All cephalopods, with the exception of the nautilus and a few deep-sea octopuses, possess ink sacks located between their gills. This ink sack, which is capable of repeatedly discharging a large amount of dark ink, is one of the primary forms of cephalopod defense.

Different cephalopod species produce different coloured ink. Generally, octopus ink is black, squid ink is blue-black, and cuttlefish ink is sepia-brown.

For many centuries it was thought that the ink discharge worked simply as an aquatic smoke-screen, hiding the startled cephalopod while it escaped. More recently, it has been found that the ink can actually be used to create a decoy. Jacques-Yves Cousteau and other divers have observed that the cloud of ink forms a shape roughly corresponding to that of the cephalopod, and due to its viscosity, it maintains this shape for a short period before it begins to dissipate. The common octopus can make up to six of these "decoy ejections" in succession. Most commonly, the startled octopus will quickly change colour to match its surroundings when discharging, so its attacker will often attack the ink cloud instead of the octopus.

However, other divers have observed ink discharges that behaved more like smoke screens, with the ink spreading rapidly as a dark cloud which is not quite opaque, but obscures vision like smoke or fog on land. It seems that at least some varieties of cephalopods can discharge their ink in either one of these forms according to their need.

In addition to its camouflaging properties, cephalopod ink seems to have a numbing effect on the olfactory sense of moray eels, which are one of the main predators of octopuses. Morays that have been exposed to ink discharges have been seen to actually touch octopuses without attacking them, apparently numbed to the point where they could not sense the octopus unless it moved. It is unknown whether the ink has a similar effect on other predators.

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