ANEMONE MINE: The Amazing Partnership between Sea Anemones and Anemonefishes

[The Hemispheres]

You wouldn’t think that a primitive, boneless, animated tube would be anybody’s pick for a best friend, but that’s exactly what the sea anemone is to the anemonefish. Scientists have only recently begun to understand the fascinating relationship between anemonefishes and their invertebrate hosts.

Sea anemones are marine invertebrates of varied and often beautiful appearance, not unlike the flower from which they take the Greek part of their name: the anemone, or “daughter of the wind.” The nearly 1,000 species of sea anemone are members of the phylum Cnidaria (sometimes called Coelenterata instead). Cnidaria refers to the cnidae, or nematocysts – the stinging cells that are unique to this phylum.

Sea anemones belong to the class Anthozoa, which also includes the corals. Unlike the colony-building corals, however, sea anemones are often solitary dwellers, living anchored to a hard surface, such as a rock, or imbedded in the ocean floor. They are generally tube-shaped (a form called a polyp), with a single opening at one end. The pedal, or “foot,” end of the body is either pointed, for digging into sediments, or disk-shaped, for attaching to something firm. The disk oral end bears the mouth opening, through which all water, food and gametes pass. The mouth is surrounded by nematocyst-bearing tentacles, with which the anemone captures food and defends itself. The internal cavity of the sea anemone, which serves as stomach, lungs, intestines, circulatory system, etc., is laced with vertical partitions called mesenteries.

Anemones can grow up to a metre in width and live up to 100 years. Their reproductive methods are varied and not entirely understood. Although all anemones are sexed – male or female – and can release eggs or sperm into the water, depending on their gender, they can also reproduce by splitting off a piece of their pedal disk, or even dividing in half. In the case of reproduction from eggs, some anemones incubate them inside their body cavity until fully formed baby anemones emerge. The young swim until they find a likely spot to attach themselves to.

Ten species of sea anemone have evolved a symbiotic relationship with the 28 species of fish collectively known as anemonefishes. Twenty-seven of the anemonefish species belong to the genus Amphiprion, while the remaining one is the only recognized species of the genus Premnas. Anemonefishes are popular in aquariums because of their bright colors, often striped patterns and bobbing motion in the water, all of which gave rise to their commercial name of clownfish.

Although sea anemones are found everywhere – from the equator to the poles – the ten species of host anemone, as they are known, live only in the tropical latitudes of the Indian and Pacific Oceans and in current-warmed areas such as the east coast of Japan. Interestingly, the host anemone species are not closely related to one another. They come from three different families, the Actiniidae, Stichodactylidae and Thalassianthidae, none of which are made of exclusively of symbiotic species. Actiniidae is the largest sea anemone family, containing most of the common shore species. The Strichodactylidae family is found only in the tropics and includes most of the host anemones. Thalassianthidae is also tropical.

Anemones and anemonefishes live in coral reefs, sand-flats near reefs and even at distances from reefs. They are found only to depths of 50 m – coincidentally convenient for divers and scientists – because the algae that live in the anemone’s tentacles and oral disc need sunlight for photosynthesis. The surplus energy generated by these microscopic, single-celled zooxanthellae fuels the anemone, making it worth the invertebrate’s while to stay in shallow, clear waters.

The first published account of the symbiotic relationship between sea anemones and anemonefish appeared in 1868, in Rambles of a Naturalist on the Shores and Waters of the China Sea by Dr. Cuthbert Collingwood, who made his observations off the coasts of Borneo. Since then, the details of this remarkable living arrangement have slowly come to light. With their stinging nematocysts, the sea anemones would seem to be a forbidding choice for a home. The nematocysts are microscopic capsules found inside the cells of the anemone’s tentacles and body. Each capsule contains a length of tube that shoots out, when the necessary stimuli are given, to sting the stimulatory object. Some types of nematocyst contain toxins. In the face of such weaponry, how and why do the anemonefishes choose to make the sea anemone their home?

The why would seem to be for protection, as well as to feed on scraps of food from the anemone’s meals. The anemonefish shelters in the sea anemone’s stinging tentacles, limiting its territory pretty much to the immediate vicinity of its host. How it is able to do so with impunity has elicited a variety of theories from biologists. Some of the ideas that have turned out to be false include that some anemones do not have nematocysts; that anemonefishes have unusually thick skin; that the fishes never touch the tentacles; and that the anemone does not use its stingers while the fish is within its tentacles. In fact, it would appear that there is something about the mucus coating of anemonefishes that differs from the mucus coating of other fishes and protects them from stings.

Which, of course, leads to the question of what that difference is? There are two main hypotheses among scientists: one, that the fish have evolved a mucus coating that does not contain substances that would stimulate the nematocysts to sting, and the other, that the fish pursues a “courtship” of the anemone that involves enduring some initial stinging, until the fish is coated with the anemone’s own mucus, which camouflages it and allows it to move freely in and around its host. This would explain why anemonefishes tend to return to the shelter of the host’s tentacles about once per minute; in this scenario, they are renewing their protective coating.

Separations of fish and host have been observed in captivity. If the separation is prolonged, the fish will be stung when it tries to reunite with its host. In such cases, the fish re-courts the anemone, letting the tentacles first touch its fins and then its belly, before moving back in. The performance is ritualized and unvarying.

There may be no single correct answer to the mechanism by which anemonefishes are able to live with anemones. Some anemonefish species are host-specific, whereas others can dwell with many species of anemones. Some anemones play host to more than one anemonefish at a time, while others will drive away all but a single resident fish. With so many variables at work, it seems likely that the entire spectrum of explanations, from adaptation to behavior, comes into play, in varying degrees.

The next question, of course, is why sea anemones have evolved as hosts to anemonefishes. What advantage is it to harbour flashy fishes in one’s tentacles? First of all, the fish keep the anemone clean, ridding it of food scraps and parasites. Secondly, the fish chase away butterfly fish (which are immune to the anemone’s sting) and other predators that feed on anemone, such as turtles, starfish and nudibranches.

Some species of anemonefish live in mated pairs in one anemone, while others live in larger groups. There is a strict social structure, presumably evolved to ensure survival. At the top of the group is a large, dominant female, who mates only with the large males. She lays her eggs inside or close to the anemone, searching out and bringing in a suitable surface for laying her eggs, if necessary. Anemonefishes can change gender, when circumstances require it. When the top female dies, the largest male in the group will shift to female and take over her role. The advantage is obvious: since anemonefish cannot live for long away from the shelter of the anemone, leaving home to seek new mates is too risky. However, it has been observed that the member of a single group are not all related, which means that baby anemonefishes do travel from their birth site and find a home in a new anemone.

Anemonefishes are not the only sea creatures to take advantage of the sea anemone’s stinging power. Crabs, shrimps and other fishes, including young damselfish, may live in or with anemones. In one of the more interesting adaptations to have evolved, the boxer crab (Lybia tessellata) dons anemones like boxing gloves and uses them to beat away predators. However, no other group of species has so thoroughly united itself with the primitive but powerful sea anemone as the anemonefishes have.

The Ten Species of Host Anemone and their Anemonefish Species:

Cryptodendrum adhaesivum Adhesive Sea Anemone
Fish: A. clarkii

A very colorful species, with short, dense extremely sticky tentacles, often brightly and variously colored in combinations of green, fuchsia, pink, blue, etc. Found in Australia, southern Japan and Polynesia, Micronesia and Melanesia to Thailand, the Maldives, and the Red Sea.

Entacmaea quadricolor Bulb-Tentacle Sea Anemone
Fish: A. akindynos, A. allardi, A. bicinctus, A. chrysopterus, A. clarkii, A. ephippium, A. frenatus, A. mccullochi, A. melanopus, A. omanensis, A. rubrocinctus, A. tricinctus, Premnas.

The most widespread of the host anemones, this species is unique for the bulbs it often has on the ends of its tentacles. It may live as an isolated polyp or in a cluster with other polyps, especially in shallow water. It is found from Micronesia and Melanesia to South Africa and the Red Sea and from Australia to Japan.

Hecteractis aurora Beaded Sea Anemone
Fish: A. akindynos, A. allardi, A. bicinctus, A. chrysogaster, A. chrysopterus, A. clarkii, A. tricinctus.

This species takes its name from the lumps or swellings strung along its tentacles. It can burrow completely into sea sediment if its foot is attached to a solid surface below. Found from Micronesia and Melanesia to East Africa and the Red Sea, and Australia to the Ryukyu Islands of Japan.

Heteractis crispa Leathery Sea Anemone
Fish: A. akindynos, A. bicinctus, A. chrysopterus, A. clarkii, A. ephippium, A. latezonatus, A. leucokranos, A. melanopus, A. omanensis, A. percula, A. perideraion, A. polymnus, A. sandaracinos, A. tricinctus.

This species is distinguished by the quantity of its tentacles – up to 800 – which shrivel and turn green or grey when the anemone is disturbed. Found from French Polynesia, Micronesia, and Melanesia to the Red Sea, and Australia to Japan.

Heteractis magnifica Magnificent Sea Anemone
Fish: A. akallopisos, A. akindynos, A. bicinctus, A. chrysogaster, A. chrysopterus, A. clarkii, A. leucokranos, A. melanopus, A. nigripes, A. ocellaris, A. percula, A. perideraion.

A large, robust animal, well-known through photographs, thanks to its preference for exposed perches and its dazzling colors – including blue, green, red, white and brown. Found from French Polynesia to East Africa, and Australia to the Ryukyu Islands of Japan.

Heteractis malu Delicate Sea Anemone
Fish: A. clarkii

Characterized by sparse, short, usually magenta-tipped tentacles. Capable of retracting its entire, slender column into sediment. Found from the Hawaiian Islands to Australia and Japan.

Macrodactyla doreensis Corkscrew Tentacle Sea Anemone
Fish: A. chrysogaster, A. clarkii, A. perideraion.

This species has sparse, long tapered tentacles that often assume a corkscrew shape. Found from Japan to New Guinea and northern Australia.

Stichodactyla gigantea Gigantic Sea Anemone
Fish: A. akindynos, A. bicinctus, A. clarkii, A. ocellaris, A. percula, A. perideraion, A. rubrocinctus.

This species has a deeply folded oral disc and short, sticky tentacles that tend to vibrate constantly. Found from Micronesia to the Red Sea, and Australia to the Ryukyu Islands of Japan.

Stichodactyla Haddoni Haddon's Sea Anemone
Fish: A. akindynos, A. chrysogaster, A. chrysopterus, A. clarkii, A. polymnus, A. seba.

This species lives burrowed in sediment, with its oral disc just at the surface, and can disappear completely beneath the sand when alarmed. Found from the Fiji Islands to Mauritius, and Australia to the Ryukyu Islands of Japan.

Stichodactyla Mertensii Mertens' Sea Anemone
Fish: A. akallopisos, A. akindynos, A. allardi, A. chrysogaster, A. chrysopterus, A. clarkii, A. fuscocaudatus, A. latifasciatus, A. leucokranos, A. ocellaris, A. sandaracinos, A. tricinctus.

This is the largest anemone species, as far as oral disk diameter goes, with some specimens reaching more than one metre across. Found from Micronesia and Melanesia to East Africa, and Australia to the Ryukyu Islands of Japan.