Placostegus Philippi, 1844

Ten Hove, Harry A. & Kupriyanova, Elena K., 2009, Taxonomy of Serpulidae (Annelida, Polychaeta): The state of affairs, Zootaxa 2036, pp. 1-126 : 74-75

29. Placostegus Philippi, 1844

( Fig. 34)

Type-species: Serpula tridentata Fabricius, 1780

Number of species: 7 (-8)

Tube triangular in cross-section, with denticulate keels, transparent or semi-transparent, often only attached to substratum at the 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 first and second 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, branchial eyes, and stylodes absent. Mouth palps present. 6 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 segment. Collar chaetae absent; collar region with girdle of reddish ocelli. Apomatus 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 ( Fig. 34A). Thoracic triangular depression absent. Abdominal chaetae truly trumpet, with distal hollow triangular blade, abruptly bent ( Fig. 34C). Achaetous anterior abdominal zone present. Long posterior capillary chaetae may be present. Posterior glandular pad absent.

Remarks. Placostegus is one of three serpulid genera (see also Neomicrorbis and Vitreotubus ) with an entirely vitreous tube; completely transparent, glass-like in live animals, it may become milky-white semitransparent after preservation in formalin. The tube in P. incomptus shows a remarkable dual appearance, proximally with closely set transverse ribs, distally smoothly triangular. Fauvel (1927 fig. 128i) and Imajima (1978 fig. 9c) mention a chitinous talon projecting from the endplate into the opercular bulb.

Placostegus has one evident diagnostic autapomorphy—the belt of bright red ocelli in the region where in other genera collar-chaetae are found (e.g., Langerhans 1884 fig. 38b, Ehlers 1887 fig. 3, Hartman 1969 fig. 2; our Fig. 1F).

1. Placostegus assimilis McIntosh, 1885 , off Bermudas; bathyal

2. Placostegus californicus Hartman, 1969 , Southern California

3. Placostegus crystallinus ( Scacchi, 1836) sensu Zibrowius, 1968a , Eastern North Atlantic, Mediterranean;? Red Sea, Indian Ocean, these reports might belong to a different species (Ben-Eliahu, pers. comm.)

4.? Placostegus grayi Baird, 1865 , no location given; generic status uncertain

5. Placostegus incomptus Ehlers, 1887 , off Cuba; bathyal

6. Placostegus langerhansi Marenzeller, 1893 , Madeira, Canary Islands

7. Placostegus tridentatus ( Fabricius, 1780) , Atlantic, Mediterranean, Indo-West Pacific.