Cephalopod ink is a dark-coloured ink released into water by most species of cephalopod, usually as an escape mechanism. All cephalopods, with the exception of the Nautilidae and the Cirrina, are able to release ink. The ink is released from the ink sacs 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. A number of other aquatic molluscs have similar responses to attack, including the gastropod clade known as sea hares.
Inking behaviours
Two distinct behaviours have been observed in inking cephalopods. The first is the release of large amounts of ink into the water by the cephalopod in order to create a dark, diffuse cloud that can obscure the predator's view, allowing the cephalopod to make a rapid retreat by jetting away. The second response to a predator is to release pseudomorphs, smaller clouds of ink with a greater mucus content, which allows them to hold their shape for longer. These are expelled slightly away from the cephalopod in question, which will often release several pseudomorphs and change colour in conjunction with these releases. The pseudomorphs are roughly the same volume as and look similar to the cephalopod that released them, and many predators have been observed attacking them mistakenly, allowing the cephalopod to escape. Furthermore, green turtle hatchlings that have been observed mistakenly attacking pseudomorphs released by Octopus bocki have subsequently ignored conspecific octopuses. Many cephalopod predators have advanced chemosensory systems, and some anecdotal evidence suggests that compounds found in cephalopod ink can irritate, numb or even deactivate such apparatus. Few controlled experiments have been conducted to substantiate this. Cephalopod ink is nonetheless generally thought to be more sophisticated than a simple "smoke screen"; the ink of a number of squid and cuttlefish has been shown to function as a conspecific chemical alarm. Octopuses have also been observed squirting ink at snails or crabs approaching their eggs. Numerous cuttlefish species add a coat of ink to their eggs, presumably to camouflage them from potential predators.
Physical properties
Sepia officinalis ink forms a polydisperse suspension composed by spheric particles with a size between 80 and 150 nm. The particles have a density of, which may be due to the amount of metals that has in its composition.
Chemical composition
Cephalopod ink contains a number of chemicals in a variety of different concentrations, depending on the species. However, its main constituents are melanin and mucus. It can also contain, among other things, tyrosinase, dopamine and L-DOPA, as well as small amounts of freeamino acids, including taurine, aspartic acid, glutamic acid, alanine and lysine.
Use by humans
Cephalopod ink has, as its name suggests, been used in the past as ink for pens and quills; the Greek name for cuttlefish, and the taxonomic name of a cuttlefish genus, Sepia, is associated with the brown colour of cuttlefish ink. Modern use of cephalopod ink is generally limited to cooking, primarily in Japan and the Mediterranean, where it is used as a food colouring and flavouring, for example in pasta and sauces, and of course calamares en su tinta. For this purpose it is generally obtainable from fishmongers, gourmet food suppliers, and is widely available in markets in Japan and Spain. The ink is extracted from the ink sacs during preparation of the dead cephalopod, usually cuttlefish, and therefore contains no mucus. Studies have shown that cephalopod ink is toxic to some cells, including tumor cells. It is being researched in mice for its antitumor activity against Meth-A fibrosarcoma. It currently remains unclear however if any of the antitumor activity of squid ink can be obtained from oral consumption, and this is indicated as an area for future investigation.
