Octopuses are relatives of shellfish and snails that lost their shells. They compensate for the absence of body armour by being strong, intelligent and able to become invisible against almost any background. In fact, no other animal has the level of sophistication that octopuses employ for camouflage.
Being boneless, octopuses can adopt a variety of shapes. They have specialised dermal muscles that change their skin from smooth to spiky, allowing them to imitate the appearance of smooth and rough surfaces. Finally, octopuses have in their skin differently coloured elements called chromatophores, iridophores, and leucophores that allow them to achieve an impressive variety of colours. They can appear red, white, brown or greenish, and the toxic blue-ringed octopus can even flash bright yellow and blue. This allows octopuses to match almost any substrate in brightness, colour and texture. All these elements operate under neural control, allowing the octopus to change appearance with exceptional speed (over several milliseconds). An octopus can be cryptic even on structure-free backgrounds because it adopts the shape, texture and colour of typical rocks from further away. It can also move slowly across sandy areas imitating a ‘moving rock’ that is driven by current.
To choose an adequate camouflage strategy, an octopus needs to be intelligent and perceive the shape, texture and colour of the objects that it imitates. Indeed, octopuses have large brains and eyes that are optically superior to ours. While human eyes suffer from spherical aberration, providing a blurry image even in the case of fully corrected vision, octopuses have an aberration-free lens, which gives a sharp image. Octopuses can see polarisation, a feature of light that is invisible to us, but they cannot discriminate colours. Even for an animal that has colour vision, matching colours of background objects is difficult because different marine predators see colours differently. While marine mammals are colour blind, different fishes have different types of colour vision and sea birds can discriminate colours that are indistinguishable to us.
So how does the colour-blind octopus apparently mimic colours? Recently, we were awarded a Marsden Grant to find the answer to this question. The team – Luis Nahmad-Rohen, a University of Auckland marine science PhD student; Yusuf Qureshi, a marine science master student, and myself – have measured octopus’ colours and demonstrated that they can produce colours that are indistinguishable from typical background colours in the eyes of marine mammals and fish. However, sea birds are likely to be able to break the colour camouflage of an octopus. We still do not know which cues a colour blind octopus uses to extract information about colours. Does the octopus’ ability to see polarisation compensate for the absence of colour vision or does an octopus ‘guess’ colour from an object’s shape or its brightness? Does an octopus choose colours differently depending on the colour vision of an animal that is likely to look at it? To tackle these questions, we are conducting behavioural experiments with octopuses in captivity and observing octopuses in their natural habitat.
Luis Nahmad-Rohen checking an octopus trap in Omaha Bay.