Hydroides rhombobulus Chen and Wu, 1980

Sun, Yanan, Ten, Harry A. & Qiu, Jian-Wen, 2012, Serpulidae (Annelida: Polychaeta) from Hong Kong, Zootaxa 3424, pp. 1-42 : 19

Hydroides rhombobulus Chen and Wu, 1980

( Figure 3 D–J)

Hydroides rhombobulus Chen and Wu, 1980: 248 –249; Mak 1982: 605–606; Sun and Yang 2000: 128.

Material examined. AM W41407, 1 specimen (22°21’N, 114°17’E, Sharp Island, on dead coral, 3 m, May 21, 2009).

Description. TUBE: lilac, up to 1.1 mm wide with lumen 0.8 mm, thin, trapezoidal in cross-section with three long longitudinal ridges and numerous transversal ridges. Collar-like rings absent.

BRANCHIAE: each lobe with 9 radioles arranged in semicircle. Interradiolar membrane absent. Terminal filament long.

PEDUNCLE: long, smooth with slight constriction near funnel, circular in cross-section, inserted below left branchial lobe. Pseudo-operculum not found.

OPERCULUM: Opercular funnel with 16 radii, distally brown, tips black, pointed and curving outward. Interradial grooves 1/3 of funnel length. Verticil almost similar in width to funnel, consisting of 5 broad valvular spines, smooth, similar in size and shape, each spine transparent light brown marginally, with a convex outside and rounded tip; spines fused basally ( Figure 3 D–E). Central tooth absent. Length of the operculum about 0.6 mm, width 0.4 mm.

COLLAR AND THORACIC MEMBRANES: collar high, thoracic membranes well-developed, apron present.

THORAX: with 7 chaetigers, 6 uncinigerous. Collar chaetae of two types: bayonet chaetae with two pointed teeth at base of finely serrated blade, and capillaries ( Figure 3 F, G). Subsequent chaetae of two sizes (limbate, Figure 3H, and capillary). Uncini along entire thorax saw-shaped with 7 curved teeth including fang ( Figure 3 I). Tori of similar size. Ventral depression present.

ABDOMEN: abdominal chaetigers 70. Uncini saw-shaped with 4 to 5 teeth anteriorly ( Figure 3 J); rasp-shaped posteriorly with 2 to 3 rows of teeth and 7 to 8 teeth in profile view including fang. Chaetae flat trumpet-shaped. Capillaries not observed. Posterior glandular pad absent.

SIZE: length up to 8.9 mm, thoracic width 0.8 mm. Branchiae and operculum accounting for one fifth of entire length.

COLOUR: in ethanol white.

Habitat. Depth: 3 m, on dead corals.

Distribution. South China Sea. Reported from Chinese coasts only.

Remarks. The number of radioles shows some variation, maybe ontogenetic: 12 pairs for holotype from Fujian, China ( Chen & Wu 1980); 7 pairs for specimens from Hong Kong, Hoi Ha Wan (Mak 1982), and 9 pairs for specimen from Sharp Island, present study.

Many Indo-West Pacific Hydroides species are characterized by a low number of rather undifferentiated valvular verticil spines, pointed funnel radii numbering between 16 and 30, and rather undifferentiated bayonet chaetae with two large teeth: H. kimberleyensis Pillai, 2009 , perezi Fauvel, 1918, rhombobulus , trilobulus Chen and Wu, 1978, tuberculatus Imajima, 1976 , uniformis Imajima and ten Hove, 1986 and xishaensis Chen and Wu, 1978. These nominal species have been distinguished from each other by (partly presumed) differences in numbers (3, 4–5, 7–8) or shape of verticil spines (“valvular” versus “triangular”), white versus bluish tubes, bayonet collar chaetae with or without accessory teeth (e.g. Imajima 1976: 133, Imajima & ten Hove 1986: 7–8). Though the latter not even considered H. tuberculatus in their differential diagnosis of H. uniformis , apparently being too clearcut different, in later and larger samples it sometimes was difficult if not impossible to distinguish between the two nominal species (ten Hove 1994: 108). It is not known whether the blue tube colour may disappear in preservative, nor is it clear if the other characters used for distinction are robust or merely due to small sample sizes. Since a revision, only worthwhile if based on comparison of actual material, falls outside the scope of present paper, we have used the “local” name H. rhombobulus for our specimens.

Figures

FIGURE 3. A – B: Hydroides albiceps, AM W 40534, operculum, lateral and apical view. C: Hydroides operculatus, AM W 41406, operculum, lateral view. D – J: Hydroides rhombobulus, AM W 41407. D – E, operculum, lateral and apical view. F, G, collar chaetae. H, thoracic limbate chaeta. I, thoracic uncinus. J, abdominal uncinus. K – O: Protula bispiralis, AM W 41409. K, dorsal view; L, limbate collar chaeta. M, limbate chaeta from chaetiger 6. N, abdominal sickle-shaped chaeta. O; abdominal uncinus. P – Q: Serpula cf. granulosa, AM W 40554, operculum, lateral and apical view. (Scale bars: A – E = 0.2 mm, F – G, O = 50 μm, I – J = 10 μm, K = 5 mm, L – N = 100 μm, P – Q = 1 mm)