Cephalopod ink

Cephalopod ink is a dark pigment released into water by most species of cephalopod, usually as an escape mechanism. All cephalopods, with the exception of the Nautilidae and the Cirrina (deep-sea octopuses), are able to release ink.

The ink is released from the ink sacs (located between the gills) and is dispersed more widely by accompanying its release with a jet of water from the siphon. Its dark colour is caused by its main constituent, melanin. Each species of cephalopod produces slightly differently coloured inks; generally, octopuses produce black ink, squid ink is blue-black, and cuttlefish ink is a shade of brown.

A number of other aquatic molluscs have similar responses to attack, including the gastropod clade known as sea hares.

Inking behaviours

I was much interested, on several occasions, by watching the habits of an Octopus or cuttle-fish ... they darted tail first, with the rapidity of an arrow, from one side of the pool to the other, at the same instant discolouring the water with a dark chestnut-brown ink.