Spirobranchus polytrema

Spirobranchus cf. polytrema not (Philippi, 1844), Type A sensu Imajima 1977

(Figure 14A–C, Figure 15 A–E)

Spirobranchus cf. polytrema — Imajima 1977: 102 –106; Fiege and Sun 1999: 126 –129. Spirobranchus polytrema not (Philippi, 1844) sensu Sun and Yang 2001a: 200 –201.

Material examined. AM W41412, 2 specimens (22°21’N, 114°17’E, Sharp Island, on dead coral, 6 m, May 21, 2009).

Description. TUBES: white, subtriangular with a flattened area of attachment in cross-section, with a small longitudinal additional tube (or hollow keel) on top, flanked by a row of pits, a lateral longitudinal ridge and a row of slits at either side (Figure 14A). About 1.0 mm wide with lumen 0.4 mm, however, anterior part of tube missing.

BRANCHIAE: together only 7 radioles, 4 on the right side, 3 on the left, arranged in a circle. Terminal filaments long, about three times longer than pinnules, approximately 0.5 mm. Branchial eyes not observed.

PEDUNCLE: smooth, flattened in cross-section, bearing a pair of wings, fringed along anterior margin with 4–6 digitiform processes. Inserted just below left branchial lobe.

OPERCULUM: with a fleshy, slightly translucent bulbous base and a tilted cone-shaped cap (Figure 14B–C, Figure 15 A). Length of the operculum about 1 mm, width 0.5 mm.

COLLAR AND THORACIC MEMBRANES: collar low.

THORAX: with 7 chaetigers, 6 uncinigerous. Collar chaetae bayonet type, with striated blade and numerous small teeth forming squarish emboss at base of blade, and limbate chaetae. Other thoracic notochaetae limbate capillaries (Figure 15 B). Uncini along entire thorax saw-shaped 9 to 10 saw-shaped, incidentally with 3 to 4 teeth above peg, anterior peg gouged (Figure 15 C). Tori of similar size along thorax, approaching each other posteriorly, leaving a ventral depression.

ABDOMEN: incomplete. Uncini of anterior and mid-body rasp-shaped, with 10–12 teeth seen in profile, 3 teeth in a row, anterior peg gouged and curved underneath (Figure 15 D). Chaetae true trumpet-shaped, abruptly bent distally, with two rows of denticles separated by a hollow groove and forming long lateral spine (Figure 15 E).

SIZE: incomplete specimens, up to 8.8 mm long and 1 mm wide.

COLOUR: in ethanol pale yellow, with brown operculum.

Habitat. Depth: 3–6 m, on dead coral skeletons.

Remarks. The occurrence of the temperate/subtropical Atlantic/Mediterranean species Spirobranchus polytrema (Philippi, 1844) in tropical Asian seas is rather doubtful; it is more likely that similar (cf.) material from these seas will prove to be genetically different. See discussions in Imajima (1977: 106) and Pillai (2009: 153) as well. Records of Spirobranchus cf. polytrema from the Indo-West Pacific include two morphotypes, type A and type B (Imajima 1977; Fiege & Sun 1999), both have an operculum consisting of a fleshy proximal part (ampulla) and a distal calcareous cone with rounded knobs, but differ from each other by the shape of tube and peduncular wings. Our material resembles type A in the shape of peduncular wings and tube. However, the opercula have a single tip to their calcareous cap instead of rounded knobs. Sun and Yang (2001a, Figure 10) figured specimens with and without rounded knobs on the distal calcareous cone of the operculum in their material from Hainan Island. To our knowledge this is the first report of a hollow keel, almost an additional tube, in the genus Spirobranchus (though Figure 16 E of Fiege and Sun 1999 leaves room for an interpretation as one being present).

Distribution. Indo-West Pacific, Sri Lanka, Australia, Japan.