Biologists have long used the eyes as one of their benchmark classifications between vertebrates and invertebrates. More specifically, they have looked at the photoreceptors utilized by each species to help classify it; the two major photoreceptors being rhabdomeric and ciliary. Recent studies, however, have forced many to retreat somewhat from this stand.
What are Photoreceptors?
Vision for all multi-cellular species is achieved with photoreceptors found in the retina of the eye. A photoreceptor cell uses a light-sensitive pigment derived from vitamin A, bound to a protein called an opsin. Light activates the opsin causing a change in the photo-pigment, which then binds to other proteins and becomes a chemical signal.
The two major opsin proteins common to all species are rhabdomeric and ciliary, or r-opsin and c-opsin. Most vertebrates – species with a bony skeleton and spine – utilize c-opsins. Most invertebrates utilize r-opsins. Scientists are discovering more and more exceptions to that rule, however. Bees, for instance, are invertebrates that use c-opsin photoreceptors. So are some annelids like leeches and earthworms. Nematodes (wormlike creatures with unsegmented bodies, like hookworms and pinworms) have no opsins at all, and therefore no vision.
A surprising number of species are appearing with both c- and r-opsin photoreceptors. In some, like certain marine worms, it appears c-opsins allow vision during the larval stage, giving way to r-opsin photoreceptors as adults. In others, one set of photoreceptors remains dormant. Humans are now known to have both types, although the c-opsins are dominant.
A classic example of r-opsin photoreceptors is found in arthropods, which have compound eyes. Cells throw up surface extensions called microvilli to increase the membranous area. The more area there is, the more photoreceptors are packed in. Microvilli are rod-shaped and look a lot like a flat top haircut. The apical end of the cells is rotated at a 90 degree angle from c-opsin photoreceptors, allowing free movement of the rods for optimum light-gathering capability.
The rods of r-opsins are extremely sensitive and can be triggered by very low light. This allows for vision acuity in low-light situations, although color recognition generally is reduced or absent.
C-opsin photoreceptors also achieve more membranous area, but in a different fashion. The apical membrane that contains them arranges itself in deep folds. Also called cones, they resemble a stack of flattened discs. Cones require more light to produce a chemical reaction. In humans, there are three different types of cones created by slightly different c-opsin proteins. Each collects a different wavelength of light, and work together to produce the signals that allow for color and depth perception.
Scientists are now discovering more and more species, like humans, that utilize both types of photoreceptors to varying degrees. The human eye, for instance, has about 120,000 r-opsin photoreceptors and more than 5 million c-opsin ones, explaining why they generally see much better in the daylight than at night. Nocturnal species, like owls, have significantly more r-opsin photoreceptors allowing them superior night vision.
http://scienceblogs.com/pharyngula/2006/09/rhabdomeric_and_ciliary_eyes.php - PZ Myers, Biologist and Associate Professor at University of Minnesota.
http://www.ncbi.nlm.nih.gov/pubmed/11604122 U.S National Library of Medicine.