Topic: Camera eyes in alciopid annelids
There is a striking example in the group known as the alciopids, which are pelagic polychaetes. The similarity of their camera eye to the vertebrate eye has attracted considerable comment.
Eyes in annelids
Probably all annelids are photosensitive; even those that unsurprisingly lack eyes such as the familiar earthworm, although some forms (e.g. Stylaria) appear to have regained eyes when they returned to the surface. Many, notably the polychaetes (which are very largely marine), have eye-spots, and these come in a considerable variety. The annelids belong to a larger group, known as the lophotrochozoans, and so are related to such groups as the brachiopods (which possess very similar setae) and molluscs. The precise origin of the annelids is controversial, but it is possible that eyes evolved independently in this group (although their ancestors almost certainly had photoreceptors).
Camera eyes in alciopid annelids
In the context of convergence in annelid eyes, the most striking example occurs in the group known as the alciopids, which are pelagic polychaetes. To find camera eyes in this group is not so surprising when it is realized that they are active predators. The similarity to the vertebrate eye has attracted considerable comment, and the eye structure includes a spherical lens, retina (similar to cephalopods in that the retina is above the neural complex, so known as upright) (in contrast to the inverted retina of the vertebrates) and a double-layered cornea. Another interesting convergence is the possession of a so-called accessory retina (also upright), an arrangement which is also found in a number of deep-sea fish and cephalopods. Its precise function is a matter of debate, but it may serve to help capture the limited amount of light penetrating to depth. Not surprisingly it detects long wave-length light, and this sensitivity to blue light (c. 460-480 nm) is again convergent.
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Map of Life - "Camera eyes in alciopid annelids"
January 23, 2018