Placostegus Philippi, 1844

Genus Placostegus Philippi, 1844 View in CoL

Type species. Placostegus tridentatus (Fabricius, 1779) View in CoL

Generic diagnosis (from ten Hove & Kupriyanova, 2009). Tube triangular in cross-section, with denticulate keels, transparent or semi-transparent, often only attached to substratum at base, collar-like rings absent. Granular overlay absent. Operculum inverse conical, with chitinous cup-shaped endplate. Peduncle cylindrical, smooth, without wings, gradually merging into operculum, at most with shallow constriction; inserted at base of radioles on one side between 1 st and 2 nd normal radiole and maximally covering base of first three radioles. Pseudoperculum absent. Radioles arranged in semi-circles, up to 24 per lobe; inter-radiolar membrane, radiolar eyes, and stylodes absent. Mouth palps present. Six thoracic chaetigerous segments. Collar tri- to penta-lobed, collar edge may be almost laciniate; tonguelets between ventral and lateral collar lobes present. Thoracic membranes long, forming ventral apron across anterior abdominal segments. Collar chaetae absent; collar region with girdle of reddish ocelli. Apomatus View in CoL chaetae absent. All uncini sub-rectangular, rasp-shaped with> 20 teeth in profile, and up to 8 small teeth in a row; anterior peg wide, flat, bluntly truncate, almost rectangular. Thoracic triangular depression absent. Abdominal chaetae true trumpet-shaped, with distal hollow triangular blade, abruptly bent. Achaetous anterior abdominal zone present. Long posterior capillary chaetae may be present. Posterior glandular pad absent.

Remarks. Placostegus and Vitreotubus (see below) are the only two serpulins with an entirely vitreous tube. Placostegus has one evident diagnostic autapomorphy – the belt of bright red ocelli in the region where in other genera collar chaetae are found (ten Hove & Kupriyanova, 2009, fig. 1F).

Unexpectedly, despite clear morphological similarities and identifiable synapomorphies, the genus was not recovered as a monophyletic clade in this study. However, because this phylogenetic position based on 18S sequence data alone contradicts monophyly of the genus Placostegus demonstrated in the multigene analysis of Serpulidae in Kupriyanova et al., 2023, here we did not change the nomenclature until further molecular data on Placostegus spp. become available.