Prionospio acuta, Peixoto & Paiva, 2020

Peixoto, Antônio João Malafaia & Paiva, Paulo Cesar, 2020, New apinnate Prionospio (Annelida: Spionidae) species from southeastern Brazil, Zootaxa 4853 (4), pp. 451-508 : 457-461

Prionospio acuta sp. nov.

( Figures 2–4 )

Type material. Brazil. Espírito Santo Basin. Holotype: Amb 5 B5, 20° 35’ 15.16” S, 39° 53’ 46.36” W, 02 Dec 2011 to 02 Feb 2012, 991 m, MNRJP-2745. Paratypes: Amb3 E5, 19° 36’ 25.08” S, 39° 10’ 20.15” W, 02 Dec 2011 to 02 Feb 2012, 352 m, MNRJP-2746 (4 ind); Amb11 B5, 20° 35’ 13.87” S, 39° 53’ 45.78” W, 06 Jun 2013 to 17 Jul 2013, 382 m, MNRJP-2747 (4 ind) .

Additional material examined. Amb3 E5, 19º 36’ 25.08” S, 39º 10’ 20.15” W, 352m (7 ind); Amb3 F5, 19º 34’ 20.51” S, 38º 41’ 18.75” W, 438m (1 ind); Amb3 CAND5 , 19º 33’ 20.99” S, 39º 2’ 36.2” W, 374m (1 ind); Amb5 B5, 20º 35’ 15.16” S, 39º 53’ 46.36” W, 991m (4 ind); Amb5 B6, 20º 35’ 15.16” S, 39º 53’ 46.36” W, 1315m (1 ind); Amb5 C5, 20º 14’ 22.9” S, 39º 48’ 33.06” W, 416m (1 ind); Amb5 D6, 19º 50’ 1.87” S, 39º 26’ 30.04” W, 1053m (1 ind); Amb6 D5, 19º 46’ 31.83” S, 39º 30’ 3.38” W, 402m (2 ind); Amb7 F6, 20º 4’ 9.68” S, 38º 31’ 29.01” W, 1021m (1 ind); Amb11 A5, 21º 4’ 8.5” S, 40º 13’ 7.35” W, 383m (5 ind); Amb11 B5, 20º 35’ 13.87” S, 39º 53’ 45.78” W, 382m (8 ind); Amb11B7, 20º 36’ 49.74” S, 39º 49’ 29.01” W, 1327m (1 ind); Amb11 C5, 20º 14’ 19.79” S, 39º 48’ 36.57” W, 418m (11 ind); Amb12 D5, 19º 46’ 33.06” S, 39º 30’ 4.21” W, 431m (4 ind); Amb12 E5, 19º 36’ 26.63” S, 39º 10’ 19.31” W, 349 m (4 ind); Amb12 F5, 19º 34’ 20.47” S, 38º 41’ 19.8” W, 445m (2 ind); Amb12 CANWN5 , 19º 49’ 36.9” S, 39º 35’ 42.69” W, 363m (3 ind) .

Diagnostic features: Prostomium bearing two short antero-lateral projections, low dorsal crests from chaetigers 10–11 to chaetigers 15–17, sabre chaetae absent.

Description. A medium-sized Prionospio , largest specimen about 8 mm long, 0.35 mm wide at the widest part for 63 chaetigers (incomplete); holotype 7 mm long, 0.3 mm wide at the widest point for 49 chaetigers (incomplete). Body dorsoventrally flattened on the branchial region and cylindrical afterwards, tapering towards the pygidium. Body color yellow to whitish in alcohol ( Fig. 2 ).

Prostomium triangular, rounded anteriorly, bearing two short antero-lateral projections, extending as a narrow keel almost to the posterior margin of chaetiger 1, flanked by conspicuous nuchal organs extending up to the margin of chaetiger 1 ( Figs 2 ; 3A ). Prostomial peaks absent. Eyes absent. Peristomium surrounding prostomium and partially fused to the first chaetiger, low lateral wings present. Grooved palps reaching up to chaetiger 10, lost in most specimens.

Chaetiger 1 with only a few chaetae in both rami, shorter than chaetae on succeeding chaetigers. Postchaetal lamellae rounded on both rami, directed upwards in notopodium, smaller than lamellae on succeeding chaetigers, especially on neuropodium ( Figs 3A ; 4A ). Prechaetal lamellae absent.

Notopodial postchaetal lamellae foliaceous on chaetigers 2–15 ( Fig. 4 B–D), rounded with a pointed tip directed upwards from chaetiger 16 to chaetigers 20–21 (size-dependent) and rounded afterwards, gradually reducing in size towards posterior region, present as a low flap on the last chaetigers. Notopodial prechaetal lamellae absent throughout ( Fig. 3B ). Dorsal crests low, from chaetigers 10–11 to chaetigers 15–17 ( Fig. 3B ). Ciliary bands present on branchiate chaetigers ( Fig. 3A ).

Neuropodial postchaetal lamellae elongated on chaetigers 2–15, bearing a rounded tip directed downwards ( Fig. 4 B–D), from chaetiger 16 to chaetigers 18–20 (depending on specimen size), gradually becoming rounder and reduced in size towards posterior region, present as a low flap on last chaetigers. Neuropodial prechaetal lamellae absent throughout.

Chaetae from notopodia and neuropodia organized in two rows of unilimbate (limbation almost imperceptible) and intensely granulated capillaries ( Fig. 4E ). Chaetae from both rows of equal length, although neuropodial chaetae slightly shorter than notopodial chaetae. Towards posterior region, capillaries become progressively elongate, non-limbate, thinner, and less numerous ( Fig. 4F ).

Hooks in notopodia starting from chaetigers 24–42, up to two per fascicle, accompanied by row of 1–4 short non-limbate capillaries ( Fig. 4G ). Hooks in neuropodia starting from chaetigers 13–24, up to five per fascicle, accompanied by 3–6 non-limbate capillaries. All hooks multidentate, with six small secondary teeth arranged in two rows above the main tooth ( Fig. 4H ). Secondary hood absent ( Fig. 4H ). Sabre chaetae absent throughout.

Up to 11 pairs of smooth, robust and flattened branchiae, starting from chaetiger 2, up to 2.5 times longer than notopodial postchaetal lamellae, gradually reducing in length towards last branchial pair. Branchiae moderately ciliated towards the tip, completely free from notopodial postchaetal lamellae ( Fig. 4I ).

Pygidium of unknown morphology.

Oocytes from chaetiger 13, measuring up to 120 µm.

Methyl green staining pattern: No specific pattern observed; light green pigment diffused throughout the body, except for anterior prostomial margin and branchial tips.

Remarks. The prostomial shape of Prionospio acuta sp. nov., bearing antero-lateral projections, is uncommon among Prionospio species, but similar to the prostomium observed in specimens of Spiogalea Aguirrezabalaga & Ceberio, 2005 (although lacking chitinous plates). The observed prostomium forms antero-lateral projections rather than typical antero-lateral horns, as found in the genera Malacoceros Quatrefages, 1843 , Rhynchospio Hartman, 1936 , Spiophanes Grube, 1860 and Scolecolepides Ehlers, 1907 .

Among Prionospio species, only Prionospio cornuta Hylleberg & Nateewathana, 1991 , described from Thailand and P. paradisea Imajima, 1990a , described from Japan also bear antero-lateral projections, although both species bear sabre chaetae (lacking in Prionospio acuta sp. nov.) and only four pairs of branchiae, which are pinnate on chaetigers 2 and 5. In addition, the antero-lateral projections (which the authors referred to as horns) of P. cornuta are actually short and rounded ( Hylleberg & Nateewathana 1991, Fig. 5 K–N). Prostomial horns are known from a single Prionospio species, P. cerastae Radashevsky, 2015 , a species that also lacks sabre chaetae, although the species possess only four pairs of branchiae, with the last pair bearing pinnules.

The lack of sabre chaetae is uncommon among Prionospio species, being observed only in Prionospio perkinsi , P. fosterae sp. nov., P. cerastae , described from Lizard Island, Australia, and P. hermesia Neal & Paterson in Paterson et al., 2016 , a deep-sea species, described from Portugal. Prionospio acuta sp. nov. is most similar to P. perkinsi and P. fosterae sp. nov. in having a similar number of branchiae (10 branchial pairs in P. perkinsi and 11 pairs in P. fosterae sp. nov. and P. acuta sp. nov.), but different morphology, as P. acuta sp. nov. bears robust flattened branchiae whereas P. perkinsi and P. fosterae sp. nov. bear cirriform branchiae. Additionally, P. acuta sp. nov. bears dorsal crests, present from chaetigers 10–11 to chaetigers 15–17, which are absent in P. fosterae sp. nov. and P. perkinsi .

Prionospio acuta sp. nov. is similar to Prionospio cerastae , which is the only known Prionospio species to bear antero-lateral horns. These species can be distinguished by the lack of dorsal crests in P. cerastae and the branchial morphology and distribution, as this species possesses only four pairs of branchia, the last pair bearing digitiform pinnules, whereas P. acuta sp. nov. bears up to 11 pairs of apinnate branchiae.

Etymology. The specific epithet, acuta , refers to the antero-lateral projections on the prostomium ( acuta, Latin for sharp or pointed).

Habitat: Muddy sand to mud, 349–1327 m depth.

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

Figures

FIGURE 2. Prionospio acuta sp. nov., anterior end and mid-body chaetigers, dorsal view (MNRJP-2745, holotype), showing characteristic color in ethanol. Abbreviations: pe, peristomium; pr, prostomium.

FIGURE 3. SEM of Prionospio acuta sp. nov., dorsal view. A. Anterior end. B. Mid-body chaetigers. Abbreviations: br, branchia; dc, dorsal crest; pe, peristomium; pr, prostomium.

FIGURE 4. Prionospio acuta sp. nov. A. Chaetiger 1. B. Chaetiger 2. C. Chaetiger 3. D. Chaetiger 4. E. Anterior capillary chaetae. F. Posterior capillary chaetae. G. Hooded hook. H. Companion capillary chaetae. I. Branchia.

FIGURE 5. Prionospio fosterae sp. nov., anterior end and mid-body chaetigers, dorsal view (MNRJP-2748, holotype), showing characteristic color in ethanol.Abbreviations: br, branchia; pe, peristomium; pr, prostomium.