Ultrastructure of phaosomous photoreceptors in Stylaria lacustris (Naididae, 'Oligochaeta', Clitellata) and their importance for the position of the Clitellata in the phylogenetic system of the Annelida

2009

Journal of Morphology

The phylogenetic position of Orbiniidae within Annelida is unresolved. Conflicting hypotheses place them either in a basal taxon Scolecida, close to Spionida, or in a basal position in Aciculata. Because Aciculata have a specific type of eye, the photoreceptive organs in the orbiniid Scoloplos armiger were investigated to test these phylogenetic hypotheses. Two different types of prostomial photoreceptor-like sense organs were found in juveniles and one additional in subadults. In juveniles there are four ciliary photoreceptor-like phaosomes with unbranched cilia and two pigmented eyes. The paired pigmented eyes lie beside the brain above the circumoesophageal connectives. Each consists of one pigmented cell, one unpigmented supportive cell and three everse rhabdomeric sensory cells with vestigial cilia. During development the number of phaosomes increases considerably and numerous unpigmented sense organs appear consisting of one rhabdomeric photoreceptor cell and one supportive cell. The development and morphology of the pigmented eyes of S. armiger suggest that they represent miniaturized eyes of the phyllodocidan type of adult eye rather than persisting larval eyes resulting in small inverse eyes typical of Scolecida. Moreover, the structure of the brain indicates a loss of the palps. Hence, a closer relationship of Orbiniidae to Phyllodocida is indicated. Due to a still extensive lack of ultrastructural data among polychaetes this conclusion cannot be corroborated by considering the structure of the unpigmented ciliary and rhabdomeric photoreceptor-like sense organs.

Section: Ciliary Photoreceptor-like Sense Structuresmentioning

confidence: 99%

Pigmented eyes, photoreceptor‐like sense organs and central nervous system in the polychaete Scoloplos armiger (Orbiniidae, Annelida) and their phylogenetic importance

Wilkens

2009

Journal of Morphology

“…7A, B). An increase in the number of receptive structures is achieved by evagination of the apical cell membrane, so that the sensory cell forms a mushroom-like process bearing densely arranged microvilli (e.g., Spionidae, Opheliidae, Polygordiidae; see Hermans & Cloney, 1966;Brandenburger & Eakin, 1981;Rhode, 1991;Bartolomaeus, 1993;Purschke, 2003;Fig. 8A).…”

Section: Eyesmentioning

confidence: 99%

“…That is, the two cell types form a continuous epithelium around an extracellular cavity into which the sensory processes project (Fig. 7C-E ;Purschke, 2003). In this epithelium pigmented supportive cells and sensory cells alternate.…”

Section: Eyesmentioning

confidence: 99%

Sense organs in polychaetes (Annelida)

2005

Hydrobiologia

151

Polychaetes possess a wide range of sensory structures. These form sense organs of several kinds, including the appendages of the head region (palps, antennae, tentacular cirri), the appendages of the trunk region and pygidium (parapodial and pygidial cirri), the nuchal organs, the dorsal organs, the lateral organs, the eyes, the photoreceptor-like sense organs, the statocysts, various kinds of pharyngeal papillae as well as structurally peculiar sensory organs of still unknown function and the apical organs of trochophore larvae. Moreover, isolated or clustered sensory cells not obviously associated with other cell types are distributed all over the body. Whereas nuchal organs are typical for polychaetes and are lacking only in a few species, all other kinds of sensory organs are restricted to certain groups of taxa or species. Some have only been described in single species till now. Sensory cells are generally bipolar sensory cells and their cell bodies are either located peripherally within the epidermis or within the central nervous system. These sensory cells are usually ciliated and different types can be disinguished. Structure, function and phylogenetic importance of the sensory structures observed in polychaetes so far are reviewed. For evaluation of the relationships of the higher taxa in Annelida palps, nuchal organs and pigmented ocelli appear to be of special importance.

“…In the latter two hypotheses these eyes must have been lost in several lineages while larval eyes have been retained throughout the major lineages of polychaetes. An additional loss of the larval ocelli must have occurred in the stem lineage of Clitellata since the only photoreceptors present are phaosomes (Purschke 2003(Purschke , 2005Purschke et al 2006). Whether this photoreceptor is derived from one of the common photoreceptors (ciliary or rhabdomeric) or a newly evolved structure is the subject of still ongoing investigations.…”

Section: Discussionmentioning

confidence: 99%

Ultrastructure of pigmented adult eyes in errant polychaetes (Annelida): implications for annelid evolution

Suschenko

2008

Zoomorphology

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.