A brachiopod's eye-sight - Understanding the evolution of light sensitive organs in the phylum Brachiopoda

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An exceptional combination of two ciliary photoreceptors (one with a lens, the other with shading pigment) forming the larval eyes of the brachiopod Terebratalia transversa was discovered during a recent study. Additional in-situ hybridization experiments detected Pax6 and Ttr-c-Opsin expression in these ciliary photoreceptors, questioning the "rule" that light sensitive organs ("eyes") of lophotrochozoans exclusively deploy rhabdomeric photorecep-tors. Since several brachiopod species are described to have larval "eye spots" of unknown structure and show photoreactive behaviour as adults, a morphological census of all possible light sensitive structures in brachiopods (larvae and adults) is planned in this project using histological, ultrastructural, immunocytochemical and molecular/gene-expression methods. Apart from larval "eye spots", mantle tissue of adults will be examined. Using both morpho-logical and molecular data characterizing brachiopod larval eyes and adult photoreceptors, we aim at describing all possible forms of eyes/photoreceptors in Brachiopoda to hypothe-size a ground pattern of brachiopod photoreceptors and to provide a comprehensive data set for comparison with other lophotrochozoan taxa. We expect that our results will shed new light on our understanding of the evolution of light sensitive structures in lophophorates, lophotrochozoans and Bilateria including the last common bilaterian ancestor.