Ultraviolet freckles start fish fights

Reef fishes recognize rivals' discrete, discreet marks

Seen in the right light, yellow reef fish become spotty pains in the tail fin.

FIGHTING SPOTS The images on the right show the invisible (to humans) UV designs for the two species of damselfish on the left. U. Siebeck et al./Current Biology 2010

Members of one damselfish species use facial patterns of speckles and swooshes to identify the fish species they regularly attack, researchers report in an upcoming issue of Current Biology. These markings show up only in ultraviolet light, says visual ecologist Ulrike Siebeck of the University of Queensland in Brisbane, Australia.

In tests, Siebeck and her colleagues found that male Ambon damselfish could tell their own species from another just by seeing the ultraviolet markings. When UV light was blocked by filters, confused males picked fights with the wrong rivals.

The UV freckles could work as a secret, or at least pretty discreet, communications channel, Siebeck proposes. Animals need to send clear signals to important compatriots, such as possible rivals or mates. Yet signals that get too clear can attract the wrong kind of attention from hungry predators. As Siebeck puts it, “How can you be colorful and not colorful at the same time?”

Both Ambon and lemon damselfish can see UV light. But plenty of their major predators, such as wrasses and cod, typically can’t, Siebeck says. So she argues that damselfish could use their spots to send a covert message.

This encrypted messaging sounds plausible for another reason, says Innes Cuthill of Bristol University in England. Short UV wavelengths scatter more readily when they hit small particles than do the longer wavelengths that people call visible light. So plankton and other bits floating in seawater make UV markings harder to detect from a distance than visible-light color patterns. “For a predator, even if it can see in the UV, the patterns will be a blur,” Cuthill says.

The lens in the human eye blocks UV wavelengths, but plenty of fish, birds and insects carry and can see some kind of UV marking. “It’s secret to us,” says, visual ecologist Sönke Johnsen of Duke University in Durham, N.C., but “it’s not super magical.”

Before calling UV freckles private lines of communication Johnsen wants to know more, such as which other species on the damselfishes’ reef can see UV. What the damselfish experiments have demonstrated clearly, he says, is that these fish can use UV to distinguish species.

Siebeck made that discovery thanks to the scrappiness of territorial Ambon males. She first tested 28 of them to see whether they would fight a member of their own species or a lemon damselfish if she presented both. Most made more attacks on their own kind, though six, for unknown reasons, tended to attack the other species.

Once she knew their fighting preferences, Siebeck changed the experiment by placing some of the potential rivals in UV-filtering plastic tubes to hide their freckles. The males who saw these fish attacked randomly, apparently because they could not detect the UV patterns.

Siebeck says these damselfish may have “the most intricate UV patterns found so far on animals.” To see whether the fish can resolve such elaborate patterns, Siebeck trained fish to nudge a card marked with a UV spot pattern based on a real fish face. When she offered a choice of cards based on both species, trained fish mostly nudged the pattern they had learned to recognize.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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