Clownfish have evolved a neat trick to live safely among stinging anemones

For years, scientists and ocean lovers alike have marveled at the unique relationship between clownfish and sea anemones.

The small, brightly colored fish seem to move fearlessly among the stinging tentacles of their hosts, completely unharmed. Their ability to thrive in this seemingly dangerous environment has fascinated researchers for over a century.

Sea anemones use specialized cells called nematocysts to deliver powerful stings to unsuspecting prey and predators. These stings can be deadly to most marine creatures, yet clownfish remain untouched.

How do these fish avoid being stung? What makes them immune to a defense mechanism designed to kill? Scientists have now uncovered the secret behind this remarkable adaptation, revealing an intricate and finely tuned biological process.

A protective layer in clownfish mucus

Researchers from the Okinawa Institute of Science and Technology (OIST) and international collaborators made a remarkable discovery.

The team found that clownfish produce mucus with exceptionally low levels of sialic acid. This sugar molecule, present in most living organisms, plays a crucial role in cellular communication. However, it also serves as a trigger for anemone stings.

Interestingly, sea anemones themselves also lack sialic acid in their mucus. This suggests they may have evolved this trait to avoid stinging their own tentacles. By maintaining similarly low levels of this sugar compound, clownfish are essentially “invisible” to the anemone’s stinging cells.

The study, published in BMC Biology, provides strong evidence that clownfish have evolved a defense strategy that mimics their hosts.

Clownfish-anemone relationship

To explore this adaptation, the researchers examined both clownfish and damselfish – a close relative that does not live with anemones.

The team used glycobiology, which focuses on the study of sugars, along with transcriptomics, which analyzes the expression of genes through RNA.

By measuring mucus samples, the experts were able to identify key differences in composition between the two fish species.

Using liquid chromatography, an advanced technique for separating and analyzing complex mixtures, they found that clownfish have significantly lower levels of sialic acid in their mucus compared to damselfish.

While clownfish still retain sialic acid in vital organs like the brain and gut, their skin mucus remains nearly free of this compound. This prevents them from activating the anemone’s stinging response, allowing them to move freely among the tentacles.

Young clownfish get stung until they adapt

Anemonefish are not born with this special adaptation. The researchers discovered that juvenile clownfish, which have not yet established a relationship with anemones, possess normal levels of sialic acid. If they attempt to enter an anemone’s tentacles at this stage, they get stung just like any other fish.

As they grow and develop their signature bright orange and white coloration, their mucus chemistry changes. Their sialic acid levels gradually decrease, allowing them to safely integrate into anemone colonies. This transition marks a crucial stage in their survival, ensuring they receive the protection anemones offer against predators.

A similar pattern was found in the domino damselfish. As juveniles, these fish can temporarily live among anemones, and researchers discovered that their mucus also exhibits reduced sialic acid levels during this stage. This suggests that multiple fish species have independently evolved similar biological adaptations to coexist with anemones.

“Our findings represent a major advancement because it’s one of the first studies to combine glycobiology with transcriptomic analysis to investigate this mechanism,” explained Dr. Natacha Roux of CRIOBE, a former researcher at OIST’s Computational Neuroethology Unit.

Can bacteria help clownfish avoid stings?

While the absence of sialic acid in clownfish mucus explains much of their immunity, researchers are now investigating whether bacteria also play a role in this process. They have two main theories.

The first theory suggests that clownfish produce enzymes that actively remove sialic acid from their mucus. The second hypothesis points to bacteria in their microbiome that could help break down these sugars.

Previous studies show that when clownfish and sea anemones live together, their bacterial communities become increasingly similar over time. This supports the idea that bacteria may influence the fish’s ability to resist stings. If true, this would add another layer of complexity to the already intricate clownfish-anemone relationship.

Clownfish and anemones help each other survive

The symbiotic bond between clownfish and anemones is not just about avoiding stings. Both species benefit from their association in multiple ways.

Clownfish gain a safe habitat, protected from larger predators that avoid anemone tentacles. In return, the fish defend their hosts from anemone-eating species and provide nutrients by stirring up the surrounding water.

Professor Vincent Laudet, head of OIST’s Marine Eco-Evo-Devo Unit, emphasized the complexity of this relationship.

“Other factors might include the thickness of fish scales, the exchange of nutrients between species, and possible adjustments by the anemones themselves,” said Professor Laudet.

“The relationship is mutually beneficial, with anemonefish receiving protection from predators while helping to defend the anemone and providing nutritional benefits.”

Challenge of proving this hypothesis

While the research strongly supports the idea that low sialic acid levels help clownfish evade anemone stings, scientists want to confirm this by manipulating the process directly.

If they can artificially raise sialic acid levels in clownfish mucus and make them vulnerable to stings, or reduce the levels in damselfish to make them resistant, it would serve as definitive proof. However, this is easier said than done.

The process of altering mucus composition in live fish is complex, requiring precise genetic and biochemical interventions. For now, scientists continue to explore ways to test their findings in controlled environments.

A milestone in marine research

This discovery represents more than just a scientific breakthrough; it also marks the first major research paper from a new collaboration between France’s National Centre for Scientific Research (CNRS) and OIST.

The work provides new insights into one of nature’s most fascinating partnerships and could open the door to further studies on marine symbiosis.

For now, the clownfish remains a symbol of the ocean’s hidden wonders, demonstrating how nature continuously shapes life through adaptation and survival. As research continues, scientists may uncover even more about the secrets behind this remarkable fish and its unusual way of life.

The study is published in the journal BMC Biology.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–