Syllis, Lamarck, 1818

Langeneck, Joachim, Musco, Luigi, Busoni, Giulio, Conese, Ilaria, Aliani, Stefano & Castelli, Alberto, 2018, Syllidae (Annelida: Phyllodocida) from the deep Mediterranean Sea, with the description of three new species, Zootaxa 4369 (2), pp. 197-220 : 213

? Syllis sp. 2

( Figure 9a–f )

Material examined. St. 3: 1 individual.

Description. Anterior fragment, 5 mm long, 0.35 mm wide for 44 chaetigers ( Fig. 9a ). Prostomium ovate, distinctly broader than long, with four small eyes in trapezoidal arrangement, palps united at the basis, longer than the prostomium. Antennae, tentacular cirri and dorsal cirri articulate, similar in length, approximately ½ to ¾ of body width; antennae and tentacular cirri incomplete or broken, dorsal cirri of the same length throughout the body, with 20–25 articles. Parapodia well developed, with two aciculae similar in size anteriorly ( Fig. 9e ), becoming clearly different around chaetiger 30, larger one with squared to rounded tip, smaller one with pointed tip anterior parapodia with two aciculae of similar size ( Fig. 9e ), whilst around chaetiger 30 clearer difference in size ( Fig. 9f ). Up to 10 compound heterogomph falcigers per parapodium; anterior 4–5 parapodia dorsal chaeta with distinctly longer blade ( Fig. 9c ); blades slightly bidentate, with short spines on margin ( Fig. 9c ), midbody parapodia with a gradual gradation in size of blades ( Fig. 9d ), hooked, smooth on margin or with very short spines on margin. Blades of compound chaetae 30–20 µm long in anterior parapodia, gradually decreasing to 20–15 µm in midbody. Dorsal simple chaetae straight, with slightly curved tip and slightly serrated subdistally ( Fig. 9b ). Pharynx through five chaetigers, long and narrow, with a slightly backward pharyngeal tooth large, triangular. Proventricle through six chaetigers, with ca. 40 cell rows. Colour yellowish; chaetigers 3–11 with a thin dark bar along itsposterior edge; posterior peristomium slightly darker in the freshly fixed individual, evenly whitish some months after fixation.

Distribution. Southern Adriatic Sea, at 730 m depth.

Remarks. The examined specimen shows intermediate features between the genera Syllis Lamarck, 1818 and Opisthosyllis Langerhans, 1879 . Similarly, to Opisthosyllis it has a large tooth inserted backwards to the anterior part of the pharynx and compound chaetae sub-bidentate to unidentate; however, it does not show any trace of the occipital flap typical of Opisthosyllis . Moreover, the position of the pharyngeal tooth is relatively close to the pharynx opening for Opisthosyllis , and compatible with that of some species of the genus Syllis ( Licher 1999; San Martín 2003). Among the currently known species of the genus Opisthosyllis the herein reported specimen appears close to Opisthosyllis flaccida ( Grube, 1878) from the Pacific Ocean in the joint occurrence of bidentate compound chaetae with relatively small secondary tooth and unidentate chaetae, gradually becoming shorter towards the posterior end. It differs from this last species as the pharyngeal tooth is more massive (thin, dagger-like in O. flaccida ), the dorsal cirri have 20–25 articles (26–40 in O. flaccida ) and anterior parapodia have two aciculae (four in O. flaccida ). Moreover, O. flaccida does not have compound chaetae with distinctly longer blade in anterior parapodia ( Licher 1999; Aguado et al. 2008). These specimens most likely represents an undescribed species, but the currently available material is too scarce to proceed with a formal description.

Figures

FIGURE 9. a‒f)?Syllis sp. 2 (St. 3, 730 m): a) anterior end in dorsal view; b) dorsal simple chaeta; c) compound chaetae from the anterior part of thebody; d) compoundchaetae from the midbody; e) aciculae from the anterior part of the body; f) aciculae from the midbody. g‒j) Anguillosyllis sp. (St. 16, 2100 m): g) complete individual in dorsal view; h) dorsal simple chaeta; i) compound chaetae; j) acicula.Scale-bar:a: 0.32 mm; b‒f:30 µm; g: 0.20 mm; h‒j: 25 µm.