Arichlidon reyssi

Ecology and distribution Arichlidon reyssi is distributed in the NE Atlantic from the Cantabrian Sea (Bay of Biscay) to the Cape Verde Archipelago, at 20 – 1651 m; in the Mediterranean from the Alboran Sea to the Levant Basin, from the intertidal to 3947 m (the deepest locations in the E Mediterranean); in the Red Sea from the intertidal to a depth of 757 m (Watson Russell 1998; San Martín 2004; Watson et al. 2014). Arichlidon reyssi usually occurs in hard substrates, e. g., coarse sediments with shell debris, rock, calcareous substrates, but also in mud at the deepest localities (Watson 1998; San Martín 2004). The material studied herein was collected together with samples of the scleractinian coral, Dendrophyllia cornigera (Lamarck, 1816), at the head of a Cap Breton Canyon tributary (Cantabrian Sea, Bay of Biscay) at 214 – 221 m and from inactive carbonate chimneys at Formosa Ridge (SW Iberian margin, Gulf of Cadiz) at 1079 m.

Description Both specimens from the Bay of Biscay are very small and extremely arched, and thus difficult to examine. The anterior fragment from station M 84 - 5 _ 633 (DBUA 0002276.01) has eleven chaetigers and measures approximately 1.17 mm long. Should the two fragments (anterior and posterior) belong to the same specimen, the total number of chaetigers would be 17 and the total length 1.94 mm. Prostomium with two pairs of coalescent dark eyes. Indistinct paired structure present ventrally at mouth level. Palaeal notochaetae bright and translucent, covering entire body and forming an acute mid-dorsal ridge. Paleae broadly rounded, distally with a small acute tip and strong serrate margins; dorsal surface with raised serrate ribs. Blades of compound neurochaetae long and spinigerous, in dorsalmost position becoming falcigerous and gradually shorter ventrally.

Remarks Arichlidon reyssi has often been reported as occurring in large numbers at a given locality (e. g., Aguirrezabalaga et al. 1986, Cantabrian Sea, 100 m, numerous specimens; Watson Russell 1998, eastern Mediterranean, 196 – 199 m, 82 specimens; Watson et al. 2014, Senghor seamount, 102 – 133 m, 80 specimens). In the present study, only a couple of specimens were found at shelf-break depths in the Bay of Biscay and a dubious posterior fragment at a deeper station in the Gulf of Cadiz. These small and incomplete specimens possess lateralmost median paleae not clearly longer than the remaining ones, which is a distinguishing character for A. reyssi (Watson Russell 1998, 2000 b). Apart from that, no other obvious morphological differences were found. The absence of tall lateralmost median paleae in some specimens was previously noted for the eastern Mediterranean, Red Sea and Egyptian populations, which were nonetheless considered as belonging to the same species (Watson Russell 1998). However, the known wide bathymetric range of this species suggests a possible complex of cryptic species in need of further study (see Watson et al. 2014).

Material examined SPAIN – Bay of Biscay • 1 af, 1 pf; tributary of Cap Breton Canyon; 43 ° 31.74 ′ N, 02 ° 45.60 ′ W; 221 m; St M 84 - 5 _ 633; DBUA 0002276.01 • 1 af; tributary of Cap Breton Canyon; 43 ° 31.68 ′ N, 02 ° 45.48 ′ W; 214 m; St M 84 - 5 _ 677; DBUA 0002276.02. PORTUGAL • 1 pf (cf); Gulf of Cadiz, Formosa Ridge; 36 ° 10.263 ′ N, 07 ° 43.819 ′ W; 1079 m; St TTR 12 _ AT 388; DBUA 0000698.