Sea anemones (Heteractis magnifica) competing for habitable space on the reef.

Anemones are invertebrates that provide important habitats for other reef organisms that live in symbiosis with them. They have similar morphological and anatomical features to the polyps of their close relative - Coral. Although part of the same phylum as soft corals (Anthozoa), interestingly, they are more closely related to hard corals due to similarities in the arrangement of their tentacles

These tentacles are common features, often easily observed swaying in currents and surge. These tentacles are full of stinging cells called nematocysts, microscopic harpoon-like structures that are fired off when the tentacles come into contact with prey items such as plankton. These nematocysts contain neurotoxins that cause cell death, and are used to immobilise the anemone’s prey. Once stunned, tentacles are then used to wrap around the prey item and bring the new meal towards their large central mouth. Although catching prey like this is an effective feeding method, anemones also live in symbiosis with Zooxanthellae, a microscopic alga. Zooxanthellae provide the anemone with energy from photosynthesis and receive essential nutrients from the anemone in return.

Anemones often form additional symbiotic relationships with other marine organisms including anemone fish and certain species of shrimps. In a mutualistic manner, anemones provide anemonefish with protection as seen in the photograph above. Unlike many of their own their predators, anemone fish are immune to the neurotoxins released by nematocysts and are able to hide at the base of the tentacles when threatened. In return, the fish defend the anemone from its predators. Findings from studies on this mutualism may also suggest that the conspicuous colouring and patterns often seen on anemonefish may aid in attracting the anemone’s prey items, and that the waste products from the anemone fish provide essential nutrients to the host anemone. Furthermore, studies have also suggested that movements from the anemonefish swimming amongst the tentacles of the anemone are likely to increase the circulation of water, which would in turn assist in the exchange of dissolved respiratory gases.

Specialised tentacles called Acrorhagi are also found on several anemone species. These are used specifically for fighting. Anemones form attachments to rocky substrates. As space on the reef is a limited resource, anemones often engage in contests with other individuals. During these contests, the Acrorhagi inflate and are used as weapons to strike their opponent. Contractions of the body allow the anemone to strike their competitor with these weapons, causing localised death to the area they come into contact with. Eventually, the weaker individual will withdraw from the fight, therefore giving up its space to the stronger individual.

Like other Anthozoans, anemones are able to reproduce both sexually and asexually through fission. By forming clones through asexual reproduction, anemones have the ability to form colonies. One particular species (A.elegantissima) has demonstrated the ability to form armies to fight other colonies and establish new settlement areas. During these colonial wars, clones are sent across the colony border in search of new areas to settle. When a battle with another colony begins, a more specialised warrior individual more heavily armed with Acrorhagi weapons is cloned to fight. In the centre of the colony the more vulnerable but highly important reproducer clones work hard. These individuals are able to receive messages that will determine whether they are to produce more scouts to help in the search for new areas, or warriors to join the front-line battle.

 -- Click here to see more of Piers' underwater photos of marine organisms. --