Clownfish and Host Anemones

Roughly 28 species of clownfish associate with just 10 species of host anemone, and each clownfish species uses only a subset of those ten. The relationship is a form of mutualism: a symbiosis in which both partners benefit.

In the wild a clownfish seldom ranges more than a few meters from its anemone and retreats into the tentacles when threatened. The anemone shelters the fish and the eggs it lays beside it. In return, the fish drives off fish that prey on anemones and supplies the anemone with nutrients.

Other fish that touch the tentacles are stung by nematocysts, the stinging cells that fire a tiny venomous thread. A clownfish isn't, because of its mucus, the protective slime coat on its skin.

In aquariums neither partner depends on the other: a clownfish thrives without an anemone, and an anemone thrives without a clownfish. Most clownfish sold today are captive-bred, meaning bred and raised by people rather than collected from reefs, and many are selectively bred for color and fin shape. Longfin clownfish are one such bred form, with fins far longer than normal. Selecting captive-bred stock is covered in Module 24.

The bubble tip anemone, Entacmaea quadricolor, is the most widely kept host anemone, and in the wild it hosts more clownfish species than any other. It anchors its pedal disc (its foot) in a rock crevice with its tentacles extended into the light, in moderate flow. Large specimens may reproduce asexually by splitting into two.

Carpet anemones are larger. Stichodactyla haddoni lives on the sand in gentle flow and buries its pedal disc, so it requires a sand bed of adequate depth. S. gigantea and the magnificent anemone, Heteractis magnifica, require bright light and strong flow, and H. magnifica climbs the rockwork rather than settling on the sand.

Every host anemone harbors zooxanthellae, symbiotic algae in its tissue that photosynthesize and pass much of the food they produce to their host. Light is therefore measured at the anemone itself, and each species has its own range (Module 18). Anemones also require feeding (Module 26).

Anemones belong in a mature system, one that has run for many months with stable readings (Module 9), never a new one. Reef systems holding them are kept at 26 to 27 C.

Clownfish are protandrous hermaphrodites: every individual begins life as a male, and a male can later become female. Within a group, the largest and most dominant fish becomes the female, the second largest becomes the breeding male, and the remainder stay small and don't breed.

If the female dies, the male changes sex to become the female, and the next largest fish becomes the breeding male. The change from male to female is permanent.

A pair is usually formed by introducing two juvenile clownfish of the same species, one of which develops into the female. The female is dominant, and the male displays submission by quivering or shaking toward her. Some chasing and nipping within a pair is normal.

Keep one pair per system. Additional clownfish usually lead to conflict, and two pairs in one system typically end as one. Maroon clownfish are the most aggressive: two of similar size may fight to the death. With maroons, buy an established pair, or introduce a much smaller fish to a larger one.

A clownfish typically begins with brief contacts with the tentacles, withdrawing each time, and stays longer with each visit. Over the following weeks its mucus changes, and the anemone discharges fewer nematocysts against it.

Some clownfish host quickly, some take weeks, and some never host. Captive-bred fish that have never encountered an anemone can still host. A clownfish that doesn't host isn't unhealthy and doesn't need an anemone.

In aquariums a clownfish may host something else instead, including an anemone that isn't one of its natural hosts, the species it associates with in the wild. A hosting clownfish defends its anemone against other fish, and a carpet anemone hosting clownfish will still capture and eat other fish (Module 24).

A healthy anemone has a closed, tight mouth at the center of its oral disc. Its pedal disc is intact, with no tears, and attached to a surface. Its tentacles are full and adhere slightly to the touch, and it's fully expanded under the lights. It may be any color except white: a white anemone has lost its zooxanthellae and is starving.

Decline an anemone with an open or gaping mouth (hanging wide open), a torn pedal disc, a pedal disc attached to nothing, tentacles that don't adhere, or a white body, even as a rescue. An anemone attached to a rock is purchased with the rock, because tearing the pedal disc can be fatal. Ask how long it has been in the store.

A healthy clownfish feeds, swims normally, breathes evenly and has intact fins, with no spots or lesions (Module 24).

Before an anemone is introduced, cover every pump intake (where a pump draws water in) and every overflow (where water drains out of the display) it could reach. An anemone that wanders into a pump is likely to die (Module 22). Lift it by hand with the pedal disc supported; never pull it from a surface or catch it in a net (Module 25).

Place it near the position its species prefers, then allow it to settle. A new anemone may wander for weeks before it settles, and that alone isn't a concern. Once settled, a healthy anemone stays in place. An anemone that keeps moving is reacting to its light, its flow or the water: check all three. Never glue an anemone in place.

A new anemone is first quarantined, kept in a separate system for a period so that any disease becomes apparent before it can spread (Module 23), and it's never introduced directly into a system that holds an established carpet anemone.

Anemones can harm one another through allelopathy, the release of compounds that harm neighboring animals (Module 24). Carpet anemones kept near each other may become irritated: they fail to expand fully and deflate repeatedly. Separating them helps in some cases and makes no difference in others.

Clownfish frequently rest on the bottom and can appear unwell. Longfin clownfish rest on the bottom, or lie on their side, more than others. A healthy fish rises at once and feeds when food is added; a fish that doesn't respond to food warrants closer attention.

A male quivering toward the female is displaying submission, and some chasing within a pair is normal. A clownfish confined to a corner, with torn fins and kept from food, is being bullied.

An anemone deflates occasionally and then reinflates, and under the lights it should be expanded most of the time. A carpet anemone that deflates and reinflates repeatedly, or shows a gaping mouth, isn't behaving normally.

Cloudy water with a deflated anemone, while everything else in the system is normal, may indicate a spawning event, in which animals release eggs and sperm into the water, rather than disease (Module 27). Treating a sick anemone is covered in the next section; fish diseases are covered in Module 30.

Signs of infection. An infected anemone becomes limp. It deflates and stays deflated, or deflates and reinflates repeatedly, and its mouth hangs slack or gapes. The surrounding water may turn cloudy after the lights go out. An anemone irritated by a neighbor also deflates, but it remains firm rather than limp.

Collection and shipping are hard on host anemones, H. magnifica and S. gigantea in particular, and new specimens are often treated on arrival.

The stock solution. Ciprofloxacin is supplied as tablets. One 500 mg tablet dissolved whole in 500 mL of RO water gives a stock solution of 1 mg per mL, so 1 mL of stock per liter gives 1 mg/L. Store the stock refrigerated and protected from light, which degrades the drug.

Treatment in a container. An anemone that can be removed is treated in its own container of system water, with aeration and a heater. Ciprofloxacin kills nitrifying bacteria, so the container has no working biofilter: the water is replaced with system water, and only the new water is dosed.

Never allow the water to cloud between changes; change it before that point. A sick anemone may need a change every 6 to 8 hours. Remove anything it expels every hour with a turkey baster, a squeeze bulb on a long tube.

Dose by species. S. gigantea and H. magnifica receive 6.5 mg/L daily. S. haddoni is far more sensitive: the dose used for those species has killed most haddoni given it. Start a haddoni at 1 mg/L and raise the dose by 1 mg/L per 24 hours only if needed, to a maximum of 3 mg/L.

Treatment in the system. A haddoni that can't easily be removed from its system, and isn't showing severe signs, can be treated in place at a low dose: 0.125 mg/L every 12 hours for 7 to 14 days. Monitor every other animal in the system throughout.

Duration. Treat for at least five days, and complete the course even if the anemone appears to recover sooner. If the infection persists after several days, particularly as cloudy water after the lights go out, keep the anemone in darkness for 24 to 48 hours, because light is degrading the drug. Don't dose during the dark period unless the water has been replaced completely, and then dose only the new water.

Signs. A dying anemone has an open, gaping mouth, sometimes with internal tissue protruding. Its pedal disc detaches, its tentacles shrink and lose their stickiness, and it remains deflated for days. A dead anemone disintegrates and gives off a foul odor when lifted out.

Why it's urgent. As it decomposes, it clouds the water, consumes dissolved oxygen and releases ammonia, a nitrogen compound that's toxic to fish as it accumulates (Module 8). It can kill the fish even in a mature system.

Removal. Turn off the wave pumps, the pumps that circulate water around the display. Lift the anemone out with a fine net and siphon out every fragment, drawing it out through a tube.

Recovery. Clean the mechanical filtration (the socks, sponges or floss that trap particles) frequently over the following days. Test for ammonia, increase aeration, carry out a large water change and run fresh activated carbon, which adsorbs dissolved organic compounds (Module 17). Watch the fish for rapid breathing. When in doubt, do a water change.

An anemone in a pump. Switch the pump off first. If the anemone is intact, move it to a safe position and observe it for several days. If it has been torn apart, siphon out the pieces, clean the pump and check the fish.

Begin with what you observe. An anemone that keeps moving points to its light, its flow or the water. A pale anemone points to insufficient light, a sudden increase in light, or heat (Module 18). A shrinking anemone may be short of food or light. An anemone moving toward a pump is in immediate danger.

A clownfish that's torn and hiding points to aggression: too many clownfish for one pair, or two pairs in one system. A clownfish resting on the bottom that still rises to feed is normal; one that refuses food isn't.

Check the light at the anemone, the flow across it, the water records in the log, every intake, the other fish and any other anemones. Correct every cause you find, one change at a time, and confirm the result.