It's a snake! No, a fish. An octopus?

Whether the so-called mimic octopus could impersonate Madonna or President Bush remains unclear, but researchers say the long-armed wonder does a great sea snake and lionfish.

The octopus, too recent a discovery to have a scientific name, prowls the silty stretches where rivers spill into the sea in Indonesia, explains Mark D. Norman of Museum Victoria in Melbourne, Australia. Even with few hiding places, the octopus forages in daylight. Such boldness may come from its ability to change posture, color, and motion in impersonations of venomous animals, say Norman and his colleagues in the first scientific report of the behavior.

Plenty of animals play mimic, perhaps taking on the color of bark or sand in the background. Many octopuses amaze divers by matching a background so perfectly that the animals seem to vanish.

Other animals mimic a less tasty or more dangerous neighbor. However, shifting impersonations among very different species is new, according to coauthor Tom Tregenza of the University of Leeds in England. "The main dramatic thing about this is that it's orders of magnitude more dynamic than what's been observed in other animals," he says.

"It sounds terrific to me," comments longtime cephalopod specialist Richard E. Young of the University of Hawaii in Honolulu. Although Young cautions that the differences in the way people and other animals see things can make mimicry very hard to prove, he says he was amazed by videos of the mimic octopus.

Norman recalls that he first heard about this octopus from some underwater photographers, whose still pictures remained open to many interpretations. "At first we joked, `This one's doing a piano and this one's doing a sofa bed,'" he says.

In 1999 and 2000, British and Japanese networks sent Norman out with TV crews. He became convinced that the animals were indeed mimics. "When you follow one around, you see it doing very un-octopus-like things," he says.

Sometimes, the octopus fled with its arms aligned in a flattened, striped oval, looking much like a common poisonous flatfish. On four occasions when damselfish pestered an octopus, Norman saw it poke six of its legs down a burrow and spread the other two. They sported bands and waved gently, resembling the sea snakes that prey on damselfish.

When Norman saw a mimic octopus chugging along well above the seafloor, extended arms colored in stripes, he thought of the sunburst of striped, poisonous spines that lionfish flare.

Norman and his colleagues propose these mimicries in the Sept. 7 PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON B, but Norman suggests the octopus' repertoire probably extends much further. "The more we watched it, the more weird behavior we saw," he says.

Tregenza suggests that the octopuses may be responding to menaces with particularly appropriate kinds of threats.

That would be pretty fancy footwork for an invertebrate, but Norman notes, "We're just getting glimpses of this amazing hidden fauna lurking out there."