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Symbiosis - Actinians And Their Fish. A classic marine scenario!

There are many types of sea anemone, somewhere in the vicinity of 1000 species. Of these, there are but a few which are obvious to the casual snorkeller or SCUBA diver, often with attention only being attracted by an anemone's symbiotic 'clown fish'. Of the vast number of anemone species only ten play host to these anemone fish. It is these host anemones which will be concentrated upon here.

Members of phylum Cnidaria, class Anthozoa and order Actiniaria, host sea anemones fall into three families:

Habitat

As sunlight and warmth are vital to the survival of anemones they are generally found within tropical and temperate regions or areas where currents regularly take these warmer waters. Clear water to the normal depths available to SCUBA divers seems to provide the most hospitable environment in which to live.

Amphiprion perideraion in Heteractis magnifica. Lady Musgrave Island, Australia. (CI000640) Encounters with anemones on and amongst coral reefs are those predominantly experienced. This is due to the fact that these are the areas in which a majority of recreational diving occurs. However, many specimens can be found on the sand flats between reefs and even on areas away from reefs.

Although many anemones are found tucked into holes or cradled amongst rocky reefs this protected life is not favoured by all specimens. Some (especially Heteractis magnifica) can often be encountered on reef areas exposed to currents that make diving difficult.

Biology

The physical makeup of a sea anemone is quite simple. It basically resembles a cylinder, closed at one end and partially open at the other. The closed end - the base of the cylinder - is referred to as the 'pedal disc' which the anemone fixes to a solid perch on a reef or buried within the sand. Rising from the pedal disc - the walls of the cylinder - is the body of the anemone or 'column'. Atop the column is the 'oral disc' from which grow the tentacles. These tentacles vary in form, size, and number from species to species, providing key indicators in the identification of anemones. In the centre of the oral disc is the mouth which leads to the throat or 'actinopharynx' and the body cavity of the anemone which functions as the single 'organ' providing all required bodily functions.

Mouth Of An Anemone. Lady Musgrave Island, Australia. (CI000641) Sea Anemones are Cnidarians (Phylum Cnidaria) and like their coral and jelly fish relatives they have specialised cells that contain stinging barbs, or nematocysts. The sting from these nematocysts is rarely enough to be felt by humans unless they come in contact with an open cut or scratch but is deadly to the creatures upon which an anemone preys. These nematocysts exists in high propertions in the tentacles and internal digestive areas.

Sea anemones are not active hunters. They may kill and digest prey with their nematocysts but only that which strays accidentally into their grasp. A more abundant food source is the nutrients leaked from the algae which live in their oral disc and tentacles. This algae or 'zooxanthellae' provide nutrients via photosynthesis as well as giving the cells in which they live an olive/brown hue. Thus, well lit, clear water and wide oral discs (to absorb as much light as possible) are important. The anemones may also use waste products from their symbiotic partners as a source of food.

Amphiprion melanopus in Entacmaea quadricolor. Vila Harbour, Vanuatu. (CI000264) Generally anemones reproduce sexually with each individual having only one sex. Eggs and sperm are released separately into the water where they mix and the eggs become fertilized. A fertilized egg develops into a larva which floats on the currents until it eventually settles as a small anemone. Some species, however, do not release eggs. Rather, they retain the eggs and take in sperm with the normal flow of water through their system. The eggs become fertilized, develop, and are eventually released as tiny anemones into the water to settle in some suitable site.

Anemone Fish

Although fish associated with sea anemones are affectionately known as clown fish there is only one true Clown Fish, i.e. Amphiprion percula. The others are referred to as 'anemone fish'. They all fall into the order Perciformes and family Pomacentridae - Damsel Fishes. The 28 species of true anemone fish are members of the subfamily Amphiprioninae - 27 within the genus Amphiprion and one within the genus Premnas - P. biaculeatus. There are also two species of Dascyllus - D. trimaculatus and D. albisella - that are sometimes observed within host anemones. These are members of the subfamily Chrominae and are not dependent upon an anemone for survival in nature.

Amphiprion clarkii. Milne Bay, New Guinea. (CI000270) Anemone fish vary in size from species to species but generally never exceed about 15cm in length. They live together in a group within the confines of their host anemone. The largest within a group will be the only female and the next largest will be her partner, the breeding male. Others within the group will all be males and generally diminish in size - a reflection of their position in the 'pecking order' of the group. If the female dies the breeding male changes sex and grows accordingly. The other males also grow as their position in the group is now increased. The largest of these becomes the new breeding male. This size/order relationship is somewhat dependent upon the smaller fish being unable to get as much food as the larger ones as well as their ongoing evasion of harassment from the larger fish.

When breeding, the female may lay between 100 and 1000 eggs with older fish tending to lay more. These eggs typically incubate for six to seven days being tended and guarded by the male with sporadic help from the female. The larvae hatch about three to four millimetres in length. They launch into the water to be carried by currents over the next eight to twelve days during which time they develop into juvenile anemone fish. They then settle to the bottom and seek an anemone to be their host. There are several possibilities for this search. A juvenile may simply swim around hoping to discover a suitable anemone or may visually search for an appropriate host. Some species also follow a chemical trail released by a compatible anemone. The juvenile may be lucky enough to find a vacant anemone but more typically must fit into the bottom of an existing matriarchal group.

Symbiosis

Amphiprion percula in Heteractis crispa. Milne Bay, New Guinea. (CI000138) The word 'symbiosis' means 'living together'. This relationship may be mutual or one sided and will always benefit one party but may be neutral, beneficial, or even detrimental to the other. In the case of host anemones and their fish, the fish definitely benefit as they are unable to survive 'in the wild' without a host anemone. Although the anemones do benefit it is to a lesser degree as they can survive and are often seen without accompanying fish. An exception to this is Entacmaea quadricolor which without accompanying fish may fall prey to hungry butterfly fish.

While other fish seem deterred by the nematocysts of an anemone the anemone fish are seemingly unaffected. The reason for this arises from the mucous that covers an anemone fish. It provides a mechanism that prevents the anemone from stinging the fish. The exact cause is not yet established but possibly comes from one or a combination of two theories. Firstly, mucous generated by the anemone rubs off on the fish making it look 'anemone like' to the anemone thus preventing the fish from being stung. Secondly, the fish itself generates a mucous which subdues the stinging reaction of the anemone. Most likely a combination of these mechanisms exists. When a fish is first introduced to an anemone it can not rest comfortably within the tentacles. It seems to be stung. Over a period of several minutes to a few hours the fish performs a 'dance' that involves more and more contact with the anemone's tentacles until it can swim amongst them without hindrance.





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Gosliner T.M., Behrens D.W. & Williams G.C. Coral Reef Animals of the Indo-Pacific Sea Challengers. Monterey, CA 93940. ISBN 0 930118 21 9
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