The mimic octopus – a master of disguise

In the natural world , mimicking a more dangerous creature is a common strategy for avoiding predators. But there is only one animal that can dynamically mimic many different creatures – the incredible mimic octopus.

As you swim through tropical waters, you notice that a strange creature has entered your territory. The intruder is unfamiliar, but when you try to chase it away, it undergoes a startling transformation. Its new form is one you recognise – a banded sea-snake, highly venomous and likely to make you its next meal. You turn and flee. You are a damselfish, you are in Indo-Malayan seas, and you have just been duped by the mimic octopus.

The mimic octopus is new to science and has yet to be properly classified *. It has, though, already gained notoriety for its unique ability to impersonate venomous or distasteful animals. Politicians and pop-stars may be beyond its scope, but its repertoire includes soles, lionfish, sea-snakes, and possibly sea anemones, stingrays and jellyfish.

To transform into a sea-snake, for example, the octopus withdraws its head and six of its tentacles into a burrow and waves the other two in opposite directions in an uncannily serpentine manner. You can download some Quicktime videos of the octopus doing its thing from the Royal Society website.

(Photograph by Ken Knezick)

The rewards of mimicry

Octopus specialist Dr Mark Norman, from the University of Melbourne, Australia, first observed the mimic off the coast of Indonesia. There, it forages in open sand flats during broad daylight and its talents may have evolved to keep it safe in these vulnerable surroundings. As Norman says, when you’re caught in the open by a passing fish, “you’ve got to look either deadly or inedible”.

Mimicking deadly or inedible animals reaps obvious benefits – predators avoid you, and you need not bother making poisons yourself. It has therefore become a common strategy, used by snakes and flies, spiders and plants. But these charlatans are all one-trick ponies.

In comparison, the mimic octopus’s charades are orders of magnitude more dynamic. “No other animal has been found that is able to rapidly change between different forms of mimicry”, says Dr Tom Tregenza, from the University of Leeds, UK, co-author of the paper which first described the mimic.

A coat of many colours

Having multiple acts benefits the octopus as predators are less likely to catch on to any individual one. If too many octopuses mimic a single creature – say, a lionfish – then predators are more and more likely to encounter the fake than the real deal. They might never learn that something that looks and moves like a lionfish is not worth biting.

Like all good performers, the mimic octopus caters to its audience. It only acts like a sea-snake (right) when confronted by territorial damselfish, which are preyed upon by sea-snakes. “This is very exciting because it raises the possibility that the mimic octopus can employ different forms of mimicry to counter different threats”, says Tregenza.

But is the mimic actually mimicking or are human eyes misinterpreting these movements? To answer this, a BBC/Discovery film crew captured six hours of live footage of the mimic in 2000. The combination of colour, posture and very “un-octopus-like” movement convinced many sceptics.

For example, when mimicking the leaf-shaped sole, the octopus not only draws its tentacles and head back into a leaf shape, but also matches a sole’s colours and undulates its body to resemble its swimming style.

Evolving an act

Norman believes that octopuses as a group are the equivalent of “rump steak swimming around”. Their bodies lack any sort of protective shell or skeleton and they have had to evolve other incredible defences to compensate. Soft bodies make them vulnerable, but they also make octopuses particularly well-suited to deception.

Without skeletons, they are expert contortionists, and can change shape or squeeze into tight spaces. Their remarkable skin can change texture, becoming spiky or smooth on a whim. It also contains sacs of pigment called chromatophores which can be expanded or contracted to produce rapid changes of colour and pattern. Armed with this arsenal of stealth, all octopuses are masters of disguise.

Most species are content to blend into their backgrounds. The mimic’s ancestor probably lived unnoticed in nearby coral reefs. These reefs are like busy and crowded high streets; in contrast, the sand flats are an open market, with rich pickings for any animal (provided that they can avoid being eaten).

The octopus’s inbuilt camouflage abilities would have given it a head-start. As time passed, individuals that slightly resembled poisonous animals would have lived longer without being eaten, allowing them to pass their appearances on to their offspring.

Unturned corners

The mimic’s behaviour remained undiscovered for years because its dull homelands are poorly studied. But it is precisely this barren nature that has provided the impetus to evolve such amazing behaviour.

As Tregenza says, “The mimic octopus teaches us that very bland and barren habitats may be home to the most impressive behaviours.” Even more surprising and wondrous animals may await discovery in these unexplored worlds.

This article won a runner-up prize in the 2004 Daily Telegraph Young Science Writer competition. I’m really pleased to now have it published for the first time – Ed

 

Update: *Since writing this article in 2004, Mark Norman has finally fully characterised and classified the mimic octopus. It now goes by the fitting scientific name of Thaumoctopus mimicus – the “mimicking miracle octopus”. – Ed

Reference: Norman, Finn & Tregenza. 2001. Proc Biol Sci 268: 1755-1758.

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