding and self-defence.

Sea anemones also have a long smooth body column. The other end of the body column may end in a flat muscular pedal disk that attaches to a hard surface. Some of these sea anemones can move slowly by gliding on the pedal disk. Sea anemones that burrow into the ground may have a bulbous tip instead to help it burrow and stay anchored.

Part or most of the body column may be buried or hidden. In sand, mud or crevices in rocks or coral rubble. The body column may have bumps or spots (called verrucae). In some, these are sticky. These help the animal grip the surroundings where it is buried, or to keep the oral disk spread out, flat against a hard surface.

In most sea anemones, the body column can retract towards the base to hide from predators or minimise exposure at low tide. Most can also tuck their tentacles and oral disk into the body column. When small anemones do this, they look like beads of jelly. Others can simply retract their entire bodies into a hole, crevice or into the sand. This is usually done by expelling fluids so that the tentacles and body deflate like balloons. To inflate again, sea anemones have special body structures to pump in and retain water.

Sometimes mistaken for soft corals. Some large sea anemones and large leathery and flowery soft corals may be mistaken for one another. Here's more on how to tell apart large soft cnidarians on the shore.

Some animals look like sea anemones but are not, e.g., peacock anemones, corallimorphs and zoanthids. Here's more on how to tell apart animals with a ring of smooth tentacles.

Stingers! Many sea anemones have nematocysts or stingers that can inject toxins. They may also have a kind of stinger that produces a long adhesive thread (called spirocysts). These make the tentacles sticky and are used to entangle hard-bodied prey such as crabs that may blunder into them. Stingers are concentrated on the tentacles.

What do they eat? Sea anemones may also use their tentacles or mucus to trap small particles, detritus and plankton from the water.

But large ones especially, can capture and swallow prey such as fishes whole. Sea anemones have stingers like other Cnidarians. Prey is captured and immobilised with these stingers. Tentacles may push larger prey into the central mouth. The edges of the mouth may be inflated into 'lips' that pucker to hold prey as it is swallowed. The mouth and body column can expand wide to accommodate the prey whole. Or the anemone may fold its oral disk over the prey. Like other cnidarians, the sea anemone lacks an anus. So it has to spit out any indigestible bits through the mouth.

Should I 'save' animals trapped in an anemone? Please don't. If you do, you will be depriving the anemone of a meal. It might not get so lucky again for a while. The animal that you 'saved' might also not survive if it was badly stung by the anemone.

Should I feed the anemones? Please don't. Anemones know how to feed themselves. You might hurt the anemone if you put the wrong thing on it, for example a toxic animal. If you put another living animal on an anemone you will be hurting two animals. Please don't put objects such as litter or dead crabs on an anemone either.

Farm in their arms: Many sea anemones also harbour symbiotic single-celled algae (called zooxanthellae) in their tentacles. The algae undergo photosynthesis to produce food from sunlight. The food produced is shared with the sea anemone, which in return provides the algae with shelter and minerals. The zooxanthellae are believed to give sea anemone tentacles their brown or greenish tinge.

Anemone friends: Some anemones may live with other animals such as hermit crabs and living snails. Other animals have adapted to live among the tentacles of sea anemones. The Anemonefish (Amphiprion sp.) is coated with mucus that does not trigger off the host sea anemone's stingers. Other creatures that also make their homes in sea anemones include anemone shrimps (Periclimenes sp.).

Anemone Babies: Most sea anemones are hermaphrodites, but act as one gender at any one time. That is, they produce either sperm or eggs during one reproductive period. Fertilisation may be external, or the eggs may be fertilised inside the anemone. The eggs develop into free-swimming larvae that eventually settle to the bottom and develop tentacles.

Some sea anemones can reproduce asexually by detaching a portion of their body, such as the pedal disk, or even by dividing into two. The detached portion eventually grows into new sea anemones.
The Swimming anemone (Boloceroides mcmurrichi) can regenerate a new anemone from a dropped tentacle. But not all sea anemones do this, so please don't mutilate sea anemones.

Human Uses: Unfortunately, these beautiful creatures are popular in the live aquarium trade and many are harvested from the wild for this trade. Studies of their fascinating structures have medical applications, such as the use of their stingers to inject substances through human skin.

Status and threats: None of our sea anemones are listed among the endangered animals of Singapore. However, like other animals harvested for the live aquarium trade, most die before they can reach the retailers. Without professional care, most die soon after they are sold. Those that do survive are unlikely to breed successfully. Like other creatures of the intertidal zone, they are affected by human activities such as reclamation and pollution. Trampling by careless visitors, and over-collection also have an impact on local populations.

The glass anemone has
very long tentacles.
Beting Bronok, Jun 10


The branched-tentacle anemone
has branched tentacles.
Sentosa, May 08


Sea anemones are distinguished
by features of the underside as well.
Beting Bronok, Aug 05


Some anemones are tiny and lie
half buried in the sand. Don't step on them!
Chek Jawa, Oct 04


The Swimming anemone really does swim!
Pulau Sekudu, Jun 06


The Wiggly reef star anemone has few tentacles.
Pulau Hantu, Apr 06


Carpet anemone eating a sand dollar
Chek Jawa, Feb 04


The Tomato anemonefish
lives with the Bubble tip anemone
Pulau Semakau, Aug 08



Some anemones live on
shells occupied by hermit crabs.
East Coast Park, Jun 15

So do anemone shrimps!
Kusu Island, Jul 04

Anemonefishes live happily
with sea anemones.
Kusu Island, Jun 04

Order Actiniaria recorded for Singapore
text index and photo index of sea anemones seen on Singapore shores

Links

References

  • Daphne Gail Fautin, S. H. Tan and Ria Tan. Dec 2009. Sea anemones (Cnidaria: Actiniaria) of Singapore: abundant and well-known shallow-water species. The Raffles Bulletin of Zoology. Pp. 121-143.
  • Edward E. Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
  • Pechenik, Jan A., 2005. Biology of the Invertebrates. 5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
  • Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore. National Council on the Environment. 163pp.
  • Ng, P. K. L. & Y. C. Wee, 1994. The Singapore Red Data Book: Threatened Plants and Animals of Singapore. The Nature Society (Singapore), Singapore. 343 pp.
www.flickr.com
FREE photos of sea anemones. Make your own badge here.
links | references | about | email Ria
Spot errors? Have a question? Want to share your sightings? email Ria I'll be glad to hear from you!
wildfactsheets website©ria tan 2008