3.12.2008

cephalopod camouflage

I've got some really cool stuff to show you. (This time there's video!)

So, while chameleons are well-known for their ability to change the color of their skin, they don't actually do it to camouflage themselves. Each species of chameleon is naturally colored to match their surroundings, and they really only change colors to send signals to other creatures, such as their mood or their physiological state.

Cephalopods, on the other hand, have much more advanced control over their coloration, in addition to the texture of their skin. Members of the clade Cephalopoda, particularly those in the clade Coleoidea, include octopuses (yes, octopuses), squid, and cuttlefish. Members of this clade have intellects far superior to any other invertebrates', which aids in their use of camouflage.

Octopuses, considered the smartest of invertebrates, are able to change both the texture and the color of their skin, through direct neural control of the muscles connected to pigment sacs called chromatophores.
A cephalopod chromatophore
(© www.tolweb.org)

The chromatophore is one of many small organs just under the skin, with a sac called the cytoelastic sacculus, which contains tiny pigment granules. Relaxed, the compressed sacculus is opaque, hiding the pigment granules. When the muscles surrounding the sac contract, the membrane of the sacculus stretches out (with as much as 50 times the area of its relaxed state) to expose the color of the pigment within. The skin of the cephalopod contains millons of these chromatophores, containing red, yellow, orange, brown, and/or black pigments, allowing the creature to take on as many as 50 different appearances, and change its coloration very rapidly. They also have leucophores, which display white spots, and iridiophores, which refract light and make the animal seem luminescent. All of these changes in color, in addition to the shape-shifting ability, allow it to camouflage or communicate very effectively.

A cephalopod can take in the nature of its surroundings using its highly complex eyes (cuttlefish have W-shaped pupils and two foveas, for acuity looking both forward and backward), and feel and smell the things it touches with its suckered tentacles. Here, an octopus effectively mimics its surroundings, and when threatened, displays its aggressive white color (notice how the white ring around the eye makes it look bigger and more intimidating):

video
(© Richard T. Hanlon, Marine Biological Laboratory, Woods Hole, MA)

Here are some other examples of octopuses adapting to the colors and textures of their environments:

video
(© National Geographic)


video
(© Hanlon)

Also in Coleoidea are the squid, some of which can also change the patterns of their skin. In the case below, when two males fight, they display aggressive white spots. When a male is courting a female, however, he shows his attractive brown shade. He can also split his coloration, however, to show the sexy brown side to the female, while his other half displays the aggressive white to fend off any other males in the area. Even cooler is that when she swims to the other side of him, he instantly switches the pattern, so she only sees his non-aggressive coloration:

video
(© Hanlon)

The third clade in Coleoidea are the cuttlefish, who have similar abilities to change the pigmentation and texture of their skin. Some species display bright coloration to show off to the ladies or to ward off predators, while others do their absolute best to blend in:

video
(© Hanlon)

One of the most fascinating things they can do is display moving bands of coloration on their skin, making it appear as if rays of light are moving across their bodies. I think the only other place I've seen this is on a space ship in Star Trek (yes this is real):

video
(© NG)

Here's some more fascinating video from PBS' NOVA series, with guest Mark Norman, marine biologist and curator of Museum Victoria in Australia (sound on):

video
(© NOVA/WGBH)

(side note: British people and their colonies say Cephalopod with a "kef" instead of "sef" like we Americans.... they still ain't figured out all of the English language yet)

video
(© NOVA/WGBH)

When the Giant cuttlefish of Australia come together to breed, the biggest, toughest males engage in intrasexual selection, where the most impressive guy wins. They always put on a dazzling show as they duke it out for mating rights (whether or not the females actually care):

video
(© NOVA/WGBH)

The little guys can't compete with the machismo of the larger, stronger males, so they've developed a new tactic to get to the waiting females: pretend to be one. In an amazing display of invertebrate guile, smaller male cuttlefish will change their body size and shape to match that of a female, and slip past the fighting males:

video
(© NOVA/WGBH)

And finally, the Flamboyant cuttlefish, the only one known to walk around on its legs, shows a bright and complex pattern when threatened to demonstrate to predators that it is poisonous to eat (also look at these):

video
(© NOVA/WGBH)

These cephalopods have developed complex systems of visual and tactile information-
gathering, and have the brains and skin to match, making them some of the most impressive marine invertebrates in the world. With advanced nervous systems capable of learning and problem-solving, eyesight that rivals that of the great apes, and a system of skin control that suits any environment, they are no doubt easily comparable to some of the most advanced predators that walk on land. Without, of course, the limitations of a skeleton:

video
(James B. Wood, Bermuda Institute of Ocean Sciences)


Paddy