Clownfish or anemonefishes (genus Amphiprion) are saltwater fish found in the warm and tropical waters of the Indo-Pacific. They mainly inhabit coral reefs.
Photo Credit: Sebastian Pena Lambarri
Scientific Frontline: Extended "At a Glance" Summary: Clownfish Anemone Bleaching
The Core Concept: Clownfish alter their behavior during coral bleaching events by prioritizing the defense and maintenance of their host anemones over social cohesion within their own groups. This shift occurs as a response to the stress of the anemones losing their symbiotic algae during marine heatwaves.
Key Distinction/Mechanism: Unlike ambient thermal stress, which has minimal direct impact on clownfish activity, the bleaching of the host anemone directly triggers a marked increase in both anemone-defending behaviors and intra-group aggression.
Origin/History: The research, led by Newcastle University researcher Fletcher Noble, was published in Royal Society Open Science in November 2026, based on video observations recorded during the Fourth Global Coral Bleaching Event in Kimbe Bay, Papua New Guinea, from March to July 2023.
Major Frameworks/Components:
- Behavioral Trade-offs: Energy allocated toward defending a mutualistic partner resulted in a 215% increase in aggressive behavior among members of the clownfish social group.
- Mutualistic Dependency: The symbiotic reliance between species for defense and nutrients, wherein clownfish aeration and cleaning behaviors may be critical for anemone recovery.
- Environmental Stress Response: The isolation of direct heat stress effects from the indirect behavioral effects caused by a stressed mutualistic partner.
Branches of Science: Marine Ecology, Behavioral Ecology, Symbiology, Environmental Biology.
Future Application: These behavioral metrics provide a baseline for future studies exploring how mutualistic species interactions might facilitate ecosystem recovery and resilience following severe marine bleaching events.
Why It Matters: As anthropogenic climate change increases the frequency of marine heatwaves, understanding how symbiotic species adapt their behaviors as a first line of defense is critical for predicting the survival of complex marine ecosystems.
Marine heatwaves and anemone bleaching are altering clownfish behavior, making clownfish more defensive of their host and more aggressive toward one another, new research reveals.
Led by Newcastle University and published in the journal Royal Society Open Science, the study explores how clownfish behavior changed during the Fourth Global Coral Bleaching Event. During this event, the clownfish experienced a heatwave and their anemone partners bleached, meaning they lost their symbiotic algae—a stress response to elevated temperatures.
The results indicate that while elevated temperatures only had a small effect on clownfish behavior, the bleaching response of the anemone altered how the animals spent their time. Bleaching led to more defensive behaviors, which could benefit the host anemone, and aggression between group members increased.
A group of Newcastle University students, led by PhD candidate Fletcher Noble, measured fish behavior from video recordings made during a marine heatwave in Kimbe Bay, Papua New Guinea, that lasted from March to July 2023. These recordings were made during the most extensive and intensive mass coral bleaching event in recorded history, spanning from 2023 to mid-2025 and known as the Fourth Global Coral Bleaching Event.
The team analyzed five behavioral categories: maintenance and defense, which help the anemone partner; and aggressive, neutral, and submissive behaviors, which maintain the clownfish social group.
Their findings show that bleaching did not alter the large amount of maintenance behaviors toward the anemone that clownfish displayed, such as aeration and cleaning, while defense behaviors increased. This emphasizes the crucial role of the mutualism between anemonefishes and anemones, where each partner depends on the other for nutrients and for defense against predators. The aeration and defense performed by clownfish during bleaching events may be important for anemones to recover from bleaching.
The potential trade-off, however, is that social cohesion may suffer in times of stress. Bleaching was linked to a 215% increase in aggressive behavior between members of the clownfish social group.
Study lead author Fletcher Noble, a PhD researcher at Newcastle University’s School of Natural and Environmental Sciences, said, “This research is one of the first to examine in detail how different types of clownfish behavior change under bleaching conditions in the fish's natural environment. We revealed that clownfish are prioritizing defending and maintaining their homes and securing their dominant position in the group during periods of environmental stress. In the future, we can examine these behaviors in even greater detail and explore how this may be helping the anemone recover from bleaching.”
Anemone Bleaching and Heat Stress
Anemone bleaching occurs due to the loss of symbiotic algae and may turn the animal pale, white, or translucent. It is caused by increased stress, such as higher water temperatures and solar radiation.
In this study, while some of the hosting anemones bleached and lost symbiotic algae, others did not, allowing the researchers to estimate the effects of heat stress separate from the indirect effects through stressed mutualistic partners.
The study demonstrates that the anemone's bleaching response drove behavioral changes more significantly than ambient thermal stress, as baseline clownfish activity remained relatively stable despite temperature fluctuations during the event.
The research was documented during the filming of the BBC Studios series Blue Planet III.
Study senior author Dr. Theresa Rueger, a senior lecturer in tropical marine sciences at the School of Natural and Environmental Sciences, Newcastle University, added, “As human activities continue to change ecosystems around the world, it is crucial to understand how species adapt their behavior to cope with environmental stress and habitat loss, since changing behaviors is animals’ first line of defense. The results show how important it is to consider interactions between different animal species, and particularly mutualisms, where both species benefit, when researching the effects of environmental change.”
Funding: This study was supported by Leverhulme Trust grant no. RPG-2023-186 (T.R.), Natural Environment Research Council grant no. NE/X012514/1 (T.R.), Murray Foundation UK Student Scholarship (M.V.), Prins Bernard Cultuurfonds Young Talent Award grant #40037695 (M.V.), ICRS Graduate Fellowship (M.V.) and Natural Environment Research Council studentship IAP2-22-344 (K.G.).
Published in journal: Royal Society Open Science
Title: Anemonefish prioritize mutualistic partner over social group during a marine heatwave
Authors: Fletcher John Noble, Melissa Versteeg, Kirstin Gaffney, Alexandra Grace Adams, Anoushka Pinto, Eve Stephenson-Archibald, Laura Sullivan, and Theresa Rueger
Source/Credit: Newcastle University
Edited by: Scientific Frontline
Reference Number: eco111126_02
