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.
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) which 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 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").
However, 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. Unfortunately, few controlled experiments have been conducted to substantiate this. Cephalopod ink is nonetheless generally thought to be more sophisticated than a simple "smokescreen"; the ink of a number of squid and cuttlefish has been shown to function as a conspecific chemical alarm.
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, and small amounts of free amino acids, including taurine, aspartic acid, glutamic acid, alanine and lysine.
The typical defence response of the sea hare to a predator is the release of chemicals such as free amino acids, ink from the ink gland and opaline from the opaline gland. Chemical deterrence involves the release of toxic chemicals which are noxious to predators and rapidly dissuades them from feeding. Ink creates a dark, diffuse cloud in the water which disrupts the sensory perception of the predator by acting as a smokescreen and as a decoy. The opaline, which affects the senses dealing with feeding, causes the predator to instinctively attack the cloud of chemicals as if it were indeed food.
Use by humans
Cephalopod ink has, as its name suggests, been used in the past as ink; indeed, 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, where it is used as a food colouring and flavouring, for example in pasta and sauces. For this purpose it is generally obtainable from fishmongers or gourmet food suppliers. The ink is extracted from the ink sacs during preparation of the dead cephalopod, usually squid, and therefore contains no mucus.
- Roger T. Hanlon, John B. Messenger: Cephalopod Behaviour, page 2. Cambridge University Press, 1999, ISBN 0-521-64583-2
- Roy L. Caldwell (2005), "An Observation of Inking Behavior Protecting Adult Octopus bocki from Predation by Green Turtle (Chelonia mydas) Hatchlings" http://muse.jhu.edu/journals/pacific_science/v059/59.1caldwell.pdf
- G.E. MacGinitie, N. MacGinitie (1968) Natural History of Marine Animals, Pages 395-397, 2nd ed. McGraw-Hill, New York.
- Charles D. Derby (2007), "Escape by Inking and Secreting: Marine Molluscs Avoid Predators Through a Rich Array of Chemicals and Mechanisms" http://www.biolbull.org/cgi/reprint/213/3/274.pdf
- Derby, Charles D.; Kicklighter, Cynthia E.; Johnson, P. M. & Xu Zhang (29 March 2007). "Chemical Composition of Inks of Diverse Marine Molluscs Suggests Convergent Chemical Defenses" (PDF). Journal of Chemical Ecology. 2007 (33): 1105–1113. doi:10.1007/s10886-007-9279-0. Archived from the original (PDF) on 29 March 2007. Retrieved 9 May 2015.
- Inman, Mason (29 March 2005). "Sea Hares Lose Their Lunch". Sciencemag.org. Retrieved 10 May 2015.