Abstract
BackgroundSabellarids, also known as honeycomb or sandcastle worms, when building their tubes, produce chemical signals (free fatty acids) that are responsible for larval settlement and the formation of three-dimensional aggregations. The larval palps and the dorsal hump (becoming the median organ in adults) are presumed to participate in such a substrate selection during settlement. Notably, the sabellariid median organ is an apparently unique organ among annelids that has been attributed with a sensory function and perhaps with some affinities to the nuchal organs of other polychaetes. Nevertheless, detailed investigations of this prominent character complex including ultrastructural examinations are lacking so far.ResultsOur comprehensive investigations provide data about the anterior sensory organs in Sabellariidae and inform about their transformation during pelagic larval development. We used a comparative approach including immunostaining with subsequent confocal laser scanning microscopy (clsm), histological sections as well as electron microscopy in a range of larval and adult stages of two sabellariid species. We find that the neuronal innervation as well as the ultrastructure of the sabellariid ciliary structures along the median organ are highly comparable with that of nuchal organs known from other polychaetes. Furthermore, the myoinhibitory protein (MIP) – a protein known to be also involved into chemo-sensation - was detected in the region of the larval median organ. Moreover, we reveal the presence of an unusual type of photoreceptor as part of the median organ in Idanthyrsus australiensis with a corrugated sensory membrane ultrastructure unlike those observed in the segmental ocelli of other polychaetes.ConclusionsWe are describing for the first time the nuchal organ-like structures in different developmental stages of two species of Sabellariidae. The external morphology, neuronal innervation, developmental fate and ultrastructure of the newly-discovered median organ-based ciliary pits are comparable with the characteristics known for annelid nuchal organs and therefore indicate a homology of both sensory complexes. The presence of myoinhibitory peptide (MIP) in the respective region supports such a hypothesis and exhibits the possibility of an involvement of the entire sabellariid median organ complex, and in particular the prominent ciliated pits, in chemo-sensation.
Highlights
Sabellarids, known as honeycomb or sandcastle worms, when building their tubes, produce chemical signals that are responsible for larval settlement and the formation of three-dimensional aggregations
Sabellariids have been the object of several studies aiming to decipher the molecular cues that induced gregarious settlement, and the sensory organs potentially involved in this process
In order to elucidate further details of morphological structures and the putative role of the sabellariid dorsal hump/median organ, we examined various developmental stages of the sabellariid species Sabellaria alveolata (Linnaeus, 1767) and Idanthyrsus australiensis (Haswell, 1883) using a comparative approach with diverse microscopic techniques including immunostaining with subsequent confocal laser scanning microscopy, as well as histological sections and electron microscopy
Summary
Sabellarids, known as honeycomb or sandcastle worms, when building their tubes, produce chemical signals (free fatty acids) that are responsible for larval settlement and the formation of three-dimensional aggregations. Molecules present in the cement secreted by benthic juveniles and adults when building their tubes (free fatty acids), are known to be responsible for larval settlement [7, 10,11,12,13,14,15]. In this respect, the larval palps and the dorsal hump (becoming the median organ in the adults) are presumed to participate in substrate selection during settlement [16,17,18]
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