Hydroides operculatus (Treadwell, 1929)

(Figure 3 C, Figure 9 A–H)

Eupomatus operculata Treadwell, 1929: 12.

Hydroides inornata —Huang and Mak 1982: 780; Mak 1982: 605; Wang and Huang 1993: 1 –12. Hydroides operculatus —Ben-Eliahu and ten Hove 1992: 44.

Material examined. AM W41406, 3 specimens (22°21’N, 114°17’E, Sharp Island, on dead coral, 3–6 m, May 21, 2009); AM W40550, 2 specimens (22°25’N, 114°16’E, Tolo Harbour, on artificial substrate: fouling plates in fish farm, 10 m, April 15, 2009).

Description. TUBE: white, trapezoidal in cross-section with two to three long longitudinal ridges and numerous transversal ridges. Collar-like rings absent. External tube diameter: from 0.7 mm to 1.3 mm (n=3, µ=1.2±0.5), lumen diameter from 0.3 mm to 1.3 mm (n=3, µ=0.8±0.5).

BRANCHIAE: radioles on each side arranged in semicircle, numbers ranging from 10 to 11 (n=3, µ=10.7±0.6). Interradiolar membrane absent. Terminal filament long.

PEDUNCLE: long, smooth, at most slightly constricted near funnel, circular in cross-section, inserted below right branchial lobe. Pseudo-operculum short club-shaped.

OPERCULUM: numbers of radii ranging from 28 to 38 (n=4, µ=34.3±5.1); pointed and curving outward. Interradial grooves 1/2 of funnel length. Verticil dark brown, consisting of 8 to 9 smooth spines (n=5, µ=8.8±0.4), hooked, tips all curving inwards; one to three spines larger; other spines similar in shape and size; one short basal internal spinule arising from inner base of each spine (Figure 3 C, Figure 9 A). External and lateral spinules or knobs absent. Central tooth absent. Opercular length from 0.9 mm to 1.3 mm (n=5, µ=1.1±0.2), width from 0.5 mm to 1.0 mm (n=5, µ=0.7±0.2).

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

THORAX: with 7 chaetigers, and 6 uncinigerous. Collar chaetae of two types: bayonet chaetae with two pointed teeth at base of smooth blade, and capillaries with minute denticles (Figure 9 B). Subsequent chaetae of two sizes (Figure 9 C). Uncini along entire thorax saw-shaped with 5 curved teeth including anterior fang (Figure 9 D). Tori of similar size throughout. Ventral depression present.

ABDOMEN: total number of abdominal chaetigers varies from 64 to 79 (n=2, µ=71.5±10.6). Uncini sawshaped with 4 to 5 teeth anteriorly (Figure 9 E–F); rasp-shaped posteriorly with 2 to 3 rows of teeth and 6 to 7 teeth in profile view (Figure 9 G–H). Chaetae flat trumpet-shaped. Capillary chaetae not observed. Posterior glandular pad absent.

SIZE: total worm length: 18.0 mm to 28.0 mm (n=4, µ =23.8±4.2), thoracic width: 0.8 mm to 1.0 mm (n=4, µ=0.9±0.1). Branchiae and operculum accounting for one fifth of entire length.

COLOUR: in ethanol white.

Habitat. Depth: 3 m, on dead corals and artificial substrate in fish farm.

Distribution. Indian-Pacific Ocean, Lessepsian migrant in E. Mediterranean.

Remarks. One of our specimens has 9 verticil spines, three of them large, which is regarded to be an opercular variation. Hydroides operculatus has been recorded as a ship-transported species (Bastida-Zavala & ten Hove 2002). It is recorded herein from fouling communities of the fish farm in Tolo Harbour.