Clownfish, aka anemonefish, are a group of snazzy reef-dwelling species made famous by Finding Nemo. They live in sea anemones, brightly colored cnidarians with powerful stings, which the fish are immune to thanks to a mucous coating on their skin.
Nemo himself is a common clownfish (Amphiprion ocellaris), while the new research focused on one of his “cousins”, the Barrier Reef anemonefish (Amphiprion akindynos). A team from the Universities of Queensland (UQ) and Maryland analyzed the fish’s genes, the proteins they code for, and their physical anatomy to work out what they can see. Their findings are reported in Scientific Reports.
"In the part of the anemonefish's eye that looks forward, the photoreceptors detect a combination of violet light and ultraviolet light," said Dr Fanny de Busserolles, of UQ’s Queensland Brain Institute (QBI), in a statement.
"They seem to be very good at distinguishing color, and very good at seeing UV – it looks like they use it a lot."
The team thinks the ability has two uses – spotting friends and finding food. The characteristic white stripes of anemonefish reflect UV light, which makes them stand out to fish that can see UV wavelengths. Most of the bigger fish that prey on anemonefish can’t see UV, meaning that clownfish’s white stripes could act as subtle signals that allow for clandestine communication between members of the same species, like a kind of invisible ink.
"They can be as flashy as they want and they won't be seen – and it might be how Nemo's cousin finds its friends."
The team note in their paper that anemonefish are territorial, and will act aggressively towards intruders who venture too close to their tentacled home. Seeing UV light might help them to assess potential intruders in the distance from the safety of their anemone.
In addition to recognizing each other, the researchers think the species’ visual prowess helps them identify the zooplankton they feed on. These teeny tiny critters don’t absorb UV light, meaning that through the eyes of an anemonefish, they appear dark against the background, making them stand out.
Anemonefish live near the water’s surface, where UV light penetrates, and the anemones in which they live require UV light to grow. Future research will tell us whether other species in this group have similarly sneaky visual systems. The researchers note that the adaptation could allow different species of anemonefish to recognize each other when sharing an anemone, so there’s a good chance they do.