Mastobranchus loii Gallardo, 1968, amended

(Fig. 2A–F; Table 1)

Mastobranchus loii Gallardo, 1968: 118, figs. 5–9

Type material. Holotype (LACM-AHF POLY 317) 12°13'24''N 109°14'13''E, 11 Jan 1960, 14m, Bay of Nha Trang, South Vietnam, North Pacific.

Redescription. Incomplete holotype 10.5 mm long, 1.2 mm wide, for 112 chaetigers. Body triangular (slightly larger ventrally than dorsally); anterior thorax slightly wider than posterior thorax. Yellowish color in alcohol.

Prostomium and peristomium very short, regenerating (Fig. 2A). Thorax with 10 chaetigers. Chaetiger 1 regenerating (Fig. 2A–B). Capillaries on noto- and neuropodia of chaetigers 2–9, chaetigers 10 and 11 with notopodial capillaries and neuropodial hooded hooks. Chaetigers 2–7 with a deep lateral groove. All chaetigers biannulate, straight, with shallow intersegmental division (Fig. 2B). Chaetigers 9–11 with mid-lateral lobes (MLL) (Fig. 2D). Dorsal chaetal bundles inserted dorso-laterally, ventral bundles inserted ventro-laterally; chaetae in a double row. Lateral organs throughout thorax. Transition between thorax and abdomen subtle, marked by chaetal change, distance of chaetal bundles, and Methyl Green staining pattern (Fig. 2B–D).

First nine abdominal chaetigers with hooded hooks on neuropodia and mixed hooded hooks and capillaries on notopodia (Fig. 2D), remaining chaetigers with only hooded hooks. Anterior abdominal chaetigers as wide as long, posterior chaetigers wider than long (Fig. 2D–E). Chaetae and hooded hooks emerge from last third of chaetiger. Both notopodial bundles of hooded hooks and capillaries emerging quite dorsally, almost together (Fig. 2C); neuropodial bundles of hooded hooks long, extending laterally and ventrally (Fig. 2D). Dorso-lateral lateral organs throughout abdomen. Anterior noto- and neuropodia with poorly developed lobes (Fig. 2C–D), posterior noto- and neuropodia with moderately developed lobes (Fig. 2E–F). Notopodial hooded hooks with three rows of teeth (2–2–1) above main fang, large main fang and slightly curved; long shaft and hood, without node. Neuropodial hooded hooks with two rows of teeth (2–2) above main fang, large main fang and slightly curved; short shaft and hood, with node. Retractile branchiae on posteriormost abdomen; branchiae dorsal, digitiform (up to seven filaments), anterior to notopodial lobe (Fig. 2E). Pygidium bearing four small cirri, forming a circular arrangement, on the dorsal region (Fig. 2F).

Remarks. The original description is very short. As the examination of the type material was possible, we decided to include the redescription of this species with optical photos in order to assist further taxonomic studies. Gallardo (1968) described Mastobranchus loii as having nine thoracic chaetigers and mentioned that the prostomium was short and somewhat retracted. What Gallardo (1968) did not realize was that the prostomium, peristomium, and chaetiger one were regenerating, that was why they were so short. It was possible to observe the discrete prostomium, and the buccal aperture between prostomium and peristomium, all of them too small in comparison with the rest of the body. The author did show these characters on drawings, but did not discuss the possibility of being in regeneration. So, we could conclude that the holotype is a regenerating specimen and the more appropriate diagnosis of the genus is “specimens with 11 thoracic chaetigers”.