How is a sea urchin like an insect eye?

EchinometraSea urchins and starfish may not be the first creatures that come to mind when pondering animal eyes, but they are full of surprises. Many echinoderms (sea urchins, starfish, brittle-stars, sea cucumbers and sea lillies) are sensitive to light. Certain brittlestars (the ophiuroids) and sea urchins (the echinoids) even have compound eye-like visual systems that in some ways rival the arthropods. So it is that along the arms of one brittlestar (Ophiocoma wendtii) we find calcitic ‘microlenses’. These are composed of modified ossicles, can be shaded using pigmented chromatophore cells, and are underlain by the photoreceptors. In sea urchins visual acuity appears to be based on photoreceptors in the tips and bases of tube feet, and here shading is not mediated by protective pigments but rather by skeletal elements (spines and tube foot pores). Visual acuity of these animals apparently approaches that of the speedy predator Nautilus.

Ophiocoma_scolopendrinaAs echinoderms have a diffuse nervous system rather than a defined brain it is not clear what sort of image the animal may ‘see’ and yet their eyes provide fascinating points of convergence. For example, the calcite microlenses of brittlestars closely resemble those found in the compound eyes (the schizochroal variety) of certain trilobites. Furthermore, the way that the sea urchin tube foot system gathers visual information parallels the structure and function of the insect compound eye in interesting ways.

Head over to the latest entry on echinoderms at the Map of Life to read the full account of this strange case of convergent evolution! You may never look at a sea urchin again in the same way…

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