Overview

Brief Summary

These sessile Cnidaria occur throughout the world oceans in deep and shallow water from the equator to the poles. They have extremely pliable bodies that can undergo drastic, though slow, shape changes, thanks to the thick mesoglea (gel) within their body wall. The properties of the mesoglea allow sea anemones to resist sudden forces, like a strong ocean current, but to adapt their shape deliberately if needed to open for feeding or to become compact for protection. (Vogel, 2003)

Sea Anemones may be best known for their symbiotic relationship with clownfishes or anemonefishes, from the Genus Amphiprion and the Genus Premnas. Only ten of the >1000 species of Actiniaria are known to associate with clownfish, and all ten of them are shallow-water dwellers, because they all have another symbiotic relationship in common- they all host single-celled algae within their tissues. These algae produce sugar through photosynthesis (which is why this partnership must live in shallow water with abundant sunlight), and share it with their sea anemone host in exchange for shelter. This kind of relationship is quite common in Cnidaria. Reef forming corals also host algae this way. (Fautin and Allen, 1992)

The relationship between sea anemones and clownfish is more unusual. Sea anemones, like corals and other Cnidaria, have stinging cells in their tentacles, but clownfish living with them are not stung. How they manage this is not well understood, but it appears to be related to a mucus coating their skin. Researchers disagree over whether this is produced by the fish, or produced by the anemone and gradually picked up by the fish. A clownfish does take some time to adapt to an anemone host at first, touching and then retreating from the tentacles until gradually it settles among them unharmed.(Fautin and Allen, 1992)

The benefit to the clownfish is clear. Predators will not pursue them among their host's stinging tentacles, and in nature clownfish are rarely seen far from a host anemone. In some cases, the anemone also benefits. When clownfish were removed from their anemone host Entacmaea quadricolor in Papua New Guinea, within a single day, the anemones had been decimated, apparently by butterfly fish, which were still hunting for leftover scraps when researchers returned. (These fish are also protected from the stinging cells of Cnidaria, like the corals they usually feed on, and evidently, also this sea anemone.) Other species of sea anemone host clownfish some of the time but thrive without them also, and may not benefit from the relationship.(Fautin and Allen, 1992)

Small crustaceans, including crabs and shrimp sometimes also associate closely with sea anemones. These small animals have proven to be nimble and elusive, so researchers don't yet know very much about these relationships.(Fautin and Allen, 1992)

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Physical Description

Diagnostic Description

Sea Anemones are benthic sessile polyps without a skeleton. They are solitary; with the exception of one colonial species, Cereus herpetodes, which is known from Chile. The proximal end of the sea anemone is either rounded, in which case the species is buried in soft substrate, or forms a more or less well developed flat pedal disc, which it uses to attach to hard substrate. The column is smooth or provided with hard structures such as verrucae, tenaculi, tubercles, vesicles, marginal spherules, marginal pseudospherules, or marginal projections.

The oral disc (at the distal end) is usually circular; in some cases it can be drawn out into lobes (eg: Anthologa achates). The tentacles are generally simple, hollow, and usually arranged in hexamerously alternating cycles. They arise from the margin and/or the oral disc, and nearly never possess spherical tips. Some species possess special fighting tenacles that can be everted or retracted; they bear large amounts of special cnidae used for defense. Colour and markings of the animals are highly variable.

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Evolution and Systematics

Functional Adaptations

Functional adaptation

Relationship protects from predators: clownfish, anemones
 

Clownfish and anemones gain protection from predators thanks to their mutualistic relationship.

     
  "For protection, clownfish seek refuge amongst the tentacles of sea anemones. The tentacles contain harpoon-like stinging capsules called nematocysts that the anemones employ to capture prey and ward off predators.

In a yet-to-be resolved biological mystery, clownfish have mucus on their skin that somehow protects them against the sting of their host anemone. As a result, the clownfish are able to stick near their host which is avoided by most other fish in the sea.

'The clownfish gets protection by hiding sting-free among the tentacles. If you remove the clownfish, large butterfly fishes will eat the anemone,' said John Randall, an ichthyologist at the University of Hawaii at Manoa.

Butterfly fish are predators of the sea anemone. In certain areas of the tropics where clownfish, sea anemone, and butterfly fish exist, clownfish scare off butterflyfish from their host anemone. Research has shown that if the clownfish are removed from the anemone, butterfly fish will move in and devour the anemone. So, the protection of the anemone afforded by the clownfish is part of the mutual relationship.

In addition to scaring off predators, some scientists speculate that clownfish waste may serve as a nutrient for the anemones…

There are more than 1,000 species of sea anemones found throughout the world's oceans. Only ten of these species share their niche with clownfish, which thrive in the tropical waters of the Indian and Pacific oceans.

Each individual host anemone is home to one group of clownfish, which contain a dominant breeding pair and up to four smaller, subordinate fish. There are 28 known species of clownfish, so more than one species of clownfish may take to any given species of anemone." (Roach 2003)
  Learn more about this functional adaptation.
  • Fautin, D. G. 1991. The anemonefish symbiosis: What is known and what is not. Symbiosis. 10(1): 23-46.
  • John Roach. 2003. No Nemo: Anemones, Not Parents, Protect Clownfish. National Geographic News [Internet], Accessed August 27, 2007.
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Functional adaptation

Supportive gel enables extreme shape change: sea anemone
 

The supportive gel-like substance (mesoglea) of sea anemones allows extreme shape changing due to its viscoelasticity.

     
  "Consider a solid material with properties and role about as distant from bone as a supportive, compression-resisting material can be. The body wall of a sea anemone--which can be quite substantial in size--consists of inner and outer surface layers separated by the thick mesoglea. One doesn't go far wrong viewing the system as a tall can of seawater whose walls are mostly made of jelly…A typical anemone has a rare facility for changing shape, ranging from a low barrel to a tall cylinder with a few flourishes in between, over times ranging from seconds to hours…Obviously its mesogleal stuffing must participate in the process. Muscle drives some of the shape changes, in particular the sudden expulsion of water in the central cavity from its single apical opening. But tracts of cilia drive other changes, such as reinflation by pumping water back in. You may recall thatciliary pumps produce exceedingly low pressures, and here we're asking that they pump up creatures that may reach half a meter in height and live in moving water.

Alexander (1962) showed the crucial role of mesogleal viscoelasticity for anemones. In creep tests on samples, strain increased from an initial value of about 0.2 to a final level ten times that, achieved after around 10 hours. That means the mesoglea has a lot of viscosity relative to its elasticity--it's hard to make it do anything fast but fairly easy to make it change shape slowly. It has a retardation time (calculated by Biggs; see Vincent [1990]) of a little under an hour. How nice! The pulsating or reversing flows of waves passing above won't sweep it about very much, but after it has hunkered down, the low-pressure ciliary pump will be adequate to pump it back up again, albeit slowly. It can stand up to a single wave but deflect in a tidal current that imposes the same drag. Furthermore, the anemone's body wall can resist the stresses of its own short-term muscle contractions, so it can bend or straighten without getting an aneurysm whenever its muscles aren't active." (Vogel 2003:360-361)
  Learn more about this functional adaptation.
  • Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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Functional adaptation

Body changes shape: sea anemone
 

The central cavity of sea anemones is reinflated by water pumping in at low pressures thanks to ciliary pumps.

     
  "Consider a solid material with properties and role about as distant from bone as a supportive, compression-resisting material can be. The body wall of a sea anemone--which can be quite substantial in size--consists of inner and outer surface layers separated by the thick mesoglea. One doesn't go far wrong viewing the system as a tall can of seawater whose walls are mostly made of jelly…A typical anemone has a rare facility for changing shape, ranging from a low barrel to a tall cylinder with a few flourishes in between, over times ranging from seconds to hours…Obviously its mesogleal stuffing must participate in the process. Muscle drives some of the shape changes, in particular the sudden expulsion of water in the central cavity from its single apical opening. But tracts of cilia drive other changes, such as reinflation by pumping water back in. You may recall thatciliary pumps produce exceedingly low pressures, and here we're asking that they pump up creatures that may reach half a meter in height and live in moving water.

"Alexander (1962) showed the crucial role of mesogleal viscoelasticity for anemones. In creep tests on samples, strain increased from an initial value of about 0.2 to a final level ten times that, achieved after around 10 hours. That means the mesoglea has a lot of viscosity relative to its elasticity--it's hard to make it do anything fast but fairly easy to make it change shape slowly. It has a retardation time (calculated by Biggs; see Vincent [1990]) of a little under an hour. How nice! The pulsating or reversing flows of waves passing above won't sweep it about very much, but after it has hunkered down, the low-pressure ciliary pump will be adequate to pump it back up again, albeit slowly. It can stand up to a single wave but deflect in a tidal current that imposes the same drag. Furthermore, the anemone's body wall can resist the stresses of its own short-term muscle contractions, so it can bend or straighten without getting an aneurysm whenever its muscles aren't active." (Vogel 2003:360-361)
  Learn more about this functional adaptation.
  • Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:988Public Records:288
Specimens with Sequences:340Public Species:15
Specimens with Barcodes:239Public BINs:23
Species:91         
Species With Barcodes:28         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Actiniaria

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