( Figure 5)
Exogone (Parexogone) wolfi San Martín 1991: 726, Fig. 6; San Martín et al. 1996: 252, Fig. 3; San Martín 2003: 243 ‒244, Figs 129‒130.
Parexogone wolfi Böggemann & Purschke 2005: 223 ‒225, Fig. 2; Böggemann 2009: 408 ‒410, Figs 145‒146; Barroso et al. 2017: 406 ‒407, Fig. 3.
? Paedophylax longicirris Webster & Benedict 1887: 722, Figs 46‒50.
? Exogone longicirris Perkins 1981: 1092, Fig. 11.
? Parexogone longicirris Lucas et al. 2017: 10 ‒11, Fig. 2.
? Exogone furcifera Eliason 1962: 243 ‒246, Fig. 11.
? Exogone (Parexogone) canyonincolae Sardá et al. 2009: 15 ‒17, Fig. 7.
Material examined. St. 8: 4 individuals. St. 16: 3 individuals. St. 19: 2 individuals.
Description. All individuals lacking pygidium; two with regenerating anterior end. Best preserved individual ca. 6 mm long for 40 chaetigers, 0.20 mm wide. Prostomium wider than long, often hidden under dorsal part of peristomial segment, four rounded, relatively large eyes in trapezoidal arrangement (absent in regenerating individuals). Palps ca. twice as long as prostomium, entirely fused, clearly pointed; antennae well-developed, median one slightly longer than palps, lateral antennae ca. half as long as median one. Dorsal cirri oval, small, present in all chaetigers. Parapodia well-developed, with 8–14 compound chaetae, blades bidentate. Two spinigerlike chaetae with distinctly long, slightly sinuous blade (45–55 µm in the anterior part, 50–70 µm in the midbody, 30–45 µm in the posterior part of the body), with a strong serration in the distal part, less evident towards basal part ( Fig. 5b). Several falcigers on each parapodium, with distinctly shorter blades and strong serration, with some longer spines distally, decreasing in size from the dorsal part of parapodium to the ventral one, approximately 15– 25 µm in the anterior part of the body, 10–20 µm in the midbody, 15–8 µm in the posterior part of the body ( Fig. 5c). Pharynx slender, longer than proventriculum, through 4 segments, bearing a strong, triangular tooth on anterior margin; proventricle short, through two segments approximately, with 15–17 cell rows.
Distribution. Western Atlantic Ocean (San Martín 1991; Barroso et al. 2017); Eastern Atlantic Ocean ( San Martín et al. 1996; Böggemann 2009); Pacific Ocean (dubious) ( San Martín 2005); Eastern Mediterranean Sea ( Simboura & Zenetos 2005); from ca. 100 m depth ( Simboura & Zenetos 2005) to more than 5000 m depth ( Böggemann 2009). This is the first record of the species for the Western Mediterranean.
Remarks. Parexogone wolfi is one of the most widespread deep-water Exogoninae and shows remarkably wide depth range adaptation ( San Martín 2003). The available descriptions, however, highlight slight differences between individuals from different areas, which might represent a clue of cryptic speciation ( Barroso et al. 2017). For instance, shallow water ( 8 m depth) Pacific individuals are distinctly thinner, with a couple of additional eyespots and shorter spiniger-like chaetae in respect to the original description ( San Martín 2005), which in our opinion suggests that they may represent an undescribed species.
Parexogone wolfi closely resembles P. canyonincolae, which differs mainly in measuring 2.5 mm long for 50 chaetigers, (ca. 6 mm and 40 chaetigers in P. wolfi). Therefore, the possibility that P. canyonincolae might just represent a juvenile stage of P. wolfi cannot be ruled out. On the other hand, Exogone longicirris Webster & Benedict, 1887 has been recently assigned to Parexogone, and closely resembles P. wolfi. The main differences are only a slightly lower number of proventricular cell rows (11–14 vs 15–21) and a less pronounced spinulation along the blade edge ( Eliason 1962; Lucas et al. 2017). The spinulation of compound chaetae, however, shows geographical variability, as the Brazilian specimens examined by Barroso et al. (2017) show less pronounced spines than those reported in the original description (San Martín 1991), thus appearing closer to P. longicirris. Taking into account the current knowledge (including the similar depth range and geographical distribution), a synonymy between P. wolfi and P. longicirris cannot be ruled out.