Streptosyllis nunezi n. sp., is most closely related to S. arenae because they share the following characters in common: both the dorsal simple chaetae and the blades of the compound chaetae are covered by a hyaline hood, and the palps are not visible dorsally. S. nunezi n. sp., differs from S. arenae in that the teeth of the shaft of the compound chaetae are covered by a hyaline hood, whereas in the latter species only the shaft is covered; the hood of blades of the compound chaetae forms 1–2 lateral tips, whereas in S. arenae the hood is smooth; the dorsal simple chaetae, being smooth in S. arenae, are strongly serrated on the margin, forming 2– 4 rounded teeth when viewed laterally in S. nunezi n. sp.; the pharynx extends through about five segments in the new species instead of through 6–7 segments in S. arenae. S. biarticulata also has a hyaline hood on the shaft of the long-bladed falcigers, on the dorsal simple chaetae and on the blades of the short-bladed falcigers, however it differs from S. nunezi n. sp., in having smooth dorsal simple chaetae and the palps are visible dorsally. Five species of the genus have a hyaline hood on the blades of the compound chaetae: S. arenae, S. templadoi, S. magnapalpa, S. biarticulata and S. nunezi n. sp. S. nunezi n. sp., can clearly be distinguished from the other four species because it is the only one to have serrated dorsal simple chaetae. S. nunezi n. sp. has thus a unique combination of characters which clearly differentiate it from all other Streptosyllis species: (a) palps not visible dorsally; (b) hyaline hood covering the teeth of shafts of the compound chaetae; (c) strongly serrated margins of dorsal simple chaetae, visible as 2–4 rounded teeth; (d) hood on the blades of the compound chaetae forming 1–2 lateral tips. The latter three characters are not shared by any other Streptosyllis species.
Description based on holotype USNM 1113750:
Body ca. 2.5 mm long, for 33 chaetigers; maximal width 140 µm without parapodial lobes, 210 µm with parapodial lobes and 260 µm including chaetae (14th segment) (Figure 4). Head semi-circular, about as wide as long, anterior margin rounded. Two pairs of eyes, one pair of eyespots, latter located near anterior margin. Three smooth antennae, median antenna originating between two pairs of eyes in middle of prostomium, lateral ones near anterior margin (near eye spots). Median antenna ca. 260 µm long, lateral antennae 130 µm long. Palps directed ventrally, not visible dorsally, forming two rounded basally fused lobes; outer margins prolonged into vestigial cirrus. Peristomium not clearly separated from first segment, bearing two pairs of tentacular cirri, about as long as lateral antennae.
Dorsal cirri smooth, thin (Figure 5e), articulated in chaetigers 7, 11, 13, 14, 15, 16, 18, 20 and 30 (often only on one side of animal), longitudinally divided articles with granular inclusions (Figure 5f). Dorsal cirri about two thirds length of lateral antennae (ca. 90 µm) in anterior and middle chaetigers, becoming longer in posterior chaetigers. Ventral cirri smooth, digitiform, almost as long as parapodial lobes in anterior and middle chaetigers, then becoming longer near posterior end (Figure 4). Median anal cirrus (ca. 90 µm long) present, lateral cirri not observed. Dorsal simple chaeta present from chaetiger 1, slightly curved, ca. 23 µm long, reaching to level of shaft of compound chaetae, tip bluntly rounded, covered by hyaline hood, proximal part strongly serrated immediately below hood, shaped as two, sometimes 3–4 rounded teeth (Figures 6d–f); sometimes second simple chaeta present. Up to 8–9 hemigomph compound chaetae in each parapodium, upper chaetae in fascicles ca. 32 µm long, lower chaetae shorter, ca. 20 µm long (Figures 5a–d). Shaft of compound chaetae with up to three apical rounded teeth, sometimes notched; with hyaline hood covering these teeth and entire distal part of shaft (Figures 6g–l). Blades of two kinds, longer ones (ca. 15 µm) on dorsal chaetae, shorter ones (ca. 7 µm) on ventral chaetae. Chaetigers 1–5 with only blades of short type (Figures 6g–h), from chaetiger 6 onwards two long-bladed and 6–7 short-bladed chaetae present. Longer blades unidentate with pointed tip covered by hyaline hood extending along side of blade, forming an acute, sometimes bent, tip and with 1–2 tips further down the blade (Figure 6j). Shorter blades with blunt hood covering tip (Figure 6l). Blades in far posterior parapodia appearing very thin, slightly serrated. Aciculae not distinctively enlarged. A single acicula in each parapodium, terminating at end of parapodial lobe. Aciculae distally knobbed, knob bluntly rounded, some- times irregularly (one side of the knob larger than the other) (Figures 6a–c). Pharynx with distal crown of ten soft papillae, unarmed, extending from anterior end to the beginning of chaetiger 5. Proventriculus extending through 4–5 segments, with about 45 muscle cell rows (Figure 4). Pale yellow in alcohol.
Character variation: Lateral anal cirri present, varying in length from as long as dorsal cirri to several times longer (up to 440 µm) (paratypes 3, 4); compound chaetae with only short blades from chaetigers 1–6; longer blades unidentate with pointed tip covered by halo-shaped hyaline hood; blades of compound chaetae smooth throughout the body; enlarged aciculae present on chaetigers 2–6 (paratype 10); some individuals with sexual products and natatory chaetae, from about chaetiger 16 onward (paratypes 3, 4, 6, 9).
Mediterranean Sea (Crete, Italy), northeastern Atlantic (Canary Islands)
Fine to coarse sandy substrates, shallow waters (1–20 m).