Nereis mariellae sp.nov.

(Figs 9, 10)

urn:lsid:zoobank.org:act: 468A65B4-7A7A-4ACA-8357- 0D6BA9626DDC

Type series: Holotype coll. 23 May 2015, continental margin off Rio de Janeiro state, Campos Basin, Brazil, 25°20ʹ18″S, 39°38ʹ28.3″W, 3285 m depth; holotype (MZUSP5357), incomplete, 28 cht, 17.4 mm long, 2.4 mm wide.

Description: Holotype atokous, with cylindrical body, slightly flattened dorsoventrally from mid-body. Alive specimen with white pale colour and visible red dorsal blood vessel (Fig. 9A). Fixed specimen with beige colour (Fig. 9B–E). Pigment spots randomly distributed on body and parapodia, still visible after fixation, with a pair of characteristic black spots dorsally on peristomium (Fig. 9A, B, E).

Prostomium slightly longer than wide, with one pair of frontal antennae and one pair of biarticulated palps (Fig. 9A, B, E). Antennae subulate, slightly longer than palps and curved laterally (Fig. 9A–C, E). Palpostyles round and palpophores cylindrical (Fig. 9C). Palps pointed ventrally (Fig. 9C). Eyes absent (Fig. 9A, B, E). Peristomium one-fifth shorter than prostomium and about twice the length of chaetiger 1; peristomium with two dorsal small pigment spots in the median region (Fig. 9A, B, E). Four pairs of tentacular cirri present; posterodorsal tentacular cirri reaching middle region of chaetiger 3; anterodorsal tentacular cirri the longest, reaching anterior region of chaetiger 4 (Fig. 9B). Tips of tentacular cirri with dark pigmentation.

Pharynx with numerous black, small and easily detachable paragnaths on both rings, with the following arrangement: area I = 6, mainly in a vertical line; area II = 27–30 on each side in two or three rows; area III = 26, in three or four rows in ellipse; area IV = 37–41 on each in a triangle shape; area V = 0; area VI = 9 on each side in a circle; and area VII–VIII = ~25, with one row presenting four larger paragnaths.

First two parapodia uniramous (Fig. 10A); all following parapodia biramous (Fig. 10B, C). Uniramous parapodia with dorsal cirri slightly longer than dorsal ligule. Dorsal ligule almost twice the length of the ventral ligule; both ligules stout, slightly rounded, ventral ligule rounded. Dorsal ligule two-fifths longer than acicular lobe and three-fifths longer than ventral ligule. Ventral cirri one-fifth longer than ventral ligule. Uniramous parapodia presenting small pigment spots (approximately four per parapodium), with one spot always presents at the ventral ligule (Fig. 10A).

Biramous parapodia with dorsal cirri twice the length of notopodial ligule. Notopodial ligule slightly longer than notopodial lobe on anterior parapodia (Fig. 10B) and 3/10 shorter than notopodial lobe on posterior parapodia (Fig. 10C); notopodial ligules pyriform, with globular base and tapering end, almost subdivided. Notopodial lobes one-fifth longer than neuropodial lobes on anterior parapodia (Fig. 10B) and slightly longer than neuropodial lobe on posterior parapodia (Fig. 10C); notopodial lobes with rounded tips (Fig. 10B) at anterior parapodia and tapering to digitiform at posterior parapodia (Fig. 10C), and neuropodial lobes with tapering end, slightly thinner on posterior parapodia (Fig. 10C). Neuropodial lobes two-fifths longer than neuropodial ligules; neuropodial ligules sub-conical, with rounded tips on anterior parapodia (Fig. 10B) and digitiform on posterior parapodia (Fig. 10C). Ventral cirri slightly longer than neuropodial ligule. Biramous parapodia also presenting small pigment spots (Fig. 9D), more randomly distributed than in uniramous parapodia.

Uniramous parapodia with supracicular chaetae (Fig. 10D) homogomph spinigers (Fig. 10I) and heterogomph falcigers (Fig. 10F), and with subacicular chaetae (Fig. 10E) heterogomph spinigers and falcigers.

Biramousparapodiawithnotochaetae(Fig.10G) homogomph spinigers. Blades of homogomph spinigers 1.5× the length of blades of heterogomph spinigers. Supracicular neurochaetae (Fig. 10H) homogomph spinigers and heterogomph falcigers. Subacicular neurochaetae (Fig. 10H) heterogomph spinigers and falcigers. Blades of heterogomph falcigers serrated and slightly thinner at anterior parapodia.

Pygidium and pygidial cirri not seen.

Variation: This species is described based on only one specimen; therefore, it is not possible to describe intraspecific variations. It is not ideal to describe a species based on a single specimen, but considering the scope of the present manuscript and the morphological and molecular data, in addition to the difficulty of collecting specimens from organic falls, we decided to describe the species.

Remarks: The species Nereis mariellae is described from a single eyeless specimen that was inhabiting whale bones. Nereis mariellae differs from most of the Nereis shallow-water species by the absence of eyes and the presence of small and fragile paragnaths, easily detachable from the pharynx. Nereis mariellae also differs from the other Nereis deep-sea species by a combination of morphological features and by the paragnath formula (Tables 3 and 4).

Nereis mariellae is more similar to the species Nereis eugeniae, Nereis tricirrata and Nereis anoculepitoka . Still, Nereis mariellae differs from Nereis eugeniae in the habitat (organic falls for Nereis mariellae and cobbles and shells for Nereis eugeniae) and the depth (3285 m for Nereis mariellae and 156 m for Nereis eugeniae) (Table 3). However, considering that only one Nereis mariellae specimen is available, the information about occurrence, habitat and size might change in the future. The number of paragnaths in each area of the pharynx also differs between both species, as seen in Table 4. Nereis mariellae differs from Nereis tricirrata in the habitat (organic falls for Nereis mariellae and soft sediment for Nereis tricirrata), in depth (3285 m for Nereis mariellae and 1766 m for Nereis eugeniae), and the species differ especially in paragnath numbers and position (Table 4), with Nereis tricirrata lacking paragnaths in areas I, III and V and presenting paragnaths in few numbers in the other areas, whereas Nereis mariellae lacks paragnaths only in area V and presents numerous paragnaths in the remnant areas (Table 4).

Nereis mariellae differs from Nereis anoculepitoka in parapodial morphology, with the neuropodial lobes and ligules of Nereis mariellae shorter than the notopodial ones throughout, whereas in Nereis anoculepitoka this pattern is observed in the anterior region, but the posterior neuropodial lobes and ligules are longer than the notopodial ones. Heterogomph falcigers of both species are also different, being longer and thinner in Nereis mariellae in comparison to Nereis anoculepitoka . The number and position of paragnaths of both species are also different, as shown in Table 4. Specimens from both species present pigment spots distributed along the body, but only some specimens of Nereis anoculepitoka present these spots, in lower numbers and more restricted to the parapodia, whereas the Nereis mariellae specimen presents larger and more numerous pigment spots distributed along the whole body (Fig. 9E), including double pigment spots on the peristomium (Fig. 9B).

The holotype of Nereis mariellae is incomplete, damaged on the posterior part of the body; therefore, homogomph falcigers were not found, because they are usually present in posterior notopodia. The presence of homogomph falcigers is an important morphological feature to differentiate Nereis from Neanthes, and the morphological differences on the blade of homogomph falcigers are important to differentiate species within Nereis. Still, this species shares most of the morphological characteristics with other Nereis species inhabiting organic falls described in the present manuscript, and the molecular analyses placed this species within a Nereis clade. Therefore, Nereis mariellae is considered a Nereis species, and the absence of homogomph falcigers is probably associated with the damage on the posterior body.

Etymology: The epithet ‘mariellae’ is a tribute to the councilwoman of the city of Rio de Janeiro, Marielle Franco, an important fighter for human rights, who was found to have been murdered in 2018.

Occurrence: Reported only in whale bones, in the transition between the continental slope and the abyssal zone at 3285 m depth, in the Campos Basin, SW Atlantic.