Cephalopod eye

Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions. They have a camera-type eye which consists of an iris, a circular lens, vitreous cavity (eye gel), pigment cells, and photoreceptor cells that translate light from the light-sensitive retina into nerve signals which travel along the optic nerve to the brain. For the past 140 years, the camera-type cephalopod eye has been compared with the vertebrate eye as an example of convergent evolution, where both types of organisms have independently evolved the camera-eye trait and both share similar functionality. Contention exists on whether this is truly convergent evolution or parallel evolution. Unlike the vertebrate camera eye, the cephalopods' form as invaginations of the body surface (rather than outgrowths of the brain), and consequently they lack a cornea. Unlike the vertebrate eye, a cephalopod eye is focused through movement, much like the lens of a camera or telescope, rather than changing shape as the lens in the human eye does. The eye is approximately spherical, as is the lens, which is fully internal.

The crystalins used in the lens appear to have developed independently from vertebrate crystalins, suggesting a homoplasious origin of the lens.

Most cephalopods possess complex extraocular muscle systems that allow for very fine control over the gross positioning of the eyes. Octopuses possess an autonomic response that maintains the orientation of their pupils such that they are always horizontal.

It has been documented that several types of cephalopods, most notably squid and octopuses, and potentially cuttlefish, have eyes that can distinguish the orientation of polarized light. This sensitivity is due to the orthogonal organization of neighboring photoreceptors. (Cephalopods have receptor cells called rhabdoms similar to those of other molluscs.) To illustrate, the vertebrate eye is normally insensitive to polarization differences because visual pigment in rods and cones is arrayed semi-randomly, and is thereby equally sensitive to any orientation of the e-vector axis of the light. Because of their orthogonal organization, the visual pigment molecules in cephalopod eyes have the highest light absorption when aligned properly with the light e-vector axis, allowing sensitivity to differences in polarization. The precise function of this ability has not been proven, but is hypothesized to be for prey detection, navigation, and possibly communication among the color-changing cephalopods.

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