Weekly Research: March 21-27, 2011

Photoreception in Animals

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Most animals have receptors allowing them to detect light – photoreceptors. Depending on how advanced the photoreceptors are, this can allow them to sense just the presence of light, going through to the direction of light, to the highest levels allowing animals to see predators and other objects. We can distinguish between several types of photoreceptors. The most general of these distinctions is whether the photoreceptor is ciliary or rhabdomeric, the difference being whether the light-sensitive membrane derives from cilia (ciliary; e.g. in vertebrates) or whether many microvilli of apical cells form a rhabdom (e.g. flatworms, molluscs, arthropods). Keep in mind though that these types coexist in most organisms; this is why there is such a bewildering array of photoreceptor types, from isolated photoreceptor cells to simple eyespots to proper eyes with lenses. Some photoreceptors have no real morphological features: the retinal ganglion cells in vertebrates and photoreceptors in the neural ganglia of several invertebrates are examples of these. We do not know how (or if!) they work.