Subject:
A Day Octopus (Octopus cyanea) rapidly changes colour. Metachrosis is the physical ability to change color voluntarily, and it is often used as a type of camouflage. This ability is not unique to cephalopods; other animal species also possess it.
A well-known example is the chameleon. Chromatophores are specialised cells situated just under the skin's surface that allow for rapid color change. These pigment-containing and light-reflecting cells are found in octopuses, with pigment granules enclosed in an elastic sac located inside the chromatophores. To change color, the animal distorts the form or size of this sac through muscular contraction controlled by nerves. A good way to visually describe this mechanism is to compare it with a group of umbrellas viewed from above that open and close simultaneously.
Some species of cephalopods also possess iridophores and leucophores. Iridophores are cells made up of stacks of thin protein plates that function as multilayer reflectors, creating iridescent greens, blues, silvers, and golds. Leucophores, on the other hand, contain spherical protein assemblages that scatter light, reflecting the environment's colours and making the animal less noticeable.
Another trick these highly intelligent animals have up their sleeves is the ability to change their physical appearance, such as adapting the texture of their skin to objects in their environment. This perfect camouflage mechanism is achieved through a unique muscular hydrostatic system in their skin that alters the size and shape of their dermal bumps (called papillae).
Besides serving as an almost perfect camouflage system, chromatophores also serve as a means of communication between different individuals.
Some species of octopus use this system to ward off potential predators. The Blue-ringed Octopus (Hapalochlaena), a group of highly venomous octopus species, will flash iridescent blue rings as a warning display. Advertising through the use of bright warning colours to potential predators that an animal is poisonous, venomous, or otherwise dangerous is called “aposematism”.
Other octopus species, such as the Mimic Octopus (Thaumoctopus mimicus), use their chromatophores in combination with specific postures to fool potential predators by impersonating other marine animals like soles, lionfish, and the Banded Sea Krait (Laticauda colubrina), a venomous marine snake.
Technique:
Octopuses always make interesting subjects for underwater videographers and scuba divers in general. Observe this intelligent creature for a while before bombarding it with your video lights. Try to position yourself lower than your subject and, if possible, get some of the ocean's blue in the background.
Let the camera roll and anticipate; only this way can you capture any action. It may take several minutes of filming before anything happens, but if you wait long enough, you will be rewarded with some nice images.
Do not stress the animal as it may result in a big squirt of dark ink and the octopus's hasty retreat in the first small gap or hole on the reef. The non-interesting sections or parts without any action are easily removed once you have transferred the images to your computer. Octopuses always make interesting subjects for underwater videographers.
For a more in-depth description about aposematism please go to our vlog post 111 or click on this link: https://www.beyondscuba.com/post/inimicus-lll-aposematism
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