"Secret Communication Channel" Found in Reef Damsels

Lemon Damsel, left, and Ambon Damsel, right, appear very similar to the human eye, but sport distinguishing facial markings that reflect UV light that is invisible to us but that some fishes can see.

The color patterns of coral reef fishes has long entranced and puzzled human observers, now scientists have found that some of these same fishes may be using their ability to detect visual highlights that are invisible to humans and some of their predators.

Primates are believed to have lost the ability to detect ultraviolet light (UV) millions of years ago, while other seeing organisms are known to be able see reflected UV light.

Now a new finding by Australian scientists reports that certain coral reef fish use ultraviolet (UV) vision to tell the difference between their own and other similar species.
 
Indeed, the otherwise rather plain-looking Ambon Damselfish (Pomacentrus amboinensis) may even be able to recognise individuals by their faces, in much the same way as humans do, the researchers say in a paper in the latest issue of the journal Current Biology.

Facial Markings Seen in UV Only
The UV vision may act as a secret channel of communication because it is invisible to the fishes’ predators, the researchers speculate.

“We observed that certain fish had very distinctive ultraviolet markings on their faces – and we wondered what they were using them for,” says lead author, Dr. Ulrike Siebeck, below, at The University of Queensland's Vision Centre in Brisbane, Australia. The Ambon Damsel, left top, and its look-alike cogenor, the Lemon Damsel, for example appear superficially very much alike to the human eye, but have UV-reflective facial patterns that the fishes themselves recognize and use in their daily interactions of defending territories and reproduction.

In a series of carefully controlled experiments, the team exposed male Ambon Damsels to males of the same and a different species (Lemon Damsels, Pomacentrus moluccensis) with similar overall body coloration but different UV facial patterns) in conditions in which the UV markings could and could not be seen.

They observed that there was only a difference in territorial reaction when the UV facial patterns could be seen, demonstrating that these patterns are necessary for the fish to discriminate between their own and another species.

In further experiments, in which the fish were shown pictures of the different facial markings, the team demonstrated that they were reacting to the specific shape of the UV patterns on the intruder’s face, not simply to their ultraviolet colour, Siebeck says.

“We came to the conclusion that the fish are using ultraviolet reflecting facial patterns to discriminate between their own species and other, similar-looking fish species. Also that they are reacting to the actual pattern, not simply the UV light they were seeing.”

“Differences between patterns on the faces of individuals suggest that Ambon Damselfish may also be able to use the patterns for the discrimination of individuals, in a manner directly comparable to the face-based recognition of individuals performed by humans,” the team says in the paper. (The same specimen, above, in full-spectrum lighting and viewed with a UV filter showing its facial markings.)

Not True for All Reef Fishes
However major predatory reef fish like coral trout (Plectropomus spp.), wrasse and, groupers do not seem to have the ability to see ultraviolet markings.

The team considers that predators like coral trout may have lost the ability to see in the ultraviolet in order to detect contrast more easily, an important factor in detecting prey at greater distances. Also, they are generally longer-lived and may need to avoid u/v damage to their eye cells more than do the shorter-lived damselfish.

“This means the damselfish are effectively exploiting a secret channel of communication among themselves and with other similar, but harmless species – one which cannot be detected by the fish that prey on them.

“This ability to see in the ultraviolet seems to have been retained in some coral reef fishes, whereas carnivorous fish and many higher animals – including humans – seem to have lost it,” Ulrike says.

“It means damselfish can see a wider spectrum of colours than we can, which is remarkable when you consider how colourful coral reefs are naturally. They must find them even more colourful places than we do.”

From materials released by ARC Centre of Excellence in Vision Science, University of Queensland, Australia. Images courtesy Ulrike E. Siebeck.

Reference: “A species of reef fish that uses ultraviolet patterns for covert face recognition” by Ulrike Siebeck, Amira N. Parker, Dennis Sprenger, Lydia Mathger and Guy Wallis. Current Biology (25 February 2010).

Abstract:

• Highlights
• Some species of reef fish possess complex UV-reflecting facial patterns
• The fish are able to use these patterns to perform species discrimination
• Unlike other animals, the fish can discriminate complex patterns using their S cones
• UV light offers a “secret communication channel,” unseen by UV-blind predators

• Summary
• The evolutionary and behavioral significance of an animal's color patterns remains poorly understood [1,2,3,4], not least, patterns that reflect ultraviolet (UV) light [5]. The current belief is that UV signals must be broad and bold to be detected because (1) they are prone to scattering in air and water, (2) when present, UV-sensitive cones are generally found in low numbers, and (3) long-wavelength-sensitive cones predominate in form vision in those species tested to date [6]. We report a study of two species of damselfish whose appearance differs only in the fine detail of UV-reflective facial patterns. We show that, contrary to expectations, the Ambon damselfish (Pomacentrus amboinensis) is able to use these patterns for species discrimination. We also reveal that the essential features of the patterns are contained in their shape rather than color. The results provide support for the hypothesis that UV is used by some fish as a high-fidelity “secret communication channel” hidden from predators [7,8]. In more general terms, the findings help unravel the details of a language of color and pattern long since lost to our primate forebears, but which has been part of the world of many seeing organisms for millions of years.