Structure and ultrastructure of the central nervous system of the polychaete Nephtys, with special reference to photoreceptor elements

Jouin-Toulmond

1993

Acta Zoologica

Three different types of presumed unpigmented ocelli have been found in the anterior end of Parenterodrilus taenioides, a small gutless interstitial polychaete. The type‐1 ocelli are located in the palps and four ocelli have been found along the length of each palp. The type‐2 and type‐3 ocelli lie close together in the head segment and are located in posterior ganglionic expansions of the brain. There is one pair of the minute type‐2 ocelli but at least two pairs of the type‐3 organs, which are the largest ocelli. In each ocellus the sensory cells are of the ciliary type and possess two cilia whose plasma membranes branch into numerous microvilli. With the exception of the type‐1 ocelli they consist of a sensory cell and a supportive cell. In each ocellus the supportive cell forms a thin cup‐shaped envelope around the densely packed ciliary branches. The type‐1 ocelli consist of a single cell forming an intracellular vacuole (phaosome) which contains less densely packed microvillus‐like structures. In particular, the structure of these ocelli is compared with that in other polychaetes, with special emphasis on the remaining genera of the Protodrilida.

Section: Discussionmentioning

confidence: 99%

Ultrastructure of Presumed Ocelli in Parenterodrilus taenioides (Polychaeta, Protodrilidae) and their Phylogenetic Significance

Jouin-Toulmond

1993

Acta Zoologica

“…However, the placement of this clade among the annelids remains without certain phylogenetic relationships. Other phaosomes were found in species of Parenterodrilus taeniodes (Protodrilidae), Nephthys caeca (Nephtyidae) or Stylaria lacustris (Naididae, Clitellata) but there are major differences to S. armiger (Purschke and Jouin Toulmond, 1993;Zahid and Golding, 1974;Purschke, 2003). Recent evidence suggests that the phaosomes of Clitellata represent modified rhabdomeric photoreceptor cells or receptor cells newly evolved in Clitellata (Purschke et al, 2006).…”

Section: Ciliary Photoreceptor-like Sense Structuresmentioning

confidence: 96%

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

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.

“…Thus they differ from other small eyes, such as are present in Sphaerosysllis hystrix Claparède, 1863, which show the structural peculiarities of adult eyes as described above although composed of only a few cells (see Purschke et al 2006). This likewise applies to the small eyes of Nephtys Cuvier, 1817 species (see Zahid and Golding 1974). However, besides structural differences these eyes may be distinguished by their molecular fingerprint and clearly represent two different types (Arendt et al 2002, unpubl.…”

Section: Distinguishing Larval Eyes and Adult Eyesmentioning

confidence: 82%

“…The larval eyes may persist or, in other polychaetes, be replaced by adult eyes of similar simple structure. Thus far such multicellular eyes have only been found in errant polychaete species and their fine structure has been described in certain taxa of Phyllodocida: Polynoidae, Aphroditidae, Sigalionidae, Syllidae, Nereididae, Nephtyidae, Phyllodocidae and Alciopidae Bröckelmann 1965, 1966;Hermans and Eakin 1974;Whittle and Golding 1974;Zahid and Golding 1974;Singla 1975;Bocquet 1976Verger-Bocquet 1983, Nicolas and Bassot 1984Eakin and Brandenburger 1985;Rhode 1991). According to Rouse and Fauchald (1997) Phyllodocida is part of Aciculata and Eunicida s. l. is its sister group; the latter comprises Eunicida s. str.…”

Section: Introductionmentioning

confidence: 97%

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.