| "At that moment I enjoyed the astonishing spectacle of the brilliant spots, which appeared upon the skin of this animal, whose remarkable form had already impressed me; sometimes it was a ray of sapphire blue which blinded me; sometimes of opalescent topaz yellow, which rendered it still more striking; at other times these two rich colours mingled their magnificent rays. During the night these opalescent spots emitted a phosphorescent brilliance which rendered this mollusc one of the most splendid of Nature's products." - Jean Baptiste Verany, on viewing a captured Histioteuthis bonelli (from "Mollusques Mediterraneens", 1851)
Most cephalopods, and squids in particular, produce brilliant displays of bioluminescence with organs called photophores. The photophores are a network of photogenic cells found in the skin, just underneath the layer of chromatophores used to change the skin colour. Some of them are simple light-producing cells which can only be revealed or hidden as neccessary by dilating the chromatophores that cover them. Others are complicated organs, with reflectors, colour filters, lenses and light guides. These can be used to create dazzling displays of variable colours and patterns.
Interestingly, while most deep-sea cephalopods produce their own light by combining luciferin and luciferase within photocytes inside the photophores, most neritic cephalopods (those that live in the shallow areas of continental shelves) do not produce their own light. Instead, they rely on a symbiosis with bacteria known as Vibrio fischeri, which are attracted to the light organs while the cephalopods are still infants. About 90% of a squid's symbiont V. fischeri will be discharged from the cells every morning, to be replaced by newborn bacteria.
Uses of Bioluminescence:
Bioluminescence has many uses for cephalopods. For many species, it is used to attract mates. It also has camouflaging properties, and can be used to distract enemies when ejected in a light-enhanced cloud of ink. Some varieties that live near the boundary of light penetration have ink sacs containing light-producing cells, and they can emit either a cloud of ink or a cloud of illumination according to their need.
Some species of squid use luminescence for camouflage. They can regulate the amount of light produced by their anterior photophores to match the level of moonlight falling on them, thereby blurring their outline from beneath and nullifying their shadow to hide themselves from predators underneath them.
Other species, and the octopus variety Stauroteuthis syrtensis use luminescence to lure their prey. These most commonly have photophores dotting their arms (not the tentacles).
At least one species, Taningia danae, uses enormous photophores on the ends of two of its arms to confuse its predators with strobe flashes of light. These photophores can be as large as lemons, and are equipped with black "eyelids" which open and shut rapidly when the squid is threatened. While this is of little use against sonar-hunting whales, it can be effective against tuna and lancetfish.
Finally, there are many species that appear to glow to attract mates and identify themselves. These typically have clusters of photophores all over their bodies, in patterns unique to each species and sex.
Sources:
- "Kingdom of the Octopus", Frank W. Lane, 1960
- "The Universe Below", William J. Broad, 1997
- http://phylogeny.arizona.edu/tree/eukaryotes/animals/mollusca/cephalopoda/cephalopoda.html
- http://www.imm-km.unibe.ch/projekte/symbiosis/VfEs/EsVfmain.htm
- http://info.bio.cmu.edu/Courses/03441/TermPapers/97TermPapers/lux/bioluminescence.html
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