<P> 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 . </P> <P> The ink is released from the ink sacs (located between the gills) and is dispersed more widely when its release is accompanied by 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 . </P> <P> A number of other aquatic molluscs have similar responses to attack, including the gastropod clade known as sea hares . </P>

Where is the ink located in a squid