Erinaceusyllis barbarae Langeneck & Musco & Busoni & Conese & Aliani & Castelli 2018, n. sp.

Erinaceusyllis barbarae Langeneck, Musco & Castelli n. sp.

(Figures 1–2)

Material examined. Holotype (MSNP: P/236/V): St. 4, Southern Adriatic Sea, 1200 m depth; Paratypes: same data as holotype: 1 individual (MSNP: P/257/V); St. 8: 2 individuals (MSNP: P/243/V). St. 11: 2 individuals (MSNP: P/252/V; P/256/V).

Description. Holotype atokous, entire, with relatively stout and robust body, 26 chaetigers for ca. 2 mm long, maximum width 0.18 mm (Fig. 1a, b); paratype entire, epitokous male, 30 chaetigers, ca. 2 mm long, 0.20 mm maximum width (Fig. 1c). Dorsum covered by small, scattered papillae. Prostomium oval, with four reddish eyes in trapezoidal arrangement and two anterior black eyespots (apparently absent in some paratypes). Antennae with bulbous base, last 2/3 briskly shrunken in a small tip; central antenna inserted slightly posteriorly to posterior eyes, as long as prostomium; lateral antennae ca. half long as median one, inserted on anterior margin (Fig. 1b). Palps shorter than prostomium, lacking noticeable papillae. Peristomium shorter than subsequent segments, covering dorsally prostomial posterior margin; single pair of peristomial cirri, similar in size and shape to lateral antennae. Dorsal cirri similar to antennae on anterior chaetigers, becoming more elongated towards pygidium, with gradually less shrunken tip (Fig. 1a), absent at chaetiger 2 (Fig. 1a, b, c). Parapodia rectangular to conical, well developed, with 7–8 compound and two simple chaetae; number of chaetae per parapodium diminishing posteriorly to five compound and two simple chaetae in posterior parapodia. Compound chaetae heterogomph, with shafts marginally smooth; blades all unidentate, distally slightly hooked (Fig. 2a, 2b). A single, most dorsal compound chaeta with distinctly elongated, slightly sinuous blade on each parapodium (Fig. 2a, b); 40 µm long on anterior parapodia, provided with several thin spines basally, 35 µm long, smooth, on posterior parapodia. Remaining falcigerous chaetae with smooth blade ranging from 25 to 18 µm on anterior parapodia, 23 to 15 µm on posterior parapodia (Fig. 2a, 2b). Dorsal simple chaetae occurring throughout the whole body, unidentate, robust, slightly sinuous and with very short distal spines (Fig. 2c); ventral simple chaetae only in posterior parapodia, thinner than dorsal one, smooth, unidentate (Fig. 2d). Acicula solitary, acuminate, with slightly enlarged sub-distal part (Fig. 2e). Pharynx short, approximately half as wide as the proventricle, across three chaetigers, tooth not visible (holotype) or relatively small, triangular, pointed, near to pharynx opening (paratype). Proventricle long, wide, barrel-shaped, and extending across four chaetigers, with about 19–20 muscle cell rows. Pygidium small, with two elongated cirri, approximately 2½ long as posterior dorsal cirri (Fig. 2f). Paratype with long capillary notochaetae and large intracoelomic sperm packages from chaetiger 8 to 23 (Fig. 1c).

Etymology. This species is friendly dedicated to Dr. Barbara Mikac, in recognition of her important contribution to the knowledge on Mediterranean polychaetes, especially from the Adriatic Sea. Distribution. Mediterranean Sea: Adriatic Sea (type locality), Tyrrhenian Sea, Sea of Sardinia; from 110 to 1200 m depth.

Remarks. Based on the chaetae with unidentate sabre-like blades, E. barbarae n. sp. resembles Erinaceusyllis serratosetosa (Hartmann-Schröder, 1982) and Erinaceusyllis ettiennei San Martín, 2005. It differs from the former in lacking spinulation in most compound chaetae and as antennae and dorsal cirri in the anterior part of the body are shorter and thicker. Erinaceusyllis ettiennei differs from the new species in having all blades with a more pronounced spinulation, which gradually decreases dorsoventrally. Moreover, E. ettiennei is smaller than the new species and has longer antennae and dorsal cirri in the anterior part of the body. Erinaceusyllis barbarae n. sp. can be easily distinguished from the remaining Mediterranean Erinaceusyllis based on their compound chaetae: Erinaceusyllis belizensis (Russell, 1989) and Erinaceusyllis cryptica (Ben-Eliahu, 1977) have bidentate blades (San Martín 2003), while Erinaceusyllis erinaceus (Claparède, 1863) has unidentate, distinctly shorter blades, with a less pronounced dorso-ventral gradation (Verdes et al. 2013). Moreover, contrary to the new species, the species of Erinaceusyllis typically occur in shallow water environments (San Martín 2005; Ramos et al. 2010; Lucas et al. 2017), even if some abyssal species originally assigned to the genus Sphaerosyllis Claparède, 1863, as Sphaerosyllis ridgensis Blake & Hilbig, 1990, and Sphaerosyllis ruthae San Martín, 2004 could actually belong to Erinaceusyllis (G. San Martín, pers. comm.). Sphaerosyllis ruthae, originally described from abyssal depths (> 4000 m) in Alaska shows some similarities with E. barbarae n. sp. as regards the shape of chaetae, that are very long, unidentate and with poorly developed ventral spinulation, but it clearly differs from the new species in the absence of eyes and in the proventricle shorter than the pharynx (longer than the pharynx in E. barbarae n. sp.).