Hyalopomatus biformis Hartman 1960

Hyalopomatus biformis (Hartman, 1960)

(Figs. 5 & 6)

Vermiliopsis biformis — Hartman 1960: 177 –178.

Hyalopomatus biformis — Zibrowius 1971: 1374 (transferred to the genus Hyalopomatus), Bastida-Zavala 2008: 21, in ten Hove & Kupriyanova 2009: 51.

Material studied. FMNH 6201 (tubes only), FMNH 6202 (3 specimens, including one intact in the tube, one with abdomen used for sequencing and one prepared for SEM), FMNH 6583 (1 specimen). For detailed collection data see Table 1.

Additional material studied. Holotype, LACM-AHF 180, 33 º26’21”N, 118º52’41”W, Santa Catalina Basin, 13.6 km of S. Light. Santa Barbara Island, St. 2130, on dead shells of brachiopods Lacqueus, mud, 1,280 m, June 26, 1952, as Vermiliopsis biformis.

Description. TUBE: white circular in cross-section, up to 1 mm wide, without flaring peristomes or circular collar-like rings, but with interruptions (Fig. 5 A) like those described for H. madreporae by Sanfilippo (2009).

BRANCHIAE: each lobe with 6 pairs of branchial radioles, arranged in semicircles, not connected by an interradiolar membrane. Pinnules long, terminal filament thin and long, approximately 0.5 mm. Branchial eyes not observed. Stylodes absent.

PEDUNCLE: smooth, more or less circular in cross section, with slight regular constrictions anteriorly (Fig. 5 B), inserted just below first and second normal radiole. Showing clear constriction just below ampulla, as thick as normal radioles. Pseudoperculum absent. Pair of lateral wings proximal to opercular bulb absent.

OPERCULUM: elongated with flattened top slightly differentiated into thicker (sclerotized) convex distal cap (no chitinous or calcareous reinforcement) and softer basal bulb (as revealed by staining with methylene blue, Fig. 5 B).

COLLAR AND THORACIC MEMBRANES: collar high, completely covering branchial lobes, with more or less laciniate edge; trilobed, with ventral lobe slightly lower than lateral ones (Fig. 5 C). Continuous with thoracic membranes, which continue until segment 2, but form narrow flaps reaching between segments 3 and 4 (Fig. 5 C). Pairs of small, wart-like protuberances of collar chaetiger absent; tonguelets between ventral and lateral collar parts absent.

THORAX: with collar chaetiger and 5 uncinigerous chaetigers. Collar chaetae of two types: fin-and-blade with distal blade continuous with basal fin and simple limbate (Fig. 6A). Subsequent chaetae limbate, of two sizes, Apomatus -chaetae absent (Fig. 6B). Uncini along entire thorax rasp-shaped, with approximately 200 small teeth, with 6–7 teeth in row above anterior peg; with flat anterior peg clearly made of two rounded lobes (Fig. 6D). Pair of prostomial eyes absent. Triangular depression absent, thoracic tori almost parallel to midventral line of thorax (Fig. 5 C).

ABDOMEN: with up to 45 abdominal chaetigers. Uncini rasp-shaped with approximately 20 teeth in profile and up to 9 rows of teeth (Fig. 6E); anterior peg flat divided into 3–6 rounded lobes (crenulated).

Chaetae nearly capillary with only narrow geniculate tip made of at least two rows of pointed teeth (Fig. 6C). Capillary chaetae present in posterior chaetigers. Posterior glandular pad absent.

SIZE: length up to 10 mm, width of thorax 0.7 mm. Branchiae and operculum accounting for approximately one third of entire length. Achaetous anterior abdominal zone present. Posterior glandular pad absent.

COLOUR: no records.

Remarks. According to the original description, Hyalopomatus biformis generally has triangular attached tube parts with a high longitudinal keel, but the distal free tube parts are circular, like in other species of the genus Hyalopomatus. H. biformis appears to be most similar to H. marenzelleri Langerhans, 1884 that has the operculum with a slightly differentiated fortified cap, the bilobed pegs of the thoracic uncini and 4–5-lobed pegs of the abdominal uncini. However, H. marenzelleri has a distinctly smooth shining tube surface and very thin peduncle without annulations (Zibrowius 1970), whereas in H. biformis, the tube is white opaque and the peduncle is annulated. H. marenzelleri is reported from temperate east Atlantic Ocean (ten Hove & Kupriyanova 2009) and H. biformis is distributed from Alaska to California in the Pacific Ocean (Bastida- Zavala 2008). Because of this, we attributed the specimens from the Patton-Murray Seamount collection to H. biformis even though no attached tube parts were available in the collection to guide this decision.

Kupriyanova et al. (2010) suggested emending the diagnosis of the genus Hyalopomatus because SEM of the abdominal chaetae of H. mironovi Kupriyanova, 1993 revealed that their tips have the teeth arranged in two rows, at least at the base of the chaetal tip and are not “flat narrow geniculate with pointed teeth”. Thus, abdominal chaetae appear to be similar to the “true trumpet-shaped” (sensu ten Hove & Kupriyanova 2009) chaetae characterised by two rows of denticles separated by a hollow groove and extended into a long lateral spine. The tips of abdominal chaetae in H. biformis examined here are similarly not flat, but are arranged into at least two irregular rows, confirming the observations of Kupriyanova et al. (2010) for H. mironovi. Further investigation of the abdominal chaetal structure using SEM in all other Hyalopomatus spp. is needed.

FIGURE 6. SEM micrographs of chaetae in Hyalopomatus biformis (FMNH 6202), A—collar chaetae of two types: finand-blade with the distal blade continuous with basal fin and simple limbate, B—chaetae of the 3rd thoracic segment, Cabdominal chaeta, close-up view, D—uncini of the 5th thoracic segment, E—posterior abdominal uncini. Scale. A—10 µm, B–E—5 µm.