Meeting Abstract
Sabellariids (Annelida) are widely distributed, occurring in all oceans and depths. They reproduce by free spawning gametes and have planktonic larvae, but are strictly tubicolous during benthic juvenile and adult stages. The tube, made of secreted cement and sand grains, can form conspecific aggregations of few individuals to several kilometer reefs. Adults of some species bear an anterior appendage, the median organ (MO), with seemingly sensory function and involved in settlement (recognizing conspecific tubes) and/or reproductive processes (stimulating spawning). We have followed a multifaceted approach to unravel the role of this organ, describe its external and internal morphology throughout ontogenetic development, and assess its taxonomic value. In order to achieve this we: 1) reviewed the literature for absence/presence and morphology of this organ and the species ecological traits in all sabellariids ; 2) investigated if the dorsal hump described in some larvae is the primordial form of the MO by studying the pattern of changes throughout the ontogenetic development in Sabellaria alveolata using immunohistochemical stainings and confocal laser scanning microscopy; 3) re-examined specimens identified as Idanthyrsus australiensis that revealed diversity in the MO external morphology in order to test the its taxonomic value. Thus, our results provide new insights into the sabellariid anterior morphology at different developmental stages, the potential sensory role of the MO and its supposed phylogenetic signal.
