It is often said that size does not matter… but clownfish prove otherwise. In terms of hierarchy, for example, these fish form small clans – consisting of a breeding pair and a few subordinates – where size determines social rank. "The breeding female is the largest of them all, and she rules over the territory of the anemone [around which the group lives]," said Melissa Versteeg, a researcher at Newcastle University in the United Kingdom. Next comes the breeding male, who is slightly smaller, followed by the subordinates, juvenile fish who would be wise not to outgrow their superiors. "That could disrupt the established hierarchy," added Versteeg. "To minimize the risk of social conflict, a lower-ranking clownfish will therefore maintain a size that is about 80% of that of its immediate superior."

The researcher, who focuses extensively on these questions of size, recently made a surprising discovery: Clownfish can shrink during marine heatwaves, which increases their short-term chances of survival.

To obtain these results, published Thursday, May 22, in Science Advances, the researchers donned their wetsuits to observe breeding pairs of Pacific clownfish living in the coral reefs of Kimbe Bay, Papua New Guinea. At that time, between February and August 2023, a severe global coral bleaching event was underway, during which sea temperatures in the study area exceeded 32°C.

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