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.
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
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
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.
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.
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