Dysponetus caecus

Dysponetus caecus (Langerhans, 1880)

Chrysopetalum caecum Langerhans, 1880: 278, pl. 14, fig. 9 (type locality: Madeira Island).

Chrysopetalum caecum – Laubier 1964: 125, figs 1–2 (Mediterranean Sea, 32 m).

Dysponetus caecus – Dahlgren & Pleijel 1995: 159, figs 2–3 (Scotland, Sweden and Denmark, intertidal to 85 m). — Böggemann 2009: 283, figs 20–21 (Angola and Guinea Basins, 5048–5494 m). — Watson et al. 2014: 315, fig. 3a–b (Senghor Seamount, 3241 m).

FRANCE • 1 af; Bay of Biscay, Saint Nazaire Canyon; 46°14.22′ N, 04°19.56′ W; 755 m; St M 84- 5_690; DBUA 0002277.

MOROCCO • 2 af; Gulf of Cadiz, Mercator mud volcano; 35°17.916′ N, 06°38.709′ W; 354 m; St 64PE284_12750W; DBUA 0001620.

All three specimens examined are very small and incomplete; longest fragment nearly complete, with 22 chaetigers, 3.6 mm long (DBUA 0001620), in poor condition. Body semi-transparent, with orange intestine. Notochaetae long, bright and translucent, covering dorsum from chaetiger 8/ 9 in smaller fragments. Prostomium sub-quadrangular, without eyes; palps and antennae missing in all specimens. Elongate, single lobe present ventrally on posterior margin of mouth. Pharynx visible through body wall, extending to segment 7; jaws not visible. First segment reduced, with two pairs of tentacular cirri (only cirrophores present). Second segment with uniramous parapodia, notochaetae, dorsal cirri and ventral cirri. Following parapodia biramous, with well-developed chaetigerous lobes, dorsal cirri and ventral cirri. Notopodial lobes low, conical mounds. Neuropodial lobes cylindrical, elongate, much longer than notopodia. Dorsal and ventral cirri tapering to long and filiform tips, almost as long as chaetae; dorsal cirri longer than ventral cirri. Notochaetae spine-like, internally chambered, with two longitudinal rows of long alternating spinelets. Neurochaetae with internally chambered shafts and very finely serrated falcigerous blades with minute bidentate tips.

This species was originally described from Madeira Island (Langerhans 1880) and was later reported from the Mediterranean Sea (Banyuls-sur-Mer; Laubier 1964). In the absence of extant type material, Dahlgren & Pleijel (1995) designated a neotype from the Mediterranean Sea and further extended the geographical distribution of the species to the NW European margin. More recently, Böggemann (2009) and Watson et al. (2014) described the same species from deeper waters off W Africa. Watson et al. (2014) discussed some morphological differences between the NE Atlantic, Mediterranean Sea and Angola Basin abyssal specimens. According to these authors, the Senghor Seamount and Madeira Island specimens have moderate length palps comparing to the longer palps of the Mediterranean Sea specimens. Also, the Senghor Seamount specimens have biramous parapodia on segment 2 without ventral cirri, while the Mediterranean Sea and Angola Basin specimens have uniramous parapodia on segment 2, with notochaetae, dorsal cirri and ventral cirri. Furthermore, a marked increase of notochaetal length between chaetigers 9 and 13 was observed for the first time in Senghor Seamount specimens (Watson et al. 2014). In the specimens examined herein, palps are missing and segment 2 has uniramous parapodia with notochaetae, dorsal and ventral cirri, similar to the Mediterranean Sea specimens. As already mentioned by Watson et al. (2014), this taxon may be a complex of cryptic species and needs further revision, preferably with the inclusion of molecular data.

Dysponetus caecus was previously recorded from shallow depths among sponges and kelp holdfasts from the intertidal to 85 m from S Sweden to Madeira Island, including N and W Scotland and N Denmark (Dahlgren & Pleijel 1995) and in similar habitats from the Mediterranean Sea (Alboran Sea and Banyuls-sur-Mer; San Martín 2004). The species was reported from deeper waters (~ 3000 m) at Senghor Seamount, NW Africa, from fine clay sediments (Watson et al. 2014) and from muddy sand with shell and gravel in the Guinea, Angola and Cape Basins (5494 m; Böggemann 2009). The specimens studied here were found in Saint Nazaire Canyon (Bay of Biscay) (Fig. 1), in a cold-water coral environment at a depth of 755 m, and in the Mercator MV (Gulf of Cadiz), in sunken wood colonization experiments densely colonised by Xylophaga dorsalis (W. Turton, 1819) at a depth of 354 m (Cunha et al. 2013a).