Clownfish and anemone: the ocean’s most famous symbiosis
A fish just a few centimetres long lives among the tentacles of an animal capable of paralysing other fish with its venom. It sleeps there, feeds there, lays its eggs right beside it. And nothing happens to it.
The relationship between the clownfish and its anemone is one of the best known in the animal world, yet what most people know about it comes from the movie. The reality is stranger, more interesting and, in places, quite different: in a real anemone, Nemo’s dad would have ended up becoming a mum.
In this article we explain how this symbiosis works, what each partner gets out of it, how clownfish organise themselves, what threats they face and how to photograph them without disturbing them.
Who’s who: the fish and the anemone
Clownfish (or anemonefish) are a group of around 30 species from the Indo-Pacific, nearly all in the genus Amphiprion. The anemones that host them are animals, not plants: relatives of corals and jellyfish that hunt with stinging cells called nematocysts. Only ten species of anemone serve as homes for clownfish.
Each fish species has its preferences. Among those seen in Bali:
Amphiprion ocellaris: the one that looks most like the famous Nemo and one of the most photographed.
Amphiprion clarkii: the most versatile; it can live in all ten anemone species and is widespread across the Indian and Pacific Oceans. It’s often photographed in Nusa Penida.
Premnas biaculeatus: the spinecheek anemonefish, a deep red fish that has also been documented in Tulamben.
The great mystery: why don’t they get stung?
For a century it was a mystery. It was known that the mucus covering the fish protected it, but not how.
The strongest explanation today, published in 2025 by researchers at Japan’s OIST and their collaborators, points to a sugar called sialic acid. It’s a molecule that triggers the anemone’s nematocysts. The clownfish keeps very low levels of sialic acid in the mucus on its skin, and the anemone itself does too, probably so it doesn’t sting itself.
The most curious finding concerns the larvae. Clownfish larvae have normal levels of sialic acid and do get stung if they approach an anemone. Only when they transform into juveniles, with their white stripes and orange colour, do those levels drop and let them in.
And here’s an important caveat: science is still refining the answer. Another study, also from 2025, found that the mechanism is more complex than the simple presence or absence of that sugar. And researchers still have to confirm why the fish keeps its levels so low, with two hypotheses on the table: its own enzymes that break it down, or bacteria in its mucus.
What each partner gets
What the fish receives | What the anemone receives |
Shelter among tentacles that ward off predators | Protection from fish, such as butterflyfish, that feed on its tentacles |
A safe place to sleep and to lay eggs | Cleaning of parasites |
Scraps of food from the anemone | Fertilisation from the ammonia in the fish’s waste, which also feeds the anemone’s symbiotic algae |
Oxygenation during the night | |
Direct food, in some cases |
Two details from the research make it especially interesting:
The fish aerates the anemone at night. When photosynthesis stops, oxygen drops. One study measured that, in the dark, the fish and anemone together consume around 40% more oxygen than apart, and another found that the fish’s movement eliminates the low-oxygen layer that forms next to the anemone. Interestingly, that effort benefits mainly the anemone, because the fish tolerates low oxygen very well.
Some clownfish feed their anemone. A study in southern Japan documented wild clownfish carrying pieces of prey to their anemone’s tentacles, and confirmed that this speeds up its growth.
It’s what biologists call mutualism: both partners come out ahead.
Social life: a matriarchy that changes sex
Clownfish don’t live alone. Each anemone holds a small group organised by size:
The female, the largest and dominant.
The breeding male, second in size.
One to three juveniles that don’t breed, smaller still.
Clownfish are protandrous hermaphrodites: they’re all born male and some become female. The aggression of the dominant pair slows the growth and sexual maturity of the subordinates, so they don’t compete to breed.
If the female dies or disappears, the breeding male changes sex and takes her place, and the largest of the juveniles becomes the new male. The change goes one way only: a female never goes back to being male.
This system was first described scientifically in 1977 and remains one of the best-studied cases of sex change regulated by social hierarchy.
And the movie? If Marlin had lost his partner in a real anemone, he would have ended up as the breeding female, and Nemo as the next male. Clownfish also don’t stray as far from their anemone as in the film: they spend nearly their entire lives glued to it.
From egg to home
The eggs are laid on rock, very close to the anemone. The larvae drift out into open water, where they spend a pelagic phase, and then they have to find an anemone. When they transform into juveniles, with their stripes and colour, their chemistry changes and they can live among the tentacles.
It’s a critical stretch of the life cycle: without an available anemone, there’s no future for the fish.
Threats: when the home disappears
That dependence is also their weak point. Clownfish need an anemone, and anemones are suffering:
Bleaching. In water that’s too warm, anemones, like corals, expel their symbiotic algae and turn white. Increasingly frequent heatwaves reduce their size and numbers, and without an anemone the fish loses its home.
Collection for aquariums. A 2008 study observed that reefs closed to fishing and collection had many more anemones and clownfish, up to 25 times the density, than reefs where they were allowed. Researchers also warned that demand for clownfish rose after the movie.
Coastal development and reef loss.
A 2022 scientific chapter sums up the risk: clownfish are threatened by anemone bleaching, ocean warming and acidification, aquarium collection and coastal development, and their extreme specialisation makes them even more vulnerable.
There is good news: clownfish breed easily in captivity, so anyone who wants one for their aquarium doesn’t need it to be taken from the reef. If you buy, make it captive-bred.
To understand bleaching better and what we can do about it, see our article on what dead coral is telling us.
How to photograph a clownfish without disturbing it
It’s one of the most rewarding subjects in macro photography, but also one of the most delicate:
Approach slowly and from the side. They’re territorial and sometimes charge the lens, but that’s a warning: if they do, back off.
Get at their level and focus on the eye, with the anemone’s tentacles out of focus behind.
Be patient. If you stay still, the fish comes back out.
Look after the anemone. Don’t touch it, push it aside or move it to make the fish come out. It’s their home and it’s easily damaged.
Don’t feed the fish or harass it to get the shot.
More technique in our guide to how to photograph marine wildlife underwater. And if you want to get the most out of your macro shots at home, there’s the Lightroom editing course.
Where to see them in Bali
Bali is one of the best destinations in the world for macro, and clownfish are a regular sight on its reefs: from Tulamben and Amed to Nusa Penida. It’s a destination where big animals, with manta rays and mola mola, live alongside an enormous variety of small life.
If you’d like to get to know the destination better, see our guide to diving in Bali.
Dive with clownfish with Born to the Ocean: Coral & Culture
Our Coral & Culture expedition explores Bali underwater and on land, with a special focus on macro photography:
Dates: 16 to 25 November 2026 and 3 to 12 September 2027 (10 days and 9 nights).
Diving: 12 dives, including a full day in Nusa Penida (Manta Point), the USAT Liberty wreck in Tulamben with a night dive, and reefs around Bali and Amed.
Photography: macro photography workshop with Borja, a personal photo session and a contest with prizes.
Culture: two days of cultural excursions, a Full Moon ceremony night and a farewell dinner with traditional Balinese dance.
Accommodation: beachfront hotel with breakfast.
Small group, accompanied by Born to the Ocean throughout the trip.
Level: recreational diving certification and previous experience.
From €1,490 (flights not included).
The November 2026 edition is just around the corner; if you’re interested, message us and we’ll tell you whether any spots are left. The September 2027 edition is already open.
You can see all the details on the Bali trip page and book or ask questions via WhatsApp. We reply within 24 hours.
Dive, travel, protect. 🤿
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