Sea urchins use their bodies as a giant compound eye, according to a study published in a recent issue of the Proceedings of the National Academy of Sciences of the USA.
Esther Ullrich-Lüter and coauthors resolved a long-standing question of how sea urchins see despite the clear absence of eye-like structures, by taking advantage of the fact that the entire genome of the Purple sea urchin (Strongylocentrotus purpuratus) has been recently sequenced.
The unusual solution utilised by the authors involved finding out where rhabdomeric opsin Sp-opsin4 and Sp-pax6, two genes essential for photoreceptor function and development, respectively, were expressed in the sea urchin.
Using a specifically designed antibody against Sp-Opsin4 and in situ hybridisation for both genes, they detected expression in two distinct groups of photoreceptor cells located in the animal's numerous tube feet (which cover the entire body): one at the rim and the other at the base of the tube feet.
The authors propose that the differential shading of the photoreceptor cells at the base of the tube feet by the calcite skeleton of the sea urchin may afford some form of directional vision to the sea urchin.
For more information, see the paper: Ullrich-Lüter, EM, S Dupont, E Arboleda, H Hausen, and MI Arnone (2011) Unique system of photoreceptors in sea urchin tube feet. Proceedings of the National Academy of Sciences 108, pp. 8367–8372.
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