Notopodial “spinning glands” of Sthenelanella (Annelida: Sigalionidae) are modified chaetal sacs

Sthenelanella is an unusual genus of tube-dwelling scale worms, with fine fibrous threads that appear on either side of the body. These fibers emerge dorsally in long silvery bundles that are then tightly interwoven to construct the felt-like material of the tube. In the literature, these fibers are described as the products of so-called “spinning glands.” In this article, we investigated the ultrastructure of these notopodial fibers and show that they are annelid chaetae. Three or four dynamic microvilli of a basal chaetoblast form each of these feltage chaetae, making them the thinnest known annelid bristles. Our results show that the spinning glands of Sthenelanella uniformis are additional, highly modified notopodial chaetal sacs. We also show that the follicle cells, by their secretion of the enamel layer, play an active role in shaping the final chaeta. These findings not only increase the known morphological diversity of chaetae but also demonstrate the apparent plasticity of the machinery that form these chitinous structures. Our results are compared with chaetae in other annelids, with a particular focus on similar fibrous chaetae in Aphroditiformia.