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 (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 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.
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 (much like a smoke screen) 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 ("false bodies"), 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 (blanch) 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 (this behaviour is often referred to as the "blanch-ink-jet manoeuvre").
Many cephalopod predators (for instance moray eels) have advanced chemosensory systems, and some anecdotal evidence suggests that compounds (such as tyrosinase) 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.
Numerous cuttlefish species add a coat of ink to their eggs, presumably to camouflage them from potential predators.
This section needs expansion with: examples of other inks besides that of the common cuttlefish, and comparisons between them. You can help by adding to it. (July 2018)
Sepia officinalis ink forms a polydisperse suspension composed by spheric particles with a size between 80 and 150 nm (measured by TRPS and SEM). The particles have a density of 1.27 g cm−3, which may be due to the amount of metals that has in its composition (4.7% in weight).
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 free amino 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 (for more information, see sepia).
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. While it is not commonly used in China, Chinese do use the cephalopod ink to dye the dough of dumplings sometimes.
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.
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