Wednesday, January 2, 2008

7.4 Tako sashimi

If you are a Jerry Lettvin fan, please skip to paragraph 2. Mere mortals, please read on:
Remove octopus guts, eyes, and beak and freeze for at least 72 hours.
Thaw and steam for 30 minutes or until done.
Cut off the tentacles and with a very sharp knife cut them into 1/8 inch slices at a 45 degree angle. Slice the belly into 1/4 inch strips.
Mix wasabi with soy sauce.
Dip octopus and enjoy.
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Save the eyes. Or visit your local Japanese restaurant, have some sashimi, and ask for octopus eyes while there. Why? A comparative anatomical study.

It is often mentioned that the octopus eye is structurally similar to the human eye, complete with cornea, iris, lens, vitreous, and retina. Presumably, it has sharp, color, and 3D vision, and is able to differentiate shapes.

However, a close examination shows that its crystalline lens has a fixed focal length like that of a camera, so the octopus focuses by moving the lens close to or away from the retina (some say by changing the shape of the eye globe). Also, the coordination of the octopus eyes is via the statocyst so the slit pupil of each eye is always in the horizontal position. Probably the most interesting feature is the retina: the photoreceptor cells are directed towards the light source.

If you go back to Topic 2.3.1, you'll see the human rods and cones are oriented against the light source, in other words, the photoreceptors are pointing towards the choroid/sclera, while that of the octopus, towards the vitreous. In terms of the efficiency in photon capture, the octopus obviously has the upper hand. Strange, huh? In fact, this has been used as the evidence of a design flaw of the human eye.

Actually, the octopus lives in (sea)water which has a refractive index of 1.33 and the sunlight is refracted and polarized when it enters water (especially at dusk and dawn). The octopus hunts at dusk. To see a prey such as a jellyfish, it will need a crystalline lens with even higher refractive index (than 1.33) in order to focus properly - because the corneal refractive power is neutralized by the water outside and the aqueous humor inside the eye. And equally important, it will need polarization vision (which actually works well with horizontal slit pupils). Polarized light is less intense and spectrally shifted, yet with more contrast, so it allows the octopus to better appreciate the pattern that leads to the capture of, e.g., a well camouflaged crab. Polarization vision will require the photoreceptors to directly analyze the incident light or the visual information is lost. The photoreceptors therefore should be oriented towards the light.

Humans can always wear Polaroid sunglasses, e.g., to see comfortably through light reflected and polarized by the surface of water/ocean. If you are a diver, try diving with your polarizers on at dusk and see if you can spot a Portuguese Man-of-War floating by. Report back after you have recovered from the pain. An octopus on land will need heavy prescription (for high myopia) just to navigate.

The take-home message: All eyes are developed to serve the one and only purpose: to see well to survive, wherever the rest of the body may reside.

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