Comparative ultrastructure of the radiolar crown in Sabellida (Annelida)
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ORIGINAL PAPER
Comparative ultrastructure of the radiolar crown in Sabellida (Annelida) Ekin Tilic1 · Greg W. Rouse2 · Thomas Bartolomaeus1 Received: 31 August 2020 / Revised: 4 November 2020 / Accepted: 17 November 2020 © The Author(s) 2020
Abstract Three major clades of tube-dwelling annelids are grouped within Sabellida: Fabriciidae, Serpulidae and Sabellidae. The most characteristic feature of these animals is the often spectacularly colorful and flower-like radiolar crown. Holding up such delicate, feathery appendages in water currents requires some sort of internal stabilization. Each of the above-mentioned family-ranked groups has overcome this problem in a different way. Herein we describe the arrangement, composition and ultrastructure of radiolar tissues for fabriciids, sabellids and serpulids using transmission electron microscopy, histology and immunohistochemistry. Our sampling of 12 species spans most of the phylogenetic lineages across Sabellida and, from within Sabellidae, includes representatives of Myxicolinae, Sabellinae and the enigmatic sabellin Caobangia. We further characterize the ultrastructure of the chordoid cells that make up the supporting cellular axis in Sabellidae and discuss the evolution of radiolar tissues within Sabellida in light of the recently published phylogeny of the group. Keywords TEM · Histology · Fabriciidae · Serpulidae · Sabellidae
Introduction Three family-ranked clades of tube-dwelling annelids are grouped within Sabellida (Kupriyanova and Rouse 2008): calcareous tube worms, Serpulidae Rafinesque, 1815; Sabellidae Latreille, 1825, with a mucus/sediment tube and the mostly minute Fabriciidae Rioja, 1923, as the sister group to the former two (Tilic et al. 2020). Like blooming flowers, sabellid worms extend their radiolar crowns into the water column, exposed to currents and predators (Fig. 1). In many sabellid and serpulid species, the radioles are equipped with a diverse array of radiolar eyes and photoreceptors that allow them to detect threats and withdraw rapidly into the safety of their tubes (Bok et al. 2016, 2017). This often colorful, feathery radiolar crown is of prostomial origin and is the most characteristic feature of these three taxa, giving them their common names: fan worms and feather duster worms (Dales 1962; Fauchald 1977; Fitzhugh * Ekin Tilic [email protected]‑bonn.de 1
Institute of Evolutionary Biology and Animal Ecology, University of Bonn, Bonn, Germany
Scripps Institution of Oceanography, UC San Diego, La Jolla, San Diego, CA, USA
2
1989; Smith 1991). The crown is used by the animal not only for respiration and suspension feeding, but can also harbor methanotrophic symbiotic bacteria in some deep-sea sabellid and serpulid species (Goffredi et al. 2020) or can sort, capture and transfer particles with its ciliated inner margins for tube building (Nicol 1931; Fitzsimons 1965; Bonar 1972). In many small species, it can collect and move sperm from the water column to epidermal spermathecae at the base of the crown, where the
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