Prionospio fosterae Peixoto & Paiva 2020, sp. nov.

Prionospio fosterae sp. nov.

(Figures 5–7)

Prionospio perkinsi: Pardo & Peixoto (in press). Not Maciolek, 1985.

Prionospio lighti: Pardo & Peixoto (in press). Not Maciolek, 1985.

Type material. Brazil. Espírito Santo Basin. Holotype: Amb 7 D1, 19° 35’ 37.21” S, 39° 41’ 19.68” W, 02 Dec 2011 to 02 Feb 2012, 21 m, MNRJP -2748. Paratypes: Amb3 F5, 19° 34’ 21.01” S, 38° 41’ 16.99” W, 02 Dec 2011 to 02 Feb 2012, 438 m, MNRJP-2749 (4 ind).

Additional material examined. Amb2 Foz5, 19º 32’ 28.16” S, 39º 41’ 35.15” W, 11m (4 ind); Amb2 Foz12, 19º 52’ 32.45” S, 39º 49’ 8.3” W, 41m (1 ind); Amb3 E5, 19º 36’ 26.24” S, 39º 10’ 17.35” W, 352m (3 ind); Amb3 F5, 19º 34’ 21.01” S, 38º 41’ 16.99” W, 438m (7 ind); Amb6 D4, 19º 45’ 55.39” S, 39º 30’ 25.74” W, 121m (3 ind); Amb7 D1, 19º 35’ 37.21” S, 39º 41’ 19.68” W, 21m (3 ind); Amb7 F3, 18º 53’ 31.83” S, 39º 6’ 22.1” W, 43m (3 ind); Amb11 B5, 20º 35’ 13.87” S, 39º 53’ 45.78” W, 382m (2 ind); Amb12 E4, 19º 36’ 4.74” S, 39º 10’ 34.59” W, 142m (3 ind); Amb12 E5, 19º 36’ 26.63” S, 39º 10’ 19.31” W, 349m (1 ind); Amb12 CANWN4, 19º 49’ 7.49” S, 39º 36’ 9.44” W, 181 m (21 ind); Amb14 D1, 19º 35’ 35.96” S, 39º 41’ 20.61” W, 18m (5 ind); Amb14 F3, 18º 53’ 31.97” S, 39º 6’ 21.78” W, 43m (3 ind).

Diagnostic features: Tridentate hooded hooks, dorsal crests absent, and sabre chaetae absent.

Description. A medium-sized Prionospio, largest complete specimen about 8 mm long, 0.3 mm wide at widest part for 60 chaetigers; holotype complete, 7.5 mm long, 0.3 mm wide at widest part for 56 chaetigers. Body dorsoventrally flattened throughout, tapering towards pygidium. Body color whitish to light yellow in alcohol (Fig. 5).

Prostomium rounded anteriorly, slightly widened towards anterior margin and narrowing towards posterior margin, extending posteriorly as a narrow keel reaching the anterior margin of chaetiger 2. Nuchal organs reaching posterior margin of chaetiger 1 (Fig. 6 A–B). Prostomial peaks absent. One or two pairs of minute reddish eyes usually present. Peristomium surrounding prostomium, partially fused to chaetiger 1. Low lateral wings present. Palps lost in all specimens.

Chaetiger 1 with only a few chaetae in both rami, especially on the notopodium, shorter than chaetae on succeeding chaetigers. Postchaetal lamellae oval on both rami, much smaller than lamellae on succeeding chaetigers (Fig. 7A). Prechaetal lamellae absent.

Well-developed foliaceous notopodial postchaetal lamellae on chaetigers 2–9 (Fig. 7 B–D), square-shaped and drastically reducing in size towards posterior region (Fig. 6A, D). Notopodial prechaetal lamella absent throughout. Dorsal crests absent. Ciliary bands present on branchiate chaetigers (Fig. 6 A–B).

Neuropodial postchaetal lamellae elliptical in branchial region; from chaetiger 2 to chaetigers 8–12 (size-dependent) (Fig. 7 B–D), drastically reduced afterwards, present as a low flap on last chaetigers. Neuropodial prechaetal lamellae absent throughout.

Chaetae from notopodia and neuropodia organized in two rows, anterior row of short, robust and unilimbate capillaries and posterior row of long (up to two times longer than anterior row), thin, narrowly unilimbate and granulated capillaries (Fig. 7 E–F). Towards posterior region, chaetae of both rows become progressively elongate, non-limbate, thinner, and less numerous (Fig. 7G).

Hooks in notopodia starting from chaetigers 20–23, up to five per fascicle, accompanied by 1–5 short non-limbate capillaries (Fig. 7H). Hooks in neuropodia starting from chaetigers 13–16, up to nine per fascicle, accompanied by 3–7 non-limbate capillaries. All hooks tridentate, with a single pair of teeth arranged side by side above main tooth (Figs 6C, 7I). Small secondary hood present (Fig. 7I). Sabre chaetae absent throughout.

Up to 11 pairs of smooth and cirriform branchiae, tapered distally. Branchiae starting from chaetiger 2, up to four times longer than notopodial lamellae, gradually reduced in length towards last branchial pair. Branchiae densely ciliated throughout its length, completely free from notopodial postchaetal lamellae (Fig. 7J).

Pygidium bearing one long dorsal cirrus and two short ventral cirri (Fig. 7K).

Gametes not observed.

Methyl green staining pattern: Anterior margin of the prostomium and ventral side of the peristomium intensely stained; postchaetal lamellae slightly stained throughout the body.

Remarks. Prionospio fosterae sp. nov. is unusual among Prionospio species for lacking sabre chaetae—a character shared only with P. acuta sp. nov. (described above), P. alexandrae sp. nov. and Prionospio perkinsi, in addition to P. cerastae and P. hermesia, which have never been recorded in Brazil (Maciolek 1985; Paterson et al. 2016).

Prionospio fosterae sp. nov. is easily distinguished from P. acuta sp. nov. by the lack of antero-lateral prostomial projections, the morphology of the hooded hooks (one pair of secondary teeth in P. fosterae sp. nov. and three pairs in P. acuta sp. nov.), branchial morphology (branchiae up to 2.5 times longer than notopodial lamellae in P. acuta sp. nov. and up to four times longer in P. fosterae sp. nov.), and the absence of dorsal crests in P. fosterae sp. nov., present from chaetigers 10–11 to chaetigers 15–17 in P. acuta sp. nov.

The lack of sabre chaetae and the presence of tridentate hooded hooks of P. fosterae sp. nov. makes it similar to P. hermesia (which, according to the authors, bears bidentate, tridentate, and possibly multidentate hooded hooks), although both species can be distinguished by branchial morphology and arrangement (up to 11 pairs of flattened branchiae in P. fosterae sp. nov. and two pairs of cirriform branchiae in P. hermesia), and the absence of dorsal crests in P. fosterae sp. nov.

The delicate sabre chaetae of P. fauchaldi and P. corrugata sp. nov. could lead researchers to conclude that these species lack sabre chaetae (which are indeed absent in P. fosterae sp. nov.), but both species can be distinguished from P. fosterae sp. nov. by the presence of long neuropodial chaetae in the posterior row of chaetiger 3, by the prostomial shape (narrow in P. fosterae sp. nov. and rectangular in P. fauchaldi and P. corrugata sp. nov.), and branchial morphology (up to 11 pairs of smooth flattened branchiae in P. fosterae sp. nov.; four pairs of branchiae, sculptured on chaetigers 2 and 5 in P. fauchaldi; and six pairs of branchiae, sculptured on chaetiger 2 and chaetigers 5–7 in P. corrugata sp. nov.). Sabre chaetae are supposedly lacking in P. kaplani Altamira, Glover & Paterson in Paterson et al. 2016, described from Cape Verde Abyssal Plain and P. branchilucida Altamira, Glover & Paterson in Paterson et al., 2016, although the authors only examined anterior fragments reaching 20 chaetigers at most.

Prionospio fosterae sp. nov. is remarkably similar to P. perkinsi in the prostomial shape (rounded anteriorly), absence of sabre chaetae, absence of dorsal crests, and branchial morphology and distribution (up to 11 pairs in P. fosterae sp. nov. and up to 10 pairs in P. perkinsi, apinnate and cirriform in both species). The starting chaetiger of notopodial and neuropodial hooded hooks also overlaps (Prionospio fosterae sp. nov.: chaetigers 20–23 [notopodial hooks] and 13–16 [neuropodial hooks]; Prionospio perkinsi: 16–23 [notopodial hooks] and 13–18 [neuropodial hooks]). Due to those similarities, P. perkinsi has been mistakenly recorded from the Brazilian coast (Amaral et al. 2013; Pardo & Peixoto in press). However, P. fosterae sp. nov. can be distinguished from P. perkinsi by the absence of prostomial peaks, shape of notopodial postchaetal lamellae on post-branchial chaetigers (square-shaped in P. fosterae sp. nov. and triangular in P. perkinsi), shape of neuropodial postchaetal lamellae (elliptical in P. fosterae sp. nov. and square-shaped in P. perkinsi), and number of secondary teeth above the main tooth (one pair in P. fosterae sp. nov. and two pairs in P. perkinsi).

Prionospio fosterae sp. nov. is also similar to P. lighti, as both species possess an anteriorly rounded prostomium, branchiae of similar distribution (up to 11 pairs in P. fosterae sp. nov. and up to 12 pairs in P. lighti), an overlap on the starting chaetiger of neuropodial hooded hooks (chaetigers 13–16 in P. fosterae sp. nov. and chaetigers 13–18 in P. lighti), and the absence of dorsal crests. Those similarities probably led to Brazilian records of P. lighti (Amaral et al. 2013), a species originally described from the Pacific coast of the USA. Despite the similarities, the two species can be separated based on the absence of prostomial peaks and sabre chaetae in P. fosterae sp. nov., starting chaetiger of notopodial hooded hooks (chaetigers 20–23 in P. fosterae sp. nov. and chaetigers 25–40 in P. lighti), and number of secondary teeth on the hooded hooks (one pair in P. fosterae sp. nov. and three pairs in P. lighti).

The occurrence of tridentate hooded hooks is unusual among Prionospio species, documented only in Prionospio tridentata Blake & Kudenov, 1978, described from Australia and P. fosterae sp. nov., but the two species differ in the prostomial morphology (rounded anteriorly in P. fosterae sp. nov. and trapezoidal in P. tridentata), branchial morphology and arrangement (up to 11 pairs of apinnate cirriform branchiae in P. fosterae sp. nov. and four pairs of branchiae, the last pair being pinnate in P. tridentata), and in the presence of sabre chaetae in P. tridentata. In some species, the hooded hooks may look tridentate, as in P. hermesia (which may bear tridentate hooks among bidentate and multidentate hooded hooks) and P. cerastae, but a closer inspection or SEM images may reveal the actual number of secondary teeth and their arrangement.

Etymology. The specific epithet, fosterae, is a tribute to Nancy M. Foster (1941–2000), a polychaetologist who erected the (now synonymized) genus Minuspio (Foster, 1971), which included Prionospio species bearing only smooth branchiae.

Habitat: Muddy sand to mud, 11–438 m depth.

Distribution: Southeastern Brazil (Espírito Santo and Campos basins), South Atlantic Ocean.