Mediomastus biscayensis sp. nov.

Figures 2A–C, 3A–C & 4A, B.

Material examined. Holotype: MNHN-IA-2000-2086, incomplete, Northern Atlantic, Bay of Biscay, West Gironde Mud Patch M 1-4KC, 45.602°N, 1.813°E, depth 68 m. Paratype: MZUSP 5654, incomplete, Northern Atlantic, Bay of Biscay, West Gironde Mud Patch M 1-4C, 45.615°N, 1.822°E, depth 70 m (2 spms) .

Description. Holotype 8 mm long, 0.15 mm wide, for 20 segments. Body thin, elongate, first four thoracic segments short, becoming as long as wide to end of thorax; abdominal segments twice as longer as wide, moniliform (Figs 2A; 3A). Dorsal and lateral grooves absent. Colour in alcohol pale yellow.

Prostomium short, conical, with papilliform palpode (Figs 2A; 4B). Eyespots absent; nuchal organs present, postero-lateral to prostomium. Peristomium well-separated from prostomium and chaetiger 1, slightly longer than chaetiger 1 dorsally (Figs 2A; 4B). Proboscis everted in one specimen, bulbous, bilobed with basal lobe covered by numerous short round papillae (Fig. 2A).

Thorax with ten chaetigers (Figs 2A; 3A); chaetigers 1–4 biramous with notopodial and neuropodial capillaries (Fig. 2A); capillaries bilimbate (Fig. 3A) organized in two rows of four capillaries in each fascicle. Chaetigers 5–10 with long-shafted hooded hooks (Figs 2B; 3B). Notopodial and neuropodial long-shafted hooded hook numbering six per fascicle; hooks long, without distinct constriction and with at least four rows of small teeth above main fang (Fig. 3B). Lateral organs and genital pores not observed.

Transition between thorax and abdomen marked by changes in shape of segments, slight constriction, and length of hooded hooks (Fig. 4A). Abdominal segments longer than wider (Fig. 4A) and becoming longer posteriorly. Notopodial and neuropodial tori short and well-separated emerging from posterior end of segments; notopodial tori dorsal and neuropodial tori ventral. Abdominal region with only hooded hooks throughout, slightly shorter than thoracic hooks (Figs 2C; 3C), without distinct constriction and with 3–4 rows of few teeth (about six) above main fang; abdominal hooded hooks few numbering 6–10 per torus.

Pygidium not observed.

Methyl Green Staining Pattern. Chaetigers 5–9 with sparse green speckles (Fig. 4A). Chaetigers 10 and 11 with dense green speckles (Fig. 4A).

Remarks. There are 17 described species of Mediomastus, the most recent species described from Japan (Tomioka et al. 2013, 2014) and China (Lin et al. 2018) from specimens previously identified as Mediomastus californiensis Hartman, 1944 . The most comprehensive review of the genus was provided by Warren et al. (1994) together with a comparative table with diagnostic characters. An updated key to species was provided in Tomioka et al. (2014). Mediomastus biscayensis sp. nov. is distinct from its congeners by a combination of characters: i) bilimbate thoracic capillaries; ii) thoracic and abdominal hooks similar in shape and with slight difference in length; iii) distinct MGSP: chaetigers 5–9 with sparse green speckles and chaetigers 10–11 with dense green speckles (See Table 1).

Mediomastus capensis Day, 1961 is a similar species with bilimbate capillaries and by having abdominal hooks that are slightly shorter than thoracic ones (Warren et al. 1994). It differs from M. biscayensis sp. nov. in that the thoracic capillaries that are ‘curved into minute hooks at tip’ (Warren et al. 1994) whereas capillaries in M. biscayensis sp. nov. are long and bilimbate, without curved hooked tips. The Methyl Green staining pattern is also distinct with a green band around the posterior half of chaetiger 9 in M. capensis and a more intense staining in chaetigers 10–11 than in M. biscayensis sp. nov. The European species Mediomastus fragilis has records in the Mediterranean (Capaccioni-Azzati 1985) and Bay of Biscay, France (Bachelet & Dauvin 1993; de Montaudouin & Sauriau 2000) but it is distinct from the new species by the presence of a small and rounded prostomium, chaetigers 10 and 11 transitional, thoracic hooks long and abdominal hooks short, and indistinct MGSP (Warren et al. 1994).

Etymology. This species is described after its type locality Bay of Biscay.

Type locality. Bay of Biscay, West Gironde Mud Patch .

Habitat. Muddy bottoms, around 70 m depth.

Distribution. Known only from the type locality.