National Geographic : 2016 Nov
80 national geographic • november 2016 octopus opens or closes, it can instantly produce patterns such as bands, stripes, or spots. The reflecting cells come in two types. The first type reflects back the light that arrives—thus causing the skin to appear white in white light, red in red light, and so on. The second type is like a living soap bubble that presents dif- ferent colors when seen from different angles. Together, the reflectors plus the pigment organs allow an octopus to create a huge variety of colors and patterns. The second element of disguise is skin texture. By contracting special mus- cles, octopuses can change their skin from smooth to spiky. The effect can be extreme. The algae octopus, Abdopus aculeatus, generates temporary wispy structures that give the impression that the animal is just a piece of seaweed. The hairy octopus, a creature yet to be scientifically described, has evolved a permanently wispy look and is hard to tell apart from a scrap of red algae. The third part of disguise is posture. The way an octopus holds itself can make it more or less conspicuous. Some octopuses, for example, will ball themselves up like a lump of coral and, using just two of their arms, creep slowly across the seafloor. (No, no, don’t look at me—I’m just a rock...) How did octopuses get so good at this? The short answer is: evolution. Through tens of millions of years, individuals that were better at disguise were more likely to evade pred- ators and leave offspring. And plenty of animals—including eels, dolphins, mantis shrimps, cormorants, many fish, and even other octopuses—are enthusiastic eaters of octopuses. Because octopuses have no bones, predators can eat the whole animal. As Mark Norman, a world expert on living cephalo- pods at Museum Victoria, in Melbourne, Australia, puts it, “These animals are pure meat walking around—they’re filet mignons.” now let’s turn to the matter of the octopus’s nervous system. A typical pond snail has just 10,000 neurons; lobsters have around 100,000; jumping spi- ders, perhaps 600,000. Honeybees and cockroaches, which after cephalopods have a claim to be the planet’s most neuronally rich invertebrates, have around a million. So the 500 million neurons of the common octopus, Octopus vulgaris, put the animals into a completely different league. In terms of their neuron count, they are better endowed than a mouse (80 million) or rat (200 million) and almost on a par with a cat (around 700 million). Yet while vertebrates keep most of their neurons in their heads, two-thirds of an octopus’s are in its arms. What’s more, nervous systems take a lot of energy to run and can evolve to be large only when the benefits outweigh the costs. So what’s going on? Peter Godfrey-Smith, a philosopher turned octopus biologist at City Uni- versity of New York and the University of Sydney, in Australia, suggests that several forces may have helped the octopus develop a complex nervous sys- tem. The first is its body. Nervous systems, after all, evolve in tandem with bodies, and the octopus body has evolved to be unusually complex. Being boneless, an octopus can extend any arm in any direction at any point; unlike you or me, it’s not limited to moving at shoulder, elbow, or wrist. This gives An octopus on the hunt is an impressive sight—every arm stretched out over the sand, each one explor- ing, probing into holes. If one arm startles a shrimp, two more can reach out to catch it.