Geographical and Ecological Distribution
Sea anemones live throughout the world’s oceans, from poles to equator, and from the deepest trenches to the shores, as do fishes. But no one kind of either lives in all places. Of nearly 1000 species of sea anemones, only 10 are host to anemonefishes. They live in the parts of the Indian and Pacific Oceans that lie within the tropics or where warm, tropical waters are carried by currents, such as the east coast of Japan (as far north as the latitude of Tokyo!). Because the 28 species of clownfishes live only with these 10 species of sea anemones, they are found in the same places.
These anemones, and their anemonefishes, exist only in shallow water, no deeper than scuba-diving depths. That is because within the cells of an anemone’s tentacles and oral disc live microscopic, single-celled, golden-brown algae (dinoflagellates) called zooxanthellae. Like all plants, they require sunlight for photosynthesis, a process in which solar energy is used to make sugars from carbon and water. Some of these sugars fuel the algae’s metabolism, but most of them “leak” to the anemone, providing energy to it. Therefore, the anemones that are host to clownfishes must live in sunny places. The amount of light in the sea diminishes rapidly with depth because water filters out sunlight. Turbidity also diminishes light penetration. So these anemones live at depths of no more than about 50 m, generally in clear water. (Reef-forming corals also contain algae, and coral reefs occur only in shallow, mostly clear water for the identical reason.)
Anemonefishes live in habitats other than reefs, but are usually thought of as reef dwellers because that is where most tropical diving occurs. Other habitats may be less colourful and diverse than reefs, but they can be equally fascinating. About as many species of host actinians (= sea anemones) live on sand-flats surrounding coral reefs, or even at some distance from reefs, as live on reefs themselves. Individuals of some species can survive in muddy areas, but they generally lack fish symbionts. Even on reefs, most species of host actinians are inconspicuous, unlike their partner fish. Spotting the fish first, then frightening it so that it takes refuge in its anemone, or (preferably) waiting patiently for its periodic bathe among the tentacles, is often the best way to locate an actinian.
How is this relationship possible?
At the time of Collingwood’s discovery, some species of fishes and anemones involved in this relationship had been known to science for a century already. Why had nobody reported their living arrangement before? We can only speculate. Perhaps poisons had been used to collect the fish, which causes them to float to the surface, so nobody could know where they had come from. Perhaps collectors saw fish living in anemones but did not appreciate its significance. Or quite possibly it was seen and simply not believed, so unlikely is an anemone as home to a fish.
Lovely, accessible — and a most unlikely partnership. Sea anemones are related to corals and more distantly to jellyfishes. Common to all of these animals are nematocysts, the harpoon-like stinging capsules that give jellyfish their sting, fire coral their burn, and the tentacles of some sea anemones their stickiness. The microscopic nematocysts, which are manufactured inside cells (but are not themselves cells), are particularly dense in tentacles and internal structures. Those of the tentacles function in defence and prey capture; internal nematocysts are essential to digestion. Within each capsule is coiled a fine tubule many times the capsule’s length. When the capsule is stimulated to fire (a combination of chemical and mechanical stimuli is necessary to trigger most kinds; there are over 30 in all), the tube shoots out, everting like the sleeve of a coat turned inside out, to penetrate or wrap around the target. Many types of nematocysts, although probably not all, contain toxins, which are delivered to predator and prey by or through the everting tubule.
The existence and function of nematocysts were known before the anemonefish symbiosis was described. And so, when Collingwood first reported “the discovery of some Actiniae of enormous size, and of habits no less novel than striking,” his prime concern was with how the fish managed to survive in an environment that is deadly to most fishes, even some much larger than anemonefishes.
Over the years, many biologists have suggested ways in which it might be possible for the fish to survive in its hostile environment. Among the hypotheses [and reasons for discarding them] were the following.
1) Tentacles of these particular anemones do not contain nematocysts. [Not only are there nematocysts, but those of all 10 species of host actinians are typical in kind and quantity to those occurring in the majority of sea anemones.]
2) The fish do not actually touch the tentacles. [While this is certainly true of some Caribbean fish that seek protection behind and under sea anemones, genuine anemonefishes swim among tentacles, and sleep on the oral disc at night.]
3) The skin of anemonefishes is thicker than normal so nematocysts cannot penetrate it. [It differs little from that of other damselfishes, and may even be slightly thinner. Indeed, an unprotected anemonefish can be killed by its host’s sting.]
4) While a fish is present, the anemone will not fire its nematocysts. [Although a sea anemone can exert some control over firing, this cannot be the solution to the riddle, because an actinian can sting and capture prey while harbouring clownfish.]
Anemonefishes are easily kept in aquaria, many of which are as large as the fish’s normal territory. Both fishes and sea anemones survive — apparently quite well — when separated from one another. However, if the separation lasts more than a few days or weeks, depending on the species involved, when the partners are reunited and the fish swims into the host’s tentacles, it withdraws rapidly, appearing (sometimes very obviously, sometimes less so) to have been stung. Thus the protection of the fish is elicited or acquired, and can disappear. A fish that had been living alone will be stung by an anemone in which another clownfish is being harboured, so the fish, rather than the actinian, is responsible for the protection.
But a stung anemonefish returns to its host repeatedly, going through an elaborate, stereotyped swimming dance, gingerly touching tentacles first to its ventral fins only, then to its entire belly. Finally, after a few minutes to several hours of such “acclimation” behaviour, it is able to dive right in.
Some anemonefishes nibble at their host’s tentacles, which it had been speculated might immunize them against the sting. But the fish are not immune to being stung, as is sometimes stated. Immunity is a physiological response that extends throughout an animal’s body. Experiments by Davenport and Norris conclusively proved that the protective agent resides in the mucus coating that anemonefishes, like all fishes, have on their surface. But what is the source of this protective mucus?
One theory is that it comes from the host actinian. Supporters of this theory believe that during its elaborate “acclimation” swimming when contact is initially made with its host, the fish smears mucus from the anemone all over itself. Just as the sea anemone does not sting itself, it does not sting a fish, or any other object, covered in its mucus. The fish is thereby chemically camouflaged: it is, essentially, a fish in anemone’s clothing. The fish’s normal behaviour of returning to its anemone at least once a minute can be interpreted as serving to maintain its protective layer of mucus. According to this theory, what allows clownfishes to live in this peculiar habitat is their unusual behaviour.
Finding anemone mucus on many objects with which the animal regularly comes in contact, such as the rocks and algae around it, other scientists believe that its presence on a fish is the result of the fish’s being protected rather than its cause. The fish’s own mucus has evolved to lack components that stimulate nematocyst discharge, according to this theory, and “acclimation” behaviour may be an artifact of artificially separating animals that normally never are parted. The secret to clownfishes’ peculiar habitat, according to this interpretation, is their unusual biochemistry.
As in so much of science, there is probably truth on both sides. Although all anemonefishes are closely related and share an unusual habitat, they vary in some aspects of their biology, including how far they venture from their home, how many fish occupy a single anemone, and which hosts and how many host species they occupy. Similarly, they may not all adapt to an actinian in the identical manner, as is generally assumed, with behaviour and biochemistry probably both playing roles to varying degrees. We believe that for fish that live with many types of hosts, behaviour is likely to be more important to adaptation, whereas for host-specific fish, biochemistry is probably the more significant factor.
However, actinians, like all coelenterates, capture and digest animal prey with their nematocysts. We have found small fish, sea urchins, and a variety of crustaceans (shrimps and crabs) in the coelenteron of host anemones. They also appear to feed on planktonic items conveyed by the currents. Although the energy they derive from photosynthesis may be sufficient to live, the anemones need sulfur, nitrogen, and other elements in order to grow and reproduce. These animals are not voracious predators: their prey probably consists of animals that bump into them (e.g. a fish fleeing a more active predator) or stumble over them (e.g. a sea urchin, which has no eyes). Therefore, the supply is probably small and irregular. A more predictable source of these nutrients may be from wastes of their symbiotic fish. This issue deserves to be studied scientifically. Anemones of some species are capable of absorbing nutrients directly from seawater through their thin tissues, and that may be another source of nutrition for these animals as well.
It is impossible to determine age of a sea anemone, except for one that has been raised in an aquarium or tracked continuously in the wild from first settlement. A small one is not necessarily young, for coelenterates grow only if well fed and shrink if starved. Individuals of species that harbour anemonefishes have been monitored for several years with no apparent change in size (although that is difficult to measure, due to the absence of a skeleton). However, studies on other species, in field and laboratory, have led to estimated ages on the order of many decades and even several centuries. There are scattered records of temperate anemones surviving many decades in commercial aquaria, and the life-span of a small sea anemone in New Zealand has been calculated, based on actuarial tables, to be over 300 years! From such data, it is likely that most individuals of the “gigantic” sea anemones we have encountered during our field work exceed a century in age. This is also consistent with the generalization that large animals of all kinds typically are long-lived.
Coelenterates are protected quite well by their nematocysts, but some predators have developed means of evading their effect. Small tropical anemones may be eaten by butterflyfishes, but large ones appear to have few enemies, and we do not know what might ultimately kill them.
All coelenterates reproduce sexually. An individual of some species may produce both eggs and sperm; host anemones appear to have separate sexes, with an individual being either male or female its entire life. The typical coelenterate pattern is that of most marine animals, one that is fraught with dangers and uncertainty — release of eggs and sperm into the sea, where fertilisation occurs and a larva (a tiny animal looking nothing like its parent that drifts in the sea) develops for several days or weeks before settling in an appropriate habitat. Many species spawn in response to an environmental cue such as a full moon or low tide so that eggs and sperm are in the same place at the same time. Typically, marine animals produce millions of tiny larvae, but the world is not overrun with them, proving that very few survive — usually just enough to maintain a stable population. The rest of the larvae serve as food for a sea full of potential predators. Finally, the surviving larvae must find an appropriate habitat.
We do not know if host actinians follow this pattern. There is a bit of evidence that in at least some species, the eggs are not released, but are fertilised inside the mother (this is not especially rare in corals and anemones; sperm enter the mother with water that is constantly being pumped in and out, and which carries food and oxygen also), where they grow to be released as tiny sea anemones. What is certain is that we seldom see small individuals of most host actinians in nature. However, it is not unusual to find large ones with ripe eggs and sperm. Therefore, we believe that successful recruitment must be rare. Very few eggs may be fertilised, or few larvae may survive, or larval settlement may be difficult, or young anemones may have high mortality (perhaps especially when they are too small to harbour fish). The apparent rarity of successful reproduction is also biologically consistent with long life.
In addition to sexual reproduction, some coelenterates undergo asexual reproduction. Entacmaea quadricolor is one of these. A polyp can divide longitudinally, resulting in two, somewhat smaller individuals, probably within the space of a few days. Each then grows to an appropriate size, divides, and so on. All descendants of the original anemone (the result of sexual reproduction) form a clone, a group of genetically identical individuals. In this species, each polyp is relatively small, but clonemates remain next to one another so their tentacles are confluent, and the associated anemonefish apparently regard them as a single large anemone.
This is so mainly for shallow-water individuals; those in deeper water grow large, and do not divide. Several other species of actinians also have two different reproductive modes: small animals that clone and large ones that do not. This appears true of Heteractis magnifica, too. In the center of its range (i.e. in eastern Indonesia, on the Great Barrier Reef, in New Guinea), it occurs as single, large individuals. To the east and west (i.e. in western Indonesia and Malaysia, and in Tahiti), several to very many small individuals of identical colouration are typically clustered together, appearing to be a single large (or huge!) anemone. Based on their shared colour and their proximity, we infer that they are clonemates.
Once they settle from the plankton, most anemones seldom move from place to place. Although they are usually damaged when people try to collect them, actinians do have the ability to detach from the substratum, partly or entirely. Small, temperate anemones can do this in response to predators or unfavorable physical factors. Indeed, those of a few species can “swim,” awkwardly launching themselves into the water briefly, a motion that often puts them beyond reach of the predator that provoked the activity. More typically, an individual glides on its pedal disc, covering a few millimeters in a day, or it may detach entirely, and roll or be carried quite a distance. That this is not terribly rare is attested by large animals suddenly appearing in well studied areas.