Spirodiscus Fauvel, 1909

Kupriyanova, Elena K. & Ippolitov, Alexei P., 2015, Deep-sea serpulids (Annelida: Polychaeta) in tetragonal tubes: on a tube convergence path from the Mesozoic to Recent, Zootaxa 4044 (2), pp. 151-200 : 162-163

Genus Spirodiscus Fauvel, 1909

Spirodiscus Fauvel, 1909: 56 –57.— Fauchald 1977: 147.

Nogrobs .—ten Hove & Kupriyanova 2009: 68 –69.—Kupriyanova & Nishi 2011: 1–2.

Type species: Spirodiscus grimaldii Fauvel, 1909

Diagnosis (after Kupriyanova & Nishi 2011, emend.): Tube white, free-lying, polygonal in cross-section, quadrangular or octagonal (= modified quadrangular), straight to slightly curved (tusk-shaped), or initially cylindrical, then quadrangular in cross-section and coiled clockwise into a flat spiral, finally with short cylindrical straight distal part. Peristomes absent. Operculum inverse cone (ampulla) with chitinous endplate and central depression. Peduncle thick, pinnulated, without distal wings, with dorsal groove distally, with or without constriction beneath ampulla; inserted as second right radiole, up to 3 times as wide as other radioles. Pseudoperculum absent. Arrangement of radioles semi-circular, up to 8 per lobe. Inter-radiolar membrane, radiolar eyes, and stylodes absent. Mouth palps absent. 5 or 6 thoracic chaetigerous segments. Collar non-lobed with straight edge, no clear separation towards thoracic membranes that end at second chaetiger; tonguelets between ventral and lateral collar parts absent. No apron. Collar chaetae limbate. Apomatus chaetae absent. Thoracic uncini saw-to-rasp-shaped with numerous teeth (> 12) in profile, 2–3 teeth per row; anterior peg gouged. Thoracic triangular depression absent. Abdominal chaetae short, with flat triangular denticulate blade; uncini similar to thoracic ones. Achaetous anterior abdominal zone absent. Long posterior capillary chaetae absent. Posterior glandular pad absent.

Remarks. The Recent genus Spirodiscus Fauvel, 1909 , characterised by an unattached quadrangular tube coiled into a flat spiral (hence the generic name), was synonymised with the fossil Nogrobs de Montfort, 1808 by Jäger (2005). This was supported by ten Hove & Kupriyanova (2009) and Kupriyanova & Nishi (2011). However, tubes of Recent Spirodiscus grimaldii are strictly clockwise coiled (when looking from the upper side), while known populations of fossil representatives of genus Nogrobs demonstrate a mixture of clockwise and anticlockwise coiled specimens (Ippolitov, unpubl.). At the same time, although coiling direction appears to be a morphological argument against Jager’s synonymy, this character is not very important taxonomically. The results of comparative SEM studies of tube wall ultrastructures (see below) show very different crystal arrangement in Spirodiscus and in the type species of genus Nogrobs , thus indicating that these genera should not be synonymised.

Spirodiscus grimaldii and S. groenlandicus comb. nov. (formerly Ditrupa groenlandica ) described below, have very different tube morphologies: coiled tetragonal in the former and tusk-shaped octagonal in the latter. However, the animals inhabiting these tubes are very similar (except for five thoracic chaetigers in S. groenlandicus comb. nov. and six in S. grimaldii ) and have identical chaetation patterns. Their tube walls also show similar ultrastructures having a well-defined outer layer with characteristic crystal orientation (see below). All this indicates that S. grimaldii and S. groenlandicus comb. nov. are closely related and justifies placing them into one genus. The generic diagnosis here has been emended to include species with both coiled tetragonal and straight octagonal tubes.