Fish use UV to spot the difference
Healthy coral reefs come packed with colourful fish and a new study reveals that the some fish send out private messages using patterns of ultra violet light that us humans - and many other animals - can't see.
It can be very important for fish to tell the difference between different species, especially for damselfish. These feisty tiddlers defend their farms of seaweed from intruders, the worst kind being other members of the same species because they strongly compete for food and mates.
Ulrike Siebeck from the University of Queensland in Australia led a team of researchers with the enviable task of studying two species of damselfish on the Great Barrier Reef. They discovered that Ambon damselfish, a small yellow fish, can read "secret" facial patterns of UV spots and stripes to tell one species from another.
The team presented wild Ambon damselfish with two other damselfish inside clear plastic tubes: one the same species, and the other a different species, the lemon damselfish that looks similar except for different UV facial patterns. Under normal light conditions, the Ambon damsels preferentially attacked other Ambons. But, when the tubes were made from UV-blocking plastic, the Ambons showed no preference for either fish, presumably because they could no longer see the UV patterns.
Also, in the laboratory, researchers trained the damselfish using food rewards to distinguish between drawings of the UV face patterns.
Together, these findings indicate that these Ambon damselfish can see intricate UV patterns and use them to recognize other fishes' faces.
UV makes an ideal secret signal because not many other fish can see it. With its short wavelength, UV light is easily scattered in water making it of little use for precision vision. And long-lived predators often protect their eyes from UV damage by screening out these wavelengths. This means the damselfish can communicate to each other, find mates, and warn each other to keep their distance without ruining their camouflage against predators.
Next, the team wants to delve deeper into the UV vision of various fish, to find out at what distance they can see, and whether they can perhaps distinguish not only between different species but also individual fish.