Scoloplos acutissimus Hartmann-Schröder, 1991

(Figs 1, 2, 12 A)

Scoloplos acutissimus Hartmann-Schröder, 1991: 48 –49, figs 73–80.

Type material. Holotype: ZMH P–20562 (photographed).

Other material examined. AM W.44175, MI QLD 2366 (photographed); AM W.44175.001, tissue for molecular study; MI QLD 2340, unregistered posterior end used for molecular analysis; AM W.44248, MI QLD 2373; AM W.46090, MI QLD 2376 (3, 1 photographed); AM W.46091, MI QLD 2378 (6); AM W.46092, MI QLD 2422 (12); AM W.46096, MI QLD 2429 (20, 1 photographed using SEM); AM W.46097, MI QLD 2432 (3); AM W.44940, MI QLD 2433 (6); AM W.44942, MI QLD 2439 (3, 1 photographed).

Examination of type material. Holotype incomplete, thoracic width 0.8 mm, with flattened thorax. Eighteen thoracic chaetigers on right side and 19 thoracic chaetigers on left side. Branchiae starting from chaetiger 17. Three or four rows of hooks and one posterior row of capillary chaetae in thoracic neuropodia; thoracic neuropodia with one podal papilla, no subpodal papillae. Anterior abdominal neuropodia with very big subpodal flange and developed subpodal notch, well developed long subequal outer and inner lobes. Notopodial lobes narrow, digitiform. Branchiae longer than notopodia. In posterior abdominal segments neuropodia with shorter lobes, inner lobes round and wider than outer.

Description. (Lizard Island material) Body long, slender; thorax slightly flattened, abdomen cylindrical (Figs 1 A, 2A). Colour in life pale yellowish-brown with red blood vessels and yellow gut content (Fig. 12 A). Thoracic width up to 0.9 mm. Prostomium sharply conical with drawn out tapering tip (Fig. 2 B). Peristomium with pair of dorso-lateral nuchal organs (Fig. 2 B). Thoracic chaetigers numbering 14–20 (usually 17–19) (Table 3). Branchiae starting from penultimate thoracic chaetiger (Fig. 1 A, Table 3), usually chaetiger 16–18 (13 in smallest specimen). First branchiae small and digitiform; becoming larger and triangular in anterior abdomen; then long, strap-like, markedly wider and longer than notopodia, in middle and posterior abdomen (Figs 1 G, H, 2I). Thoracic postchaetal lobes well developed from chaetiger 1 (neuropodia) or 2 (notopodia) (Figs 1 D, E, 2B). Notopodial lobes short and papilliform in anterior thorax; gradually increasing in length, becoming digitiform, as long as branchiae in posterior thorax (Figs 1 A–C, E–G, 2C, D, F). Lateral organs below notopodia of all segments well developed (Fig. 2 B, D). Thoracic neuropodial postchaetal lobes round papilliform, in posterior thorax becoming elongated and arising from low ridge, more developed below papilla (mammiform shape) (Figs 1 C, E–G, 2C, D, F). No subpodal or stomach papillae. Abdominal notopodial lobes narrow, lanceolate, shorter than branchiae (Figs 1 H, 2I). Abdominal neuropodia supported one thin acicula and bilobed with subequal lobes; inner lobe rounded, slightly longer and thicker than outer one (Figs 1 H, I, 2C, I). Parapodial flange well developed, with deep notch and round upper margin without flange papilla (Fig. 1 H, I). Ciliated dorsal organs with two short ciliated strips present middorsally (Fig. 2 I, J). Thoracic notopodia bearing only crenulate capillary chaetae; neuropodia with 3–4 anterior rows of hooks and one posterior row of capillaries, neuropodial lobe located on same level as capillary chaetae in middle of row (Figs 1 C, E–G, 2B–G); hooks in anterior chaetigers slightly curved, serrated with 4 denticles; in posterior thoracic chaetigers hooks almost straight, smooth, hooded, very short in anterior row; in one or two last thoracic chaetigers hooks replaced by capillary chaetae (Figs 1 C, E–G, 2B–G). In abdomen both rami bearing thin capillaries, besides forked chaetae present in notopodia (Fig. 2 H) and flail chaetae in neuropodia (Fig. 1 H, J). Pygidium with two long anal cirri (Fig. 2 K).

Remarks. Scoloplos acutissimus was described by Hartmann-Schröder (1991) from Gladstone, Queensland and has not been recorded since this study. Re-examination of holotype revealed higher number of thoracic chaetigers (18/19 vs 17) than originally reported. Lizard Island material had up to 20 thoracic chaetigers (Table 3). The newly collected specimens correspond well with the original description and the type material which was examined. The variability in the number of thoracic chaetigers and in the location of the first pair of branchiae was investigated for 26 specimens (Table 3). The pygidium and anal cirri are described for the first time.

Type locality. Gladstone, Queensland.

Distribution. Gladstone, Lizard Island, Queensland.

Molecular analyses. The analysis of the sequence data for the 18S rRNA, 16S rRNA and CO1 gene has shown (with a good support for 18S and 16S) that all trees include the clade that contains Scoloplos armiger, S.

acmeceps, and Leitoscoloplos pugettensis (Figs 13–15). In CO1 analysis this clade also included S. acutus . In the CO1 and 16S analyses Scoloplos acutissimus joined the Scoloplos armiger-S. acmeceps-Leitoscoloplos pugettensis clade (Figs 14, 15). However the analysis of the sequence data for 18S rRNA gene (Fig. 13) showed that Scoloplos acutissimus was not included in this clade, but with low support it was likely in the same clade as representatives of the genera Nainereis, Orbinia, Leodamas, Phylo and others instead; the same clade contains Scoloplos dayi, S. normalis, Leitoscoloplos robustus and L. fragililis .