Apomatus voightae n. sp.

(Figs. 1 & 2, Table 2)

Material studied. Type series, FMNH 6185 (1 paratype, complete but broken in 3 pieces with tube fragments), FMNH 6199 (1 complete holotype), FMNH 6217 (?2 paratypes, one lacking abdomen with intact branchia and collapsed operculum, another represented by branchia with operculum in tube, as well as a thorax and posterior part of an abdomen), FMNH 6583 (1 broken paratype with tube fragments). For detailed collection data see Table 1.

Description. TUBE: up to 3.5 mm wide with lumen of up to 3.3 mm diameter. White opaque, circular in cross section, with smooth surface, circular in internal cross-section, with slight circular collar-like rings, but without flaring peristomes. Median keels absent (Fig. 1 B).

BRANCHIAE: all flat ribbon-like, wide (Fig. 1 A); each lobe with up to 17 pairs of branchial radioles (Table 2), arranged in semicircles, fused basally (connected by short interradiolar membrane) for about 1/20 of their length. Pinnules short and thin; each radiole ends into thin short filamentous tip (Fig. 1 A, not well visible here). Branchial eyes not observed in the preserved material. Stylodes absent.

FIGURE 2. SEM micrographs of chaetae in Apomatus voightae n. sp. (FMNH 6583). A—collar chaetae, entire bundle, arrows pointing to Apomatus chaetae, B—close-up of the Apomatus chaetae (lower arrow) from A, C—chaetae of the 4th thoracic segment, D—chaetae of the 5th abdominal segment, E—uncini of the 6th thoracic segment, F—close-up of the peg, uncini of the 5th thoracic segment, G—uncini of the 10th abdominal segment. Scale. A—100 µm, B–D—10 µm, F— 2 µm, G—5 µm.

PEDUNCLE: pinnulate, flat in cross section, not different from other branchial radioles. Pair of lateral wings proximal to opercular bulb absent. Showing clear constriction just below operculum. Pseudoperculum absent.

OPERCULUM: globular soft transparent vesicle without any trace of endplate (Fig. 1 A). Diameter of the operculum 1.6– 3 mm (Table 2).

COLLAR AND THORACIC MEMBRANES: collar unlobed, with entire edge, short, barely covering branchial lobes (Fig. 1 C, D); continuous with wide thoracic membranes forming an apron across anterior abdominal chaetigers 1 or 2 (Fig. 1 C, D). Pairs of small, wart-like protuberances of collar chaetiger absent, calcium-secreting glands visible on the collar.

THORAX: with collar chaetiger, and 6 uncinigerous chaetigers (Fig. 1 C, D). Collar chaetae (Fig. 2A, B) of two types: simple capillary (limbate) and flat Apomatus chaetae (Fig. 2B). Subsequent chaetal bundle with Apomatus -chaetae and simple capillary (limbate) chaetae (Fig. 2C). Uncini rasp-to-saw shaped (Fig. 2E) with approximately 30 teeth in profile, up to 6 teeth in a row above and continuing onto peg; anterior peg long, blunt, almost rectangular (Fig. 2F). Pair of prostomial eyes absent.

ABDOMEN: up to 60 abdominal chaetigers (Table 2). Uncini rasp-shaped with 3–6 rows of teeth and> 30 teeth in profile view, long blunt almost rectangular peg (Fig. 2G). Chaetae flat sickle-shaped with finely denticulate blades (Fig. 2D). Long capillary chaetae present in posterior chaetigers. Pygidium bilobed. Posterior glandular pad very distinct.

SIZE: body length (without branchia) up to 21 mm, width of thorax up to 3 mm (Table 2). Branchiae and operculum accounting for one third of entire length.

COLOUR: no records.

Remarks. The new species very clearly differs from all other species of both Apomatus and Protula as well as from all other so far described serpulid species by its very characteristic flat ribbon-like brachial radioles. Although species of the genus Metavermilia Bush, 1905 sensu Zibrowius, 1971 possess flattened ribbon-like opercular peduncle (see Nishi et al. 2007 for a review), their branchial radioles are of normal shape, not different from branchia of other serpulids. Thoracic uncini of the new species have up to 6 teeth, whereas exceptionally up to 4 teeth in a row above and continuing onto peg were previously reported for species of Apomatus . Finally, another very unusual feature of A. voightae is the presence of Apomatus chaetae in the collar chaetae bundle. Normally in this genus Apomatus - type chaetae are absent among collar chaetae, but are present in abundance starting from the 3rd chaetiger (ten Hove & Pantus 1985). In fact, this appears to be the first serpulid ever recorded with Apomatus chaetae in the collar bundle, so special attention was paid to the position of these chaetae.

Etymology. The species is named after Dr. Janet Voight (Field Museum of Natural History, Chicago, USA) who carefully collected and kindly provided the serpulid material for this study.