Fine structure of the cerebral eyes in Flabelligera affinis (Annelida, Sedentaria, Cirratuliformia): new data prove the existence of typical converse annelid multicellular eyes in a sedentary polychaete

Abstract

Sedentary polychaetes usually possess small and simple eyes with flabelligerids being an exception by possessing large pigmented eyes. These eyes have been described to represent a platyhelminth type of inverse pigmented eye instead of converse eyes typically found in polychaetes suggesting independent evolution of large eyes in Flabelligeridae. The aim of the present work was to reinvestigate this hypothesis by studying the cerebral eyes in Flabelligera affinis which has three pairs of pigmented eyes. The first and second pairs are large multicellular organs placed dorsally on the prostomium whereas the third so far unknown pair consists of minute eyes situated within the brain. Unexpectedly, the first and second pairs of eyes are typical annelid multicellular converse eyes with numerous rhabdometric photoreceptor cells intermingling with pigmented cells. At the opening of the pigment cup both eyes possess a lens-like structure which is formed by elongated unpigmented supportive cells. The third pair of eyes is converse in design as well consisting of only three cells: one pigmented cell, one rhabdomeric photoreceptor cell penetrating the pigmented cell, and one unpigmented supportive cell in front of the pigment cup opening. Thus, previous evaluations of the eyes as being of inverse design very likely represent a misinterpretation, where the lens-forming cells have been mistaken for sensory cell processes. Our results proving the presence of typical converse annelid multicellular eyes in Flabelligeridae suggest that such eyes belong to the pleistoannelid ground pattern and were retained in the last common ancestor of Sedentaria.