BIOLOGY OF SEA ANEMONES
Sea anemones that are host to clownfishes, like many tropical actinians and some temperate ones, harbour unicellular algae within the cells of their tentacles and oral disc (see Introduction). A portion of the sugars produced by these plants through photosynthesis are "leaked" to their host. This may be the anemone's major source of energy. The widely flared oral disc of many host actinians serves not only to accommodate fish, but its large surface area is well adapted for intercepting sunlight.
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 (see chapter 5), 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 (how anemonefishes might do this is discussed in chapter 4).
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 (see chapter 1). 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.
Sea anemones are very similar to corals. One of the solitary mushroom corals, Heliofungia actiniformis, extends its tentacles by day (most do so only at night) and looks very much like an actinian (hence its specific name). Could it not harbour clownfish? In an aquarium lacking an anemone, we did have a clownfish take up residence among a mushroom coral's tentacles. But it does not happen in nature. However, a small, snow-white pipefish, Siokunichthys nigrolineatus, lives among the tentacles, much like an anemonefish.
Corals and sea anemones differ not only in the respective presence and absence of a skeleton, but also in the types of nematocysts they possess and in their anatomy. Intermediate between them are corallimorpharians, skeletonless polyps having coral-like nematocysts and anatomy. In fact, some coelenterate experts regard corallimorpharians simply as corals without skeletons.
The corallimorpharian Amplexidiscus fenestrafer, the largest species known, resembles some host anemones. Bill Hamner first documented their feeding behaviour, in which prey is rapidly enveloped by the oral disc that closes like a draw-string purse. The mouth is then opened, the prey being swallowed and killed within the polyp. On several occasions he kept corallimorpharians in the same aquarium with anemonefishes, only to discover the following day that a fish was missing. He found that at night a fish settled onto the corallimorpharian's oral disc, just as it would with a host anemone, thereby provoking the draw-string response and subsequent ingestion. Their superficial similarity to some host anemones, and their living in areas of the reef where plankton accumulates, led us to speculate that these corallimorpharians might "lure" naive anemonefish larvae to attempt to settle in them. Of course, the possible mimicry of a host anemone, and the resultant predation on naive fry, requires that at least some anemonefish larvae recognize hosts at least in part visually (see chapter 4).
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Proceed to Chapter 4