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

Abstract

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.

Immunohistochemical investigations of the development of Scoloplos armiger (“intertidalis clade”) indicate a paedomorphic origin of Proscoloplos cygnochaetus (Annelida, Orbiniidae)

“…). Moreover, as expected for direct developers the arrangement of the nervous system in early juveniles of S. armiger resembles that of adults (Wilkens & Purschke ).…”

Systematics, evolution and phylogeny of Annelida – a morphological perspective

“…Interestingly, such palp nerve roots have also been reported from taxa which do not possess palps (Scalibregmatidae, Paronidae and Orbiniidae; see Orrhage and Müller, 2005;Wilkens and Purschke, 2009b). This has been taken as an indication of reduction of palps in those taxa, rather than their primary absence.…”

“…9 on the dorsal root of the circumoesophageal connective have been reported in most taxa investigated and may be promising candidates for having been present in the -br = branchia, no = notopodium, ne = neuropodium, ci = cilium, mv = microvillus, dc = dorsal cirrus, vc = ventral cirrus, la = lateral antenna, pa = palp, ma = median antenna, tc = tentacular cirrus. SEM Micrographs; originals E, F M: Nesnidal, Osnabrück, G-J S; Raabe, Osnabrück annelid ground pattern (Wilkens and Purschke, 2009b). Taxa regarded as belonging to the basal radiation (see Weigert et al, 2014) do not show a unique pattern but at least nerve root No.…”

“…adriatica and with respect to their proposed phylogenetic position more importantly in the orbiniid Scoloplos armiger (Wilkens and Purschke, 2009b;Purschke, 2011). The pigmented eyes of Sipunculida are also structurally similar to the adult eyes of polychaetes (Purschke, 2011), which are especially important in the "new annelid phylogeny" where Sipuncula are part of the annelid radiation (Dordel at al., 2010;Weigert et al, 2014).…”

Invertebrate neurophylogeny: suggested terms and definitions for a neuroanatomical glossary

“…Apart from their simple structure, larval eyes are characterized by their molecular fingerprint and can thus be distinguished with certainty from adult eyes (Arendt et al, unpublished information). However, a structural distinction between a persisting larval eye and a newly developed miniaturized adult eye is not always discernible [125,126]. …”