New York: Clownfish, the charismatic stars of coral reefs and animated films alike have revealed a remarkable survival strategy: they shrink.
A new study shows that during an intense marine heat wave off Papua New Guinea in 2023, many clownfish noticeably reduced their body size – a biological adjustment that increased their chances of survival. Out of 134 clownfish monitored in Kimbe Bay, 101 were found to have shrunk at least once during the heat stress event.
“We were shocked at first when we saw that they were shrinking at all. This kind of plasticity is unprecedented in coral reef fish,” said Morgan Bennett-Smith, the study’s lead author and a marine biologist at Boston University.
Response to Climate Pressure
Rising ocean temperatures, fueled by climate change, are triggering more frequent and intense underwater heat waves. These conditions bleach the sea anemones that clownfish depend on for shelter and survival, creating a high-stress environment.
Scientists believe that shrinking may help clownfish conserve energy, as smaller fish require less food and oxygen, precious resources in overheated ecosystems.
One theory proposed by researchers is that clownfish may be reabsorbing some of their bone matter to reduce size. Adding to the complexity, clownfish are highly social animals with rigid breeding hierarchies.
The dominant female is always the largest in the group. Remarkably, in some breeding pairs, both partners shrank, but the female maintained her size advantage, preserving the social order despite the stress.
Temporary Adaptation
While the shrinking appears to be reversible, clownfish were observed regaining size when the water cooled it remains unclear how sustainable this coping mechanism is over time.
This is not the first time marine species have been seen shrinking under thermal stress. Marine iguanas in the Galápagos, for instance, reduce their size during El Niño events. However, this is the first documented case of such adaptation in coral reef fish.
Despite the distressing context, researchers say this phenomenon is also a sign of biological resilience.
