Australia: why do fiddler crabs change colour?

From ABC in Australia:

Crabs Change Color to Avoid Hungry Birds

By Stephen Pincock

Oct. 23, 2006 — Tiny blue-shelled fiddler crabs change their color to avoid being eaten by predatory birds, researchers have discovered.

Scientists have long been intrigued by the crabs‘ ability to change the color of their top shell from bright blue to a more subdued, muddy shade.

But exactly why the crabs change their color hasn’t been clear.

“When you catch them, for instance, they go dull,” said Jochen Zeil of the Australian National University in Canberra.

Zeil’s research appears in the Journal of Experimental Biology.

To find an answer, Zeil and his colleagues studied a particular species of fiddler crab, Uca vomeris, on the mud flats of Australia’s northeast coast, south of Townsville.

“Some populations of these crabs are all very dull, but in other places they are very colorful,” Zeil said.

“We wanted to understand how this happens.”

The scientists looked at variations between crabs from three different areas: one group was dull colored, another was colorful and a third group mixed.

They found differences in the numbers of crab-eating birds near the dull colored populations.

“In the places where the crabs were colorful, there were few birds,” Zeil says.

“But in the others there were more birds actively hunting crabs.”

Researchers at The Vision Centre and Australian National University have achieved a world-first in working out how fiddler crabs perceive their world and respond to it: here.

Female fiddler crabs are sensitive to changes in the speed of a male’s courtship display, significantly preferring displays that accelerate to those that are performed at a constant speed or slow down: here.

Trapeziid crabs and coral reefs: here.