Ultrastructure of pigmented adult eyes in errant polychaetes (Annelida): implications for annelid evolution |
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Authors: | Dominick Suschenko Günter Purschke |
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Institution: | 1.Zoologie,Universit?t Osnabrück,Osnabrück,Germany |
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Abstract: | The evolution of photoreceptor cells and eyes in Metazoa is far from being resolved, although recent developmental and structural
studies have provided strong evidence for a common origin of photoreceptor cells and existence of sister cell types already
in early metazoans. These sister cell types are ciliary and rhabdomeric photoreceptor cells, depending on which part of each
cell is involved in photoreception proper. However, a crucial point in eye evolution is how the enormous structural diversity
of photoreceptor cells and visual systems developed, given the general molecular conservation of the photoreceptor cells.
One example of this diversity can be observed in Annelida. Within the polychaetes the errant forms, taxon Aciculata, constitute
the only group possessing true multicellular eyes in the adult stage. Thus far these organs have been investigated only in
taxa of Phyllodocida, a subgroup of Aciculata. Data on Eunicida and Amphinomida as well as certain phyllodocidan taxa had
been lacking. The ultrastructure of these adult eyes was investigated in various species of errant polychaetes, belonging
to Amphinomidae, Eunicidae and Hesionidae, to elucidate whether they provide any phylogenetic clues regarding either the evolution
of visual systems in Annelida or lophotrochozoan phylogeny in general. These eyes are composed of numerous supportive pigment
cells and rhabdomeric photoreceptor cells and sometimes additional cell types. As a rule the pigment and rhabdomeric cell
types form a continuous epithelium in which the two types intermingle. Presence of granules with shading pigment in sensory
cells is a common feature but is apparently restricted to a taxon comprising Phyllodocida and Eunicida s. str. Very likely
a lens-like structure does not belong to the ground pattern of annelid eyes, despite its presence in Phyllodocida. These lens-like
structures are formed by secretions or cellular processes of the pigment cells. In many species the eye cup communicates with
the exterior via a small cuticularized canal. This canal is interpreted as a rudiment due to the mode of formation in the
epidermis. With respect to current phylogenetic hypotheses, these multicellular eyes have either been developed in the stem
species of a taxon Aciculata nested within the polychaetes or have been evolved in the stem lineage of Annelida. Similarities
to gastropod eyes are interpreted as convergent and not as indication of common origin. Except for the photoreceptor cells
proper, the structure of the adult eyes in polychaetes most likely does not help to resolve lophotrochozoan phylogeny. |
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Keywords: | Aciculata Phyllodocida Amphinomida Eunicida Pigment cell Lens Rhabdomeric photoreceptor |
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