Streblosoma chiguensis sp. nov.

Figs 1 A–F, 2A–F

Material examined. Holotype (NMNS 7516 - 1), aquaculture holding tank of the Tainan Fisheries Research Institute, Sangu Village (23°07´20˝ N 120°04´50˝E), Tainan, Taiwan; on Ulva sp.; 6 January 2007.

Description. Holotype complete, colourless in alcohol (Fig. 1 A), body length 41.0 mm with 69 segments, maximum width 2.0 mm on segment 11.

Prostomium at base of upper lip; upper lip large, thick, horseshoe-shaped (Fig. 1 B); lower lip small, thick; buccal tentacles slender, basal part of prostomium short and compact; eyespots absent; peristomium forming lips and continue dorsally; ventral surface of anterior segments corrugated.

Lateral lobes absent. Three pairs of branchiae on segments 2 to 4 (Fig. 1 C). Most filaments arising from body wall near the base of notopodia and few from body wall at medial gap between pairs of notopodia, first pair filaments extending laterally beyond notopodia of segment 2 (Fig. 1 C–D); segments 2 to 4 with 14, 12 and 10 independent filaments in each band, respectively.

Notopodia present from segment 2 to 55. Anterior parapodia elongate, rectangular, with distinct glandular pre- and postchaetal lobe; notochaetae emerging from between the lobe, mid to posterior notopodia becoming trapezoid, less glandular than anterior ones; notopodia on segment 2 clearly dorsal than following ones (Fig. 1 D), reaching maximum length on segment 10, thereafter becoming progressively shorter. Notochaetae with two rows of chaetae with finely striated margins, anterior row slightly shorter than those from posterior row, all tapering to tips (Fig. 1 E–F).

Neuropodia swollen on segments 5 to 14; uncini arranged in single rows, with elongate prow and slightly unturned terminal button, dental formula MF:2:3 (Figs 1 B, 2A–B); neuropodia becoming trapezoidal processes on segments 15 to 25, thereafter short and low, extent near posterior end (Fig. 1 A). Uncini on segments 15 to 25 arranged in looped rows with dorsal edge slightly curved on middle body segments, segments 25 to 55 arranged in C-shaped rows, openings all toward dorsal side (Fig. 2 C–D); uncini with elongate prow and slightly upturned terminal button, dental formula MF:2:3 (Fig. 2 E–F).

Nephridial and genital papillae absent; pygidium distally rounded, without papillae.

Etymology. The name is derived from the Chigu lagoon, where the seawater is extracted for filling holding tanks in the Tainan Fisheries Research Institute.

Type locality. Sangu Village, Tanian, Taiwan.

Distribution. Only known from the type locality.

Remarks. Reuscher et al. (2012) provided a synoptic table for most species of the genus, which includes five morphological characters (numbers of branchial filaments on each branchial segment, the presence/absence of eyespots, number of pairs of notopodia, and dental formula of anterior and posterior tori). However, some of those characters are relatively unstable such as eyes may fade after preservation, number of pairs of notopodia based on incomplete specimens, or uncinial dental formula described were without using high resolution or scanning electron microscopes. Nogueira et al. (2004) suggested other characters, such as the presence of lateral extension of branchiae on first pair of notopodia, uncini of segments arranged in curved rows, and posterior uncini arranged in straight rows for morphological comparisons between congeners.

Up to now, only six species of the genus are known to have uncini arranged in curved rows as in the present species. They are S. curvus Hutchings et al., 2015, S. duplicata Hutchings, 1990, S. hesslei Day, 1955, S. porchatensis Nogueira, Garraffoni & Alves, 2004, S. toddae Hutchings & Smith, 1997, and S. uncinatus Kudenov, 1975 (Nogueira et al. 2004; Hutchings et al. 2015; Fig. 2 C–D). This new species differs from S. curvus and S. duplicata by the presence of lateral extension of branchial filaments on segment 2 (Nogueira et al. 2004; Hutchings et al. 2015; Fig. 1 C–D), and varies from S. hesslei, S. porchatensis, S. toddae, and S. uncinatus by number of pairs of notopodia, the occurrence of notopodia on body segments, and morphology of uncini. The new species has 69 segments with 54 pairs of notopodia occurring from segments 2–55, whereas S. hesslei, S. porchatensis, S. toddae, and S. uncinatus have 73 segments with 36 pairs of notopodia occurring from segments 2–35, 57 segments with notopodia occurring from segment 2 to near pygidium, 150 segments with notopodia occurring from segment 2 to near pygidium, and 62 segments with notopodia occurring from segment 2 to near pygidium, respectively (Nogueira et al. 2004: 99, Table 1). Uncini of S. chiguensis sp. nov. have a large subterminal dorsal button, whereas those of the other four species have either no dorsal button or with a small dorsal button (Nogueira et al. 2004: 99, Table 1; Fig. 2 A–B). Moreover, S. chiguensis sp. nov. has first pair of neuropodia from segment 5 and uncini arranged in C-shaped rows on posterior segments, but S. toddae has first pair of neuropodia from segment 4 and S. uncinatus has uncini arranged in straight rows on posterior segments (Nogueira et al. 2004: 99, Table 1; Fig. 2 D).