Topic: Compound eyes in sabellid annelids
Compound eyes have evolved convergently in the annelids, notably amongst the sabellids, where they evidently serve as an optical alarm system.
Probably all annelids are photosensitive; even those that lack unsurprisingly lack eyes such as the familiar earth-worm (see love-darts), 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 eyes evolved independently in this group (although their ancestors almost certainly had photoreceptors).
Compound eyes are best known amongst the arthropods, where they have evolved several times independently. Somewhat less well known is that compound eyes have evolved convergently in the annelids, notably amongst the sabellids, where they evidently serve as an optical alarm system (a similar system using compound eyes is also found in some bivalve molluscs) detecting movements that lead to very rapid retreat of the worm into the safety of its tube. They also occur in the so-called fan worms, or serpulids, which have a similar mode of life to the sabellids. It is not clear, apparently, whether the compound eyes evolved independently in each group.
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Map of Life - "Compound eyes in sabellid annelids"
June 25, 2017