Spirobranchus tetraceros Schmarda 1861

Spirobranchus tetraceros (Schmarda, 1861)

(Figure 2 F, Figure 14 D–E, Figure 16 A–G)

Pomatoceros tetraceros Schmarda, 1861: 30.

Spirobranchus tetraceros — Imajima 1979: 177 –178; Imajima and ten Hove 1984: 51 –52; Tan and Morton 1998: 392; Sun and Yang 2001a: 201 –202; Pillai 2009: 159 –162; Ben-Eliahu and ten Hove 2011: 88 –95 [for partial, complicated synonymy].

Spirobranchus tricornis not (Mörch) sensu Straughan 1967: 244, 246 [fide ten Hove 1970: 47]; Mak 1982: 609; Wang and Huang 1993: 1 –12.

Spirobranchus tetraceros -complex—Fiege and Sun 1999: 129 –130.

Material examined. AM W41411, 23 specimens (22°21’N, 114°17’E, Sharp Island, in living corals, 3–6 m, May 21, 2009).

Description. TUBES: thin, pink outside and white inside, almost circular in cross-section, with one longitudinal ridge. All embedded in coral skeleton (Figure 2 F). Up to 3.2 mm wide with lumen up to 2.9 mm.

BRANCHIAE: radioles arranged in a circle on each side; numbers arranging from 15 to 30 (n=19, µ=20.9±3.8). Interradiolar membranes extending to about half of radiolar length, fringes folded with 2–3 papillae present between radioles. Terminal filament longer than pinnules. Branchial eyes not observed.

PEDUNCLE: smooth, subtriangular in cross-section, inserted just below right branchial lobe. Pair of triangular wings with 4-6 digitiform processes along anterior margin.

OPERCULUM: opercular plate flat, bearing four branched spines arising from common base (Figure 14 D–E). Opercular length from 5.8 mm to 9.5 mm (n=19, µ=7.8±1.5), width from 1.1 mm to 3.0 mm (n=19, µ=2.4±0.6).

COLLAR AND THORACIC MEMBRANES: collar high, trilobed, continuous with well-developed thoracic membranes, forming apron across anterior abdominal chaetigers.

THORAX: with 7 chaetigers, 6 uncinigerous. Collar chaetae of two types: bayonet-like chaetae, with numerous small teeth forming squarish emboss at base of striated blade (Figure 16 A) and slender, almost smooth capillaries (Figure 16 B). Subsequent chaetae of two sizes, limbate and capillary (Figure 16 C–D). Uncini saw-shaped, with 11 teeth in a single row and gouged peg (Figure 16 E). Tori increasing in length along thorax, approaching each other posteriorly, leaving a ventral depression.

ABDOMEN: total number of abdominal chaetigers varies from 74 to 110 (n=14, µ=92±12.2). Uncini sawshaped, with peg and 10 to 12 teeth in a row anteriorly (Figure 16 F), increasing to 11 to 13 teeth in a row in middle and posterior abdomen. Chaetae true trumpet-shaped, with two rows of denticles separated by a hollow groove and forming long bend lateral spine (Figure 16 G). Posterior capillaries present.

SIZE: total worm length: 23.0 mm to 56.0 mm (n=14, µ=38±10.8). thoracic width: 1.2 mm to 3.6 mm (n=19, r: 1.2–3.6, µ=2.5±0.6). Branchiae and operculum accounting for approximately one tenth of entire length.

COLOUR: living worms brightly coloured, opercular plate and horns generally overgrown, light brown to greenish, peduncle and wings blue with dark blue patches, branchial radioles purplish blue or deeply red at basal two-thirds, becoming pink to white distally, thorax and anterior abdomen purplish blue, becoming pink or colourless posteriorly (Figure 2 F).

Habitat. Depth: 3– 6 m. Embedded in living corals.

Remarks. Spirobranchus tricornis sensu auct. has been reported from Hong Kong by Mak (1982: 609), Wang and Huang (1993) and by Morton and Morton (1983: 279). From Mak’s diagnosis it is evident that he was referring to S. tetraceros. Since Mak was involved in Wang and Huang’s study (1993: 10), we assume that their material belonged to the same taxon. However, Morton and Morton (1983: 278–279) clearly depict a spiralised branchial crown, referring to a member of the Spirobranchus corniculatus- complex, see above.

Spirobranchus tetraceros is considered to be a species-complex (ten Hove 1994: 113; ten Hove & Kupriyanova 2009: 98; Ben-Eliahu & ten Hove 2011: 91). Except for the white to bluish colour of the tube, the present specimens agree in general with the description of S. tetraceros as given by Pillai (2009) for specimens from the intertidal in Western Australia, tubes of which were mostly covered by corals. The present specimens were collected from living corals, especially Platygyra carnosus Veron and Porites lutea Edwards & Haime; their tubes, except the openings, were embedded in the corals.

Distribution. Widely distributed in tropical Indo-West Pacific waters. This is the first report of its common occurrence in live massive corals in local waters. Mak (1982, as Spirobranchus tricornis) reported this species from Hong Kong, but did not point out whether the specimens were collected from live or dead corals.