Tough mothers and tender fathers: New assays reveal the behavioural complexity of the common clownfish

Abstract

In the last two decades, anemonefish, especially the Ocellaris or common clownfish (Amphiprion ocellaris), have captured the public's imagination. Boosted by the 2003 Disney Pixar film, Finding Nemo, they have become the most popular marine fish on the UK market (Pinnegar & Murray, 2019). As a proud owner of a marine aquarium and two common clownfish myself [with an electricity bill to prove it… (Perry, 2023)], their popularity is easy to understand. For example, their vibrant colours and patterns, sex changing abilities, hardy nature and affinity for captive rearing make them a fascinating fish to keep. Perhaps most fascinating, however, are their big personalities, such as unwavering defence of their host anemone from other tank-mates.

Although most clownfish owners, including myself, sit and gawp at their fish for hours, with little understanding of the behavioural firework display playing out in front of them, the same cannot be said for Graham et al. (2025). In this issue, they measure 35 behaviours across 7 behavioural assays: (1) parental care, (2) small intruder aggression (hermit crab), (3) large intruder aggression [threespot dascyllus (Dascyllus trimaculatus)], (4) male-oriented aggression, (5) female-oriented aggression, (6) immediate reaction to a threat and, finally, (7) nest maintenance.

Using this broad array of assays, Graham et al. (2025) found that 33 of the behaviours were repeatable, with sex differences in (1) parental care, (3) large intruder aggression and (5) female-oriented aggression. Although both sexes display egg-caring behaviours [which cannot be said for all damselfish, with females in some species devouring the eggs at the first chance (Sakai et al., 2023)], males displayed more egg-caring behaviours. Females, on the contrary, displayed more aggressive defensive behaviours.

It was also observed that territorial aggression increased when there were no eggs in the nest, as both the male and female had more time to devote to defence. Indeed, when surrogate eggs were placed in the nest, aggression went up, which may suggest that the fish could tell when eggs were not their own. Finally, males also showed three behavioural syndromes, building on previous work (Wong et al., 2013), where each syndrome had a suite of correlated behaviours. Unexpectedly, females showed no behavioural syndromes. Graham et al. (2025) suggest that because of the unique sequential hermaphroditism and social hierarchy, which lock these fish into a final, irreversible sexual and social role, they may have reduced behavioural plasticity.

This study provides a new and unprecedented glimpse into the lives of a legendary species, star of both stage and screen, whose behavioural charisma has made them a staple in home aquariums across the globe. Perhaps most importantly, however, is the behavioural toolkit generated by this work, which is ready to probe these iconic fish under the many different stages of their fascinating life history.