Box 191 Sea Anemones As Hosts And Guests

Hosts

One of the best known and loved associations on coral reefs is that between large tropical sea anemones and the small, brightly coloured fish known as 'clownfish' or 'anemon-efish'. Fish and sea anemones live together in a close association known as 'symbiosis', where both species benefit from the arrangement and concessions are made by the partners to make it possible for the two to coexist. Ten species of sea anemone are known to play host to anemonefishes. These come from the families Stychodactylidae (species of Heteranthus and Stichodactyla), Actiniidae (Entacmaea quadricolor and Macrodactyla doreen-sis) and Thalassianthidae (Cryptodendrum adhaesivum) (see Fig. 19.3 for some of these species). Like most sea anemones, these hosts have a well-developed defensive system based on nematocysts within the ectoderm of the tentacles. The fish swim around among the tentacles despite this hazardous feature and benefit from the protection from larger predators that would be harmed by the tentacles. The anemones themselves have also been found to benefit by protection from predators due to the defensive activities of the anemonefishes and a number of other ecological advantages have been documented.

At least 28 fish species are involved in anemone associations throughout the Indo-Pacific, all from the family Pomacentridae and most from the genus Amphiprion (see Figs 19.3B, 19.4). Some anemonefish are quite host specific, but most can occur in many hosts. A hierarchy exists among the fish species themselves, which means that certain species are much more likely than others to successfully colonise and monopolise the available hosts. Other factors too, such as habitat and geography, play a role in the availability and likelihood of settlement in a host of a particular fish species. The fish species differ in the way they approach new hosts, but all have some mechanism for developing a tolerance to the sting of the anemone via a mucous surface coating developed following gradual contact with the tentacles.

Guests

There are also numerous associations of sea anemones with other host organisms, where again the anemone's stinging capabilities provide some protection for the host and the host gives the anemone access to resources. One thing that is difficult for anemones (at least after the larval phase) is moving around and some of them avoid this problem by living in association with motile animals such as slow-moving molluscs or mollusc shells inhabited by hermit crabs.

of the animal), until the animal is unresponsive to touch. Permits are required to collect, see Chapter 12.

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