Ultrastructure of pigmented eyes in Onuphidae and Eunicidae (Annelida: Errantia: Eunicida) and its importance in understanding the evolution of eyes in Annelida

Abstract

Annelida displays enormous eye and photoreceptor cell (PRC) diversity. In polychaetes, larval and adult eyes can be readily distinguished as the former are small, inverse and comprised of only two or three cells, and the latter are usually everse and multicellular. However, there are some species in which adult eyes are small, fewer in number or even absent. Recent studies show that two pairs of multicellular adult eyes belong to the ground pattern of a clade comprising Amphinomida/Sipuncula and Pleistoannelida with Errantia and Sedentaria. As ultrastructural data in one higher taxon of Errantia, Eunicida, are scarce or completely lacking for certain subgroups, we investigated the structure of pigmented eyes in three species of eunicidan bristle worms: Aponuphis bilineata (Onuphidae), Paucibranchia bellii (Eunicidae) and Leodice cf. torquata (Eunicidae). All had two pairs of pigmented eyes possessing typical adult structures: rhabdomeric PRCs (rPRC), pigmented supportive cells (PSCs) and additional cell types (in some species). The PSCs formed a shading pigment cup housing the sensory processes of the PRCs in everse orientation. Both Eunicidae species examined possess a lens-like structure formed by extensions of the PSCs as typical for members of Errantia, suggesting that lens-like structures formed by PSC processes were acquired in the stem lineage of Errantia and represent an autapomorphy of this clade. Our observations provide further evidence for the presence of two pairs of adult eyes in the ground pattern of Amphinomida/Sipuncula and Pleistoannelida.