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Newfound Octopus Impersonates Fish, Snakes

John Roach
for National Geographic News
September 21, 2001
 
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Scientists have discovered what may be the ideal partner for a game of charades: A long-armed octopus that mimics poisonous creatures of the sea to avoid its predators.

The clever creature is a brown octopus about two feet (60 centimeters) long that slithers along the muddy bottom of shallow, tropical estuaries where rivers spill into the sea. It was discovered so recently that it still doesn't have a scientific name, but scientists are intrigued by its uncanny ability to impersonate lion fish, soles, and banded sea snakes.



Octopuses are thought to be one of the most intelligent invertebrates and can change the color and texture of their skin to blend in with rocks, algae, or coral to avoid predators. But until now, an octopus with the ability to actually assume the appearance of another animal had never been observed.

"Having studied many octopus species in the wild, I am never surprised by the color and shape change capacities of these animals," said Mark Norman of the Melbourne Museum in Australia. "However, this animal stood out as it was the only one we've encountered that goes beyond camouflage to take on the guise of dangerous animals."

Norman and fellow researchers Julian Finn of the University of Tasmania in Australia and Tom Tregenza of the University of Leeds in England describe the octopus mimic in the September 7 issue of Proceedings of the Royal Society of London.

"This," Tregenza said, "is a rather dramatic animal."

Talented Impersonator

Mimicry is a fairly common survival strategy in nature. Certain flies, for example, assume the black and yellow stripes of bees as a warning to potential predators. But the adaptable octopus is the first known species that can assume multiple guises.

Each of the nine specimens that scientists saw during research dives off the coasts of Sulawesi and Bali in Indonesia impersonated more than one toxic species. The creatures they routinely mimicked were:

Sole fish. To take on the appearance of flat and poisonous sole fish abundant in the habitat, the octopus builds up speed through jet propulsion and draws all of its arms together to form a leaf-shaped wedge. It then undulates in the manner of a swimming flat fish.

Lion fish. Just above the seafloor the octopus swims with its arms spread wide and trailing from its body, mimicking the lion fish and its poisonous fins.

Sea snakes. Changing its color to imitate the yellow and black bands of the toxic sea snake, the octopus threads six of its arms into a hole and waves the other two arms in opposite directions so they look like two snakes.

The researchers think the octopus also may be able to impersonate other sea creatures such as sand anemones, stingrays, mantis shrimp, and jellyfish. For now, however, the scientists will only say for sure that the octopus can mimic sole, lion fish, and sea snakes.

"Mimicry has an element of subjectivity to it," said Tregenza. "You have to be fairly careful. No other octopus has really been shown to mimic another animal."

Long Overlooked

One reason why the researchers had not discovered the octopus previously is that it lives in a habitat that's not very appealing to scuba divers—a muddy and relatively barren landscape that lacks the variety and splendor of life found in coral reefs.

"We also think that is why it has such a dramatic [mimicking ability]," said Tregenza. "It has nowhere to hide. It could burrow, or try and mimic one of the animals also found in their environment."

"These boring environments are just the place where you might see the most exciting behavior of animals," he added.

The researchers believe the recently discovered octopus uses its mimicry to avoid predation by large fish rather than as a mechanism to trick its prey. They also think it probably evolved from another species of octopus that's active during the day in coral reefs nearby.

"It may have moved to open ground to harness the many crustaceans and fish found in these habitats," said Norman. "Individuals which looked like dangerous animals were clearly selected for, while others were quickly nailed by passing barracuda, sharks, or groupers. Hence, mimicry was selected for."

The fact that the octopus can adopt so many different animal impersonations greatly reduces its likelihood of encountering predators. This is advantageous, said Tregenza, because if a predator fish often saw a lion fish that looked suspiciously like an octopus, the predator would eventually be more willing to risk being poisoned by taking a bite of its prey—thus blowing the octopus's cover.

Tregenza said the octopus may decide which creature to impersonate depending on what particular predator is near. Evidence of such behavior came from observations showing that when the octopus was attacked by territorial damselfishes, it mimicked one of the fishes' common predators, the banded sea snake.

"If the mimic octopus can figuratively use its mimicry—that is, can choose to use a particular form in response to a particular threat—this could potentially dramatically improve the defensive value of its mimicry behavior," said Tregenza.
 

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