Nicidion megabalanicola sp. nov.

Fig. 9

Material examined. Taiwan: Holotype (NMNS 6694-1), Dawen (21°57´22˝N, 120°45´17˝E), Pingtung County; paratypes: Five specimens (NMNS 6694-2~6), Dawen (21°57´22˝N, 120°45´17˝E), Pingtung County, subtidal, in barnacle clumps, 4–6 m deep, February 12, 1997.

Description. Holotype, complete specimen, sex unknown, total body length 18 mm with 51 chaetigers; maximum width at chaetiger 5, about 1.3 mm; length through chaetiger 10, 4.0 mm; chaetiger 10 width about 1.3 mm; dorsum at chaetiger 4 with a wide white band. Paired superior anal cirri, digitiform, with 3 articulations; paired inferior anal cirri, ovate (Fig. 9 D). Body cylindrical but tapering at the end (Fig. 9 A).

Prostomium equal in length and width to peristomium, about 1/2 as deep as peristomium. Prostomial lobes frontally rounded, dorsally flattened, median sulcus shallow (Fig. 9 B–C). Eyes present, situated posteriorly to base of palps, ovate and brick red (Fig. 9 A–C). Palps and antennae arranged in semicircle, similar thickness, median antenna isolated by gap; palpophore and ceratophore base short cylindrical, with three indistinct rings; palpostyles and ceratostyles digitiform, A-II 6 articulations, A-I and II 3 articulations, palps 2 articulations; A-II longest, extending to chaetiger 3, A-I and III and palps to middle anterior peristomial ring. Peristomium lower lip distinctly muscular; separation between rings distinct dorsally; anterior ring about 2/3 of peristomial length; peristomial cirri digitiform, extending to middle of peristomial ring, without articulations, a white band present subdistally (Fig. 9 A–C).

Maxillary formula. 1+1, 6+5, 6+0, 3+8?, 1+1; part of MxIII covered by MxII; mandibles flat (Fig. 9 E).

Branchiae present, pectinate, distinctly shorter than dorsal cirri, not reduced in mid-body region, stems erect (Fig. 9 B–D). Branchiae present from chaetiger 5 to chaetiger 46, about 92% of chaetigers branchiate; first 4 pairs and last 6 pairs chaetiger with single filament, maximum 3 filaments at about chaetiger 13 to 27; branchial filaments slender, tapering, shorter than dorsal cirri (Fig. 9 B–D).

Acicular lobes withdrawn into body wall; aciculae emerge between dorsum and midline. All prechaetal and postchaetal follow outline of acicular lobes closely. First three ventral cirri digitiform, without inflation; bases of ventral cirri ovate inflated from chaetiger 4 to chaetiger 20, digitiform distally; dorsal cirri slender, tapering, with a white band subdistally (Fig. 9 A), anterior dorsal cirri with indistinct 3 rings, thereafter 2 to 3 indistinct rings.

Limbate chaetae longer than all other chaetae, narrow, margin serrated (Fig. 9 F). Pectinate chaetae flat, tapering, with one elongated marginal tooth, inner teeth 8–12 (Fig. 9 G). Shafts of compound falcigers slightly bent at middle, distally inflated on both margins, inner margin beak-like, distally with serrations; appendages thick, bidentate; proximal tooth of anterior chaetigers acute angle isosceles triangle, slightly larger than distal tooth, both directed obliquely (Fig. 9 I); proximal tooth of posterior chaetigers nearly twice as wide as and about equal in length to distal tooth, proximal tooth formed at right angle to appendages, distal tooth directed obliquely; guards on anterior chaetigers symmetrical, bullet-shaped, cutting margin serrated, without mucros, guards on posterior chaetigers symmetrical, subdistally inflated, cutting margin serrated, with mucros, (Fig. 9 H–J); psudocompound falcigers and compound spinigers absent. Aciculae amber, straight, subdistally tapering, cross-sections rounded; paired per chaetiger, superior aciculae larger than inferior aciculae, both superior and inferior aciculae of anterior chaetigers tapering, distally blunt, thereafter superior aciculae distally enlarged, hammer-shaped, inferior aciculae tapering, with mucros (Fig. 9 J, K–L); separation between core and sheath distinct in both aciculae and subacicular hooks. Subacicular hooks amber on most chaetigers, yellow on last few chaetigers, bidentate; present from chaetiger 14 or chaetiger 15 to last chaetiger, single per chaetiger; shafts of hooks straight, internal striations present; proximal tooth larger than distal tooth, directed laterally, formed a right angle to axis of shaft; distal tooth small, slightly curved, directed obliquely; guards truncate distally, covering distal head entirely (Fig. 9 M).

Etymology. The name is derived from the encrypted living condition of the worm in the crevices of the giant barnacle ( Megabalanus volcano Pilsbry, 1916) colonies.

Type locality. Dawen, Pingtung County, Taiwan.

Habitat. The worm is found associated with clumps of the giant barnacle ( Megabalanus volcano) that live on the wall of the Nuclear Power Plant cooling water intake pipe.

Distribution. Known only from the type locality.

Remarks. Morphology of this species conforms to the diagnosis of Nicidion suggested by Zanol et al. (2014), primarily because of the absence of MxVI and the presence of bidentate compound falcigers, dark and bidentate subacicular hooks with darkest shade closest to the distal end, less than four branchial filaments, and one white segment at anterior of the worm (Fig. 9 A, E, H–J, M).

The present species belongs to the B-2 group (sensu Fauchald 1970, 1992). Of all the described species, Nicidion mutilata is the only species that somewhat resemble this new species by having a white segment at the anterior of the worm (Zanol et al. 2014: 17, Fig. 16C). However, they are very different in many other characters, including morphology of prostomial appendages and peristomial cirri, branchial filament number, the length ratio of branchiae to dorsal cirri, forms of limbate chaetae, compound falciger, acicula and subacicular hooks (see Fauchald 1992: 231–233, Fig. 77f–n; Fig. 9). Moreover, Nicidion megabalanicola sp. nov., has the following diagnostic characters, the presence of hammer-headed superior aciculae and mucronate inferior aciculae, which have not been reported for any other species in this genus (Fig. 9 K–L).