The Anemonefish-Anemone Partnership: Not Just Clowning Around

Species Updated : Apr 16, 2025

Beyond the lively contrasting colours and animated behaviour of clownfish lies the complex world of the anemonefish-anemone relationship, an innovation that has revolutionised the lives of these fish

Have you heard of clownfish, the small and highly energetic fish often with orange and white bands? They’re sometimes referred to as the “Nemo fish” after the protagonist in the film Finding Nemo.

Clownfish are easily one of the most well-known and adored marine animals globally. Seemingly always happy to see you at their coral reef, dashing out of their homes as if to greet you. Clownfish are also a photographer’s delight, with their contrasting colours, animated behaviour, and no apparent fear when approached closely. But after the first few sightings of a clownfish, many people find that the novelty quickly fades. Could this be because encountering them is not rare enough, and perhaps their behaviour is too repetitive? Unsurprisingly, clownfish, like so many commonly found animals we tend to glaze over, are extraordinarily complex!

Anemonefishes typically lay their eggs on sturdy surfaces within reach of their host’s tentacles for added protection from predators. The breeding male takes responsibility and cares for the eggs by fanning, cleaning, and oxygenating the area. Video: Umeed Mistry

Cover Photo: The anemonefish-sea anemone partnership goes back millions of years. A mutualism held together by a microbial connection protecting the anemonefish from their partner’s sting. Swimming through the tentacles ensures that an anemonefish’s immunity never fades. Photo: Dhritiman Mukherjee

Why clownfish?

Clownfish are named for their bright bands and blotches that resemble the painted faces of circus clowns. As a group, clownfish are related to other small ray-finned fishes called damselfishes, with whom they form the taxonomic family Pomacentridae. Around 12-20 million years ago, the ancestor of today’s clownfish did something other Pomacentrid fish were not doing at the time. They entered a live-in partnership with a distantly related group of marine animals — sea anemones. This was an important turn of events, especially for the clownfish, who are also called anemonefish.

The partnership

Life on a coral reef comes with numerous challenges, from dealing with predators to constantly competing for food, space, and shelter. In such a cutthroat environment, innovation and cooperation can often be the key to long-term survival. The anemonefish-anemone partnership is one such innovation that revolutionised the lives of these fish. In this partnership, anemonefish behave like full-time residents in the anemone, taking shelter between the lethal tentacles of their host. In return, anemonefish aerate the anemone with their movements, keep it clean by removing fallen sediments, and do their part in deterring the anemone’s predators. Although not initially apparent, this partnership has a third stakeholder, microscopic zooxanthellae, that live inside the anemone’s tissues. These algae use the ammonia-rich faeces of resident anemonefish for photosynthesis. A good portion of the food zooxanthellae manufacture is shared with their host anemone. A win-win-win scenario.

(1) The black-finned, (2) spine-cheeked, and (3) Clark’s anemonefish are among the 28 species of anemonefish found in the tropical coral reefs of the Indian and Pacific oceans. Photo: Umeed Mistry (1), Samuel John (2), Dhritiman Mukherjee (3)

Anemone membership cards

How an anemonefish manages to escape the lethal effects of its host’s tentacles is a question that has boggled the minds of scientists for decades. It was known that the key to not getting stung lay in a protective mucous coating lining the anemonefish’s body, but the exact mechanisms at work are only now being uncovered.

Each anemonefish living within a particular anemone contains a unique collection of bacteria in its skin that is shared and recognised by its host. In essence, each anemonefish carries in its mucous a signature microbial membership card specific to its host anemone, which allows it to live unharmed. The anemone detects it as one of its own. Partnering fish and anemones share this microbiome through direct contact and chemicals the anemone releases in its surrounding waters.

The magnificent anemone has a wide oral disc with a centrally placed mouth protected by venomous tentacles. At night, when the anemone retracts itself, the anemonefish increases water flow within the anemone by constantly fanning its fins and swimming around. Photos: Samuel John (1), Vardhan Patankar (2)

For anemonefish, the benefits of living in partnership with sea anemones have clearly accrued. Anemonefish lifespans go up to 30 years. This is more than double the lifespan of other damselfish and six times greater than other reef fish of the same size. Recent genetic studies have also found that the evolution of this partnership may have allowed the anemonefish group to rapidly evolve and diversify into so many different species in the last five million years (which, geologically speaking, is a very short time).

Social lives of anemonefish

Although not immediately evident, anemonefish social groups are less like families and more like strict dominance hierarchies. There may be anywhere between 2-7 anemonefish in this social unit, and while there is a small chance that some are siblings that hatched there, most arrive as fresh, young recruits from elsewhere.

(1) From the smallest nonbreeding anemonefishes (at the back of the anemone far end of the anemone) to the largest dominant female (largest and maroon-coloured), each fish in an anemonefish hierarchy has designated roles based on their rank and works hard not to lose their rank in the system, or worse, be evicted. (2) Rank in these hierarchies is constantly enforced by the breeding female through aggressive interactions like by the breeding female charging, chasing and biting. Often, anemones extend their symbiotic relationship to crustaceans like this porcelain crab that ends up cleaning the anemone by feeding on food scraps. Photo and video: Samuel John

Right at the top of an anemonefish hierarchy is the largest individual, the dominant breeding female. The next biggest fish, the breeding male, follows her. Next in the descending order of rank comes each of the nonbreeding fish, some of whom are males and some whose sex is not yet determined. Rank is decided based on body size, physical fitness, and degree of involvement in group work.

Only the dominant breeding female and male will mate during the breeding season, and will continue to do so each season for life. But this changes the moment one of them dies. What takes place next in the lives of the remaining fish is extraordinary and scientifically known as “protandrous sequential hermaphroditism”. If either the dominant breeding female or male disappears, the fish next in the hierarchy will begin to undergo physical and physiological changes in its body and take the vacant spot. If the dominant female dies, the highest-ranking male transforms into a female. If the dominant male dies, the top-ranking nonbreeder will turn into the dominant male. A similar promotion in rank will follow all the way down to the smallest fish.

A false clownfish (*Amphiprion ocellaris*) inhabits a bleached sea anemone in the shallow tidepools of Havelock Island during the mass bleaching episode in June 2024. Photo: Sarah Abidi

High stakes

Let us pause to ask a few questions: Why do fish who rank low in the hierarchy put up with the aggression of the dominant ones? Couldn’t they quit and become the dominant ones in other anemones? As it happens, if lower-ranked males decide to leave and venture out for better prospects, life outside of the anemone is no picnic either, and chances of survival are extremely low. A nonbreeder is more likely to become a dominant male and then a female one day by sticking with the same anemone for the long haul. Good behaviour, like helping with defence, cleaning, and taking care of eggs, even when not theirs, ensures they stay safe and alive until they ascend the hierarchy. Anemonefish stick around for years, waiting for their turn to inherit the territory and have their own offspring.

Let’s not judge a clownfish merely by their cuteness or widespread occurrence. The numerous complexities in the lives of these fish, hiding in plain sight, are even more fascinating.