Prionospio nonatoi Peixoto & Paiva 2019, sp. nov.

Prionospio nonatoi sp. nov.

(Figures 6–9)

Type material. Brazil. Espírito Santo Basin. Holotype: Amb 7 D4, 19° 45' 54.56" S 39° 30' 25.23" W, 12/2011 to 02/ 2012, 144m, MZUSP 3387. Paratypes: Amb7 B4R1, 20° 35' 25.16" S 39° 54' 58.31" W, 12/2011 to 02/ 2012, 157m, MNRJP 1828 (3 ind), MZUSP 3388 (2 ind), MZUSP 3389 (2 ind).

Additional material: Amb7 B4, 20° 35' 25.16" S 39° 54' 58.31" W, 157m (47 ind); Amb7 D4, 19° 45' 54.56" S 39° 30' 25.23" W, 144m (15 ind); Amb7 E4, 19° 36' 4.32" S 39° 10' 34.07" W, 147m (6 ind).

Diagnostic feature: Branchiae absent.

Description: A small-sized spionid, largest individual about 4.8 mm long, 0.18 mm wide for 65 chaetigers, holotype 4.6 mm long, 0.18 mm wide for 62 chaetigers. Body cylindrical, slightly dorsoventrally compressed throughout body, tapered towards the pygidium (Fig. 6 A–D; 8A). Color in alcohol white. Pigmentation absent.

Prostomium narrow, slightly widened towards the anterior margin, rounded anteriorly, extending posteriorly as a narrow keel to the posterior margin of chaetiger 1 (Figs 6 A–D; 7A; 8A–B). Eyes absent. Prostomial peaks not observed. Prostomium and peristomium well-delimitated by a deep incision (Figs 6 A–D; 8A–B). Peristomium short, surrounding prostomium and partially fused to chaetiger 1, lacking lateral wings. Palps lost in all individuals.

Chaetiger 1 with few and short chaetae on both rami. Postchaetal lamellae auricular, reduced. Prechaetal lamellae absent.

Notopodial postchaetal lamellae foliaceous on chaetigers 2–5, largest on chaetiger 3 (Figs 6 A–D; 7A; 8A–B) and smaller on chaetigers 4 and 5. Lamellae rounded from chaetiger 6 to chaetiger 11–14 and reduced to a low flap afterwards. Notopodial prechaetal lamella absent throughout. Dorsal crests low, from chaetiger 8 to chaetiger 10– 15 (Fig. 8A).

Neuropodial postchaetal lamellae of chaetiger 2 well-developed, triangular and elongated ventrally, triangular and not elongated ventrally on chaetiger 3, rounded on chaetigers 4–11 and reduced to a low flap afterwards (Fig. 6D; 8A). Neuropodial prechaetal lamellae absent throughout.

Chaetae organized in two rows of sparsely granulated non-limbate capillaries (Fig. 9A). Towards the posterior region, capillaries progressively become elongate, thinner and less numerous (Fig. 9B).

Hooks in notopodia from chaetigers 32–44, up to three per fascicle, accompanied by 1–4 short, non-limbate capillaries. Hooks in neuropodia from chaetigers 11–12, up to seven per fascicle, accompanied by 1–4 non-limbate capillaries. Hooks multidentate, with 8 secondary teeth arranged in two rows above the main tooth (appearing as a single row of 4 secondary teeth in light microscopy) (Figs 7 A–B; 9D). Small secondary hood present (Fig. 9D). Hooks accompanied by 3–10 short non-limbate capillaries.

Non-limbate and sparsely granulated sabre chaetae consistently from chaetiger 10 (Fig. 9C).

Branchiae absent in all individuals (Figs 6 A–D; 7A; 8A–B). Pygidium with a pair of short rounded ventral cirri and a slightly longer mid-dorsal cirrus (Figs 6E; 7D).

Oocytes from chaetigers 10–11, measuring up to 80 µm.

Methyl green pattern: Intense staining on prostomium and peristomium.

Remarks: According to Radashevsky (2012), the late development of branchiae on anterior chaetigers is common in Prionospio, which could lead to the assumption that the species represents a juvenile stage. However, the complete absence of branchiae was observed in all 76 individuals, regardless of size (from 2.5 to 4.8 mm long; from 39 to 65 chaetigers) or sexual maturity.

Owing to the lack of branchiae, the placement of this species in any current genus is problematic, since branchial morphology and distribution are characters of great significance for spionid taxonomy (Foster 1969, 1971; Blake & Kudenov 1978; Johnson 1984; Maciolek 1985; Blake 1996; Bick 2005; Delgado-Blas 2009; Radashevsky 2012; Blake et al. 2017). For the Prionospio -complex, lack of branchiae is unusual, shared only with Aurospio abranchiata Neal, Paterson & Soto in Paterson et al., 2016, although its placement in Aurospio is questioned by Blake et al. (2017), stating that, recently, several Prionospio species have been erroneously attributed to Aurospio.

Among Brazilian species, Prionospio nonatoi sp. nov. is similar to P. cirrifera, P. delta Hartman, 1965, P. fauchaldi Maciolek, 1985 and P. multibranchiata in having only low dorsal crests and significant overlapping in starting chaetiger of notopodial and neuropodial hooded hooks. However, not only do these species possess branchiae, but P. cirrifera, P. delta, and P. multibranchiata present multiple pairs of smooth apinnate branchiae, while P. fauchaldi presents distinctly wrinkled branchiae on chaetigers 2–5. Even if branchiae are completely lost in these species, they can still be separated from P. nonatoi sp. nov. by prostomial shape.

As for Aurospio abranchiata, both species are similar in having an enlarged notopodial postchaetal lamellae on chaetiger 3, distribution of dorsal crests, lack of branchiae, starting chaetiger of sabre chaetae and hooded hooks and presence of a secondary hood on the hooks. However, they can be separated based on the shape of notopodial postchaetal lamellae from chaetigers 2–5, the shape of neuropodial postchaetal lamellae from chaetigers 2–4 and by bathymetrical distribution.

Etymology: The species name, nonatoi, is a tribute to Edmundo Ferraz Nonato (1920–2014), who dedicated his life to the study of marine worms and is considered the “father” of Brazilian polychaetology.

Habitat: Fine sand to muddy sand, at 144–153 m depth.

Distribution: Southeast Brazil (Espírito Santos and Campos Basins), and only found during the summer.