Leptoecia midatlantica Budaeva, 2012, new species

Leptoecia midatlantica, new species

Figs. 1–7, Tab. 3

Type material. NHMUK 2011.351 St. JC037/70, 4% formalin transferred to 70% ethanol (holotype); NHMUK 2011.352–361 St. JC037/70, 4% formalin transferred to 70% ethanol (11 paratypes); NHMUK 2011.362 St. JC037/ 70, 96% ethanol (1 paratype); NHMUK 2011.363–370 St. JC037/61, 4% formalin transferred to 70% ethanol (8 paratypes); NHMUK 2011.371, St. JC037/61, 96% ethanol (1 paratype); ZMBN 87901 St. JC037/61, 4% formalin transferred to 70% ethanol (2 paratypes); ZMBN 87902 St. JC037/61, 96% ethanol (1 paratype); ZMBN 87903 St. JC037/70, 4% formalin transferred to 70% ethanol (5 paratypes); ZMBN 87904 St. JC037/70, 96% ethanol (1 paratype).

Non type material examined. ZMBN, RV G. O Sars, St. 368, 4% formalin transferred to 70% ethanol (1); NOCS, RRS James Cook: St. JC011/101, 4% formalin transferred to 70% ethanol (5); St. JC011/111, 4% formalin transferred to 70% ethanol (2); St. JC037/19, 96% ethanol (5); St. JC037/61, 4% formalin transferred to 70% ethanol (73); St. JC037/61, 96% ethanol (12); St. JC037/67, 4% formalin transferred to 70% ethanol (6); St. JC037/67, 96% ethanol (2); St. JC037/70, 4% formalin transferred to 70% ethanol (61); St. JC037/70, 96% ethanol (14); St. JC037/79, 4% formalin transferred to 70% ethanol (1); SIO, RRS James Cook: St. JC037/61, 96% ethanol (10); St. JC037/70, 96% ethanol (20).

Type locality. North Atlantic, Mid-Atlantic Ridge, Charlie-Gibbs Fracture Zone, 54º13'N, 36º04'W – 54º10.5'N, 36º05'W, 2604–2615 m.

Diagnosis. Both uni- and bidentate simple falcigers exclusively on the first pair of parapodia, rarely only bidentate falcigers present; falcigers straight, without subdistal reduction in diameter; subacicular hooks from chaetiger 28–39; dorsal cirri on all chaetigers; digitiform postchaetal lobes till chaetiger 8–16; peristomium half as long as first chaetiger; tubes always dorsoventrally flattened with longitudinal ribs along both sides

Description. Holotype, a complete specimen consisting of 66 chaetigers, 31 mm long and 0.93 mm wide. Other examined specimens varying from 0.69 mm to 1.31 mm in width and from 19 mm to 41 mm in length. Longest examined specimen consists of 81 chaetigers. All specimens yellowish, lacking colour pattern.

Prostomium of variable shape, from distally pointed through conical or slightly bilobed, to distinctly bilobed with two hemispherical frontal lips (Fig. 2 A–E, 3A–F). Eyes absent. Nuchal groves absent (Fig. 3 E, G). Palps reaching chaetiger 1; lateral antennae reaching chaetiger 8 (5–13); median antenna longer and thicker than lateral antennae, reaching chaetiger 9 (7–16). Ceratophores consisting of 2 (1–3) rings equal in length (Fig. 2 B, 3E). Peristomium about half as long as the first chaetiger; peristomial cirri absent (Fig. 2 B, 3E).

First anterior pair of parapodia modified, prolonged, projecting lateroventrally and directing anteriorly, with auricular prechaetal lobes, knob-like postchaetal lobes and subulate dorsal and ventral cirri (Fig. 3 F, 4A, B). Second pair of parapodia with short and rounded prechaetal lobes, subulate postchaetal lobes and dorsal and ventral cirri (Fig. 4 C, 5A). Subulate ventral cirri replaced by flattened round glandular pads from chaetiger 3 (Fig. 2 A, 4D, 5B). Dorsal cirri present on all chaetigers including the posteriormost ones, subulate on anterior parapodia and decreasing in length posteriorly (Fig. 2 I–L, 3I, K). Postchaetal lobes subulate to digitiform on first 13 (8–16) chaetigers (Fig. 5 C), completely disappearing posteriorly. Branchiae absent (Fig. 3 B).

First pair of parapodia with simple uni- and bidentate falcigers, rarely only bidentate falcigers present; capillary, limbate or pectinate chaetae absent. Anterior falcigers with short, distally blunt paired hoods and without subdistal reduction in diameter (Fig. 5 D–H, 6D–G). Limbate (Fig. 5 I) and pectinate (Fig. 5 J–L) chaetae arranged in two fascicles starting from chaetiger 2: dorsal fascicle with 5–8 pectinate chaetae and 1–2 limbate chaetae, ventral fascicle with 3–4 limbate chaetae (Fig. 4 C–G). Simple bidentate subacicular hooks (Fig. 5 M– O, 6H) starting from chaetiger 38 (28–39) within the ventral fascicle of limbate chaetae (Fig. 4 H–J). Appearance of first subacicular hooks not size-dependent in the specimens examined (Fig. 7 A). Compound subacicular hooks representing juvenile condition in few posteriormost chaetigers (Fig. 5 P). Ventral limbate chaetae present in all posterior parapodia. Pectinate chaetae flat with straight distal margins and up to ten denticles. Aciculae yellow with pointed tips, 1–3 per parapodium (Fig. 4).

Mandibles with almost straight distal margins (Fig. 2 G, 3F). Maxillae weakly sclerotized; carriers longer than wide; distal parts of MxI pointed, dark brown in colour (Fig. 2 F). Maxillary formula (based on 3 specimens): MxI=1+1; MxII=13–18+13–17; MxIII=10–13+0; MxIV=10–12+11–17. Maxillae V absent.

Pygidium conical, covered with glandular buds (Fig. 3 H–J), with anus opening dorsally (Fig. 2 I–L). Number of anal cirri varying from 2 to 5, but most examined specimens (90%) with two anal cirri (Fig. 2 H–L, 3H, I).

Tubes translucent, thin-walled, dorsoventrally flattened with distinct longitudinal thickened ribs along both lateral margins. Holotype containing spherical oocytes in its body cavity around chaetigers 19–28.

Remarks. Leptoecia midatlantica sp. nov., can be distinguished from its congeners by the presence of both uni- and bidentate simple falcigers on the first pair of parapodia. However this character was variable in the studied material. Some specimens displayed the presence of only bidentate falcigers, but never only unidentate falcigers. L. midatlantica sp. nov., is similar to L. benthaliana in having falcigers on the first chaetiger only, dorsal cirri on all parapodia, and subacicular hooks starting in the middle of the body (around chaetiger 35). Orensanz (1990) examined ca. 60 specimens of L. benthaliana describing the variation in the shape of the prostomium (slightly bilobed or pointed) and shape of the tube (circular in cross section or with lateral ribs on one or both sides). The examined specimens of L. midatlantica sp. nov., also demonstrated the variation in the shape of the prostomium and the variable degree of frontal and upper lips development. Among 74 specimens examined, 42 had a slightly or noticeably bilobed prostomium (Fig. 2 A, B, E, 3E, F) and 32 had a conical or pointed prostomium (Fig. 2 C, D, 3A, D). All examined specimens had flattened tubes; no tubes circular in cross-section were found in the studied material. L. midatlantica sp. nov., can be distinguished from L. benthaliana by the shape of anterior falcigers. In L. benthaliana all falcigers independently on their size have subdistal reduction in their diameter (Orensanz 1990) (Fig. 6 A–C), whereas in L. midatlantica sp. nov., falcigers uniform in their diameter throughout (Fig. 6 D–G).

Distribution. Leptoecia midatlantica sp. nov., is known from the northern part of the Mid-Atlantic Ridge from the north off the Azores to the southern tip of the Reykjanes ridge (Fig. 1). Depth range 2107–2754 m.

Etymology. The specific name, midatlantica, refers to the geographical range of the new species that was reported from the deep-sea habitats of the northern part of the Mid-Atlantic Ridge.