Ultrastructure and functional morphology of the dorsal organs in Scoloplos armiger (Annelida, Sedentaria, Orbiniida)

Abstract

Annelids and particularly polychaetes possess a great variety of sensory organs and respond to numerous sensory stimuli. Although eyes and nuchal organs are comparatively well studied, the so-called dorsal organs are among the lesser-known sense organs in aquatic annelids. Moreover, they are known to be restricted to only two out of approximately 80 families of polychaetes—Orbiniidae and Spionidae—which are not closely related. These organs have been regarded as segmentally repeated nuchal organs in the latter taxon, but in Orbiniidae, data are lacking, although it is known that the organs occur almost along the entire trunk except for the anterior-most segments. Furthermore, although the nuchal organ ultrastructure is known to be comparatively uniform for many polychaete species, a comparative investigation has not been conducted in Orbiniidae. To bridge this data gap, we examined an intertidal population of the widely distributed species Scoloplos armiger. Although not completely identical, nuchal and dorsal organs show a high degree of correspondence in the examined specimens. Moreover, both organs correspond to the general structure of nuchal organs. They comprise ciliated supportive cells and bipolar receptor cells and are innervated directly from the brain. The supportive cells form subcuticular spaces and olfactory chambers apically protected by specialized microvilli that house the sensory processes—cilia and microvilli—of the monociliated receptor cells. Therefore, it can be concluded that nuchal and dorsal organs are also identical in Orbiniidae. However, despite general correspondence with spionids, convergent evolution in the two taxa appears to be the most parsimonious interpretation.

Development and structure of the anterior nervous system and sense organs in the holopelagic annelid Tomopteris spp. (Phyllodocida, Errantia)

“…As is typical in polychaete nuchal organs, the sensory cells of T. helgolandica are bipolar receptor cells terminating in clusters at the bottom of the olfactory chamber (Buhre & Purschke, 2021;Purschke, 1997Purschke, , 2005Verger-Bocquet, 1992). Most receptor cells correspond to those generally described as having only one branched cilium with a modified axoneme and additional microvilli.…”

“…In contrast to the external variability of nuchal organs, their anatomical structure is almost identical, comprising only a few but highly similar cell types throughout all taxa investigated (Buhre & Purschke, 2021;Purschke, 1997Purschke, , 2005Verger-Bocquet, 1992). These organs comprise ciliated supporting cells, bipolar primary receptor cells, a retractor muscle (if they can be retracted), and direct innervation from the brain's posterior region (Buhre & Purschke, 2021;Helm et al, 2018b;Jelsing & Jacobsen, 2010;Purschke, 1997Purschke, , 2005Purschke, , 2016Rullier, 1951;Schmidtberg & Dorresteijn, 2010;Verger-Bocquet, 1992). The supportive and receptor cells form an olfactory chamber separated from the exterior that houses the sensory processes.…”

“…Besides a few loss and reduction cases, several taxa possess extremely large and well-developed nuchal organs, which extend posteriorly over several trunk segments. Examples usually mentioned are Amphinomidae and certain Spionidae, Syllidae, and Phyllodocidae members (Buhre & Purschke, 2021;Purschke, 1997Purschke, , 2005. While known to possess enlarged nuchal organs (Fernández-Álamo, 2022;Pleijel, 2001), Tomopteridae have mostly been neglected in this respect.…”

“…In nuchal organs not situated in more or less deep pits, the olfactory chamber is often separated from the outside by a modified cover formed of microvilli or cuticles of the supportive cells (Buhre & Purschke, 2021;Purschke, 1997). However, nuchal organs without such covers have been found in certain errant polychaetes.…”