Melinnopsis nathanieli sp. nov.

Type material.

Holotype: • One specimen; NHMUK ANEA 2025.3262, incomplete, end of abdomen missing; Atlantic, Porcupine Abyssal Plain Sustained Observatory; OTSB 14; Start 49°05.43'N, 016°53.02'W, End 49°01.12'N, 016°57.87'W; depth 4843–4848 m; 17/05/23–18/05/23; RRS James Cook Cruise 247, Station 056; COI PX 149846, 16 S PX 169426. Paratypes: Total six specimens. • NHMUK ANEA 2025.3263; NHMUK ANEA 2025.3264, complete specimen; Atlantic, Porcupine Abyssal Sustained Observatory; OTSB 14; Start 48°51.21'N, 016°51.45'W, End 48°57.45'N, 016°51.85'W; depth 4831–4838 m; 07/06/24–08/06/24; RRS James Cook Cruise 263, Station 071. • NHMUK ANEA 2025.3265, buccal tentacle extended 10 mm; NHMUK ANEA 2025.3266; NHMUK ANEA 2025.3267 (SEM specimen); NHMUK ANEA 2025.3268 (SEM specimen), complete specimen broken in two fragments; Atlantic, Porcupine Abyssal Plain Sustained Observatory; OTSB 14; Start 48°54.03'N, 016°49.26'W, End 49°00.70'N, 016°50.22'W; depth 4835–4836 m; 06/06/24–07/06/24; RRS James Cook Cruise 263, Station 069.

Other material examined.

Total 35 specimens. • DISCOLL -JC 247-056-POLY-003; DISCOLL -JC 247-056-POLY-004; DISCOLL -JC 247-056-POLY-005; DISCOLL -JC 247-056-POLY-006 (COI PX 149845; 16 S PX 169425; 18 S PX 169420); DISCOLL -JC 247-056-POLY-007; DISCOLL -JC 247-056-POLY-008; DISCOLL -JC 247-056-POLY-009 (COI PX 149844; 16 S PX 169424; 18 S PX 169419); DISCOLL -JC 247-056-POLY-010; DISCOLL -JC 247-056-POLY-011 (COI PX 149843; 16 S PX 169423; 18 S PX 169418); DISCOLL -JC 247-056-POLY-012; DISCOLL -JC 247-056-POLY-013 (COI PX 149842; 18 S PX 169417); DISCOLL -JC 247-056-POLY-014; DISCOLL -JC 247-056-POLY-015 (COI PX 149841; 16 S PX 169422; 18 S PX 169416); DISCOLL -JC 247-056-POLY-016; DISCOLL -JC 247-056-POLY-018; DISCOLL -JC 247-056-POLY-019; DISCOLL -JC 247-056-POLY-020; DISCOLL -JC 247-056-POLY-021; DISCOLL -JC 247-056-POLY-022; DISCOLL -JC 247-056-POLY-023; DISCOLL -JC 247-056-POLY-024; DISCOLL -JC 247-056-POLY-025; all same collection data as for holotype. • DISCOLL -JC 247-051-POLY-039; Atlantic, Porcupine Abyssal Plain Sustained Observatory; OTSB 14; Start 49°02.63'N, 016°56.95'W, End 48°58.10'N, 016°57.81'W; depth 4844–4846 m; 16/05/23-15/05/23; RRS James Cook Cruise 247, Station 051. • DISCOLL -JC 263-069-POLY-033; DISCOLL -JC 263-069-POLY-034; DISCOLL -JC 263-069-POLY-035; DISCOLL -JC 263-069-POLY-036; DISCOLL -JC 263-069-POLY-037; Atlantic, Porcupine Abyssal Plain Sustained Observatory; OTSB 14; Start 48°54.03'N, 016°49.26'W, End 49°00.70'N, 016°50.22'W; depth 4835–4836 m; 06/06/24–07/06/24; RRS James Cook Cruise 263, Station 069. • DISCOLL -JC 263-071-POLY-023, complete specimen (5 branchiae); DISCOLL -JC 263-071-POLY-024 (16 S PX 16942); DISCOLL -JC 263-071-POLY-025, 2 fragments; DISCOLL -JC 263-071-POLY-027; DISCOLL -JC 263-071-POLY-028; DISCOLL -JC 263-071-POLY-029; DISCOLL -JC 263-071-POLY-030; Atlantic, Porcupine Abyssal Plain Sustained Observatory; OTSB 14; Start 48°51.21'N, 016°51.45'W, End 48°57.45'N, 016°51.85'W; depth 4831–4838 m; 07/06/24–08/06/24; RRS James Cook Cruise 263, Station 071.

Additional comparative material.

BMNH 1885.12.1.330, holotype of Melinnopsis atlantica McIntosh, 1885, off Chesapeake Bay, North America, 37°25.002'N, 71°40.002'W, HMS Challenger, Stn. 44, 1700 fathoms (3109 m), 02 / 05 / 1873, 2 short anterior fragments, 1 posterior fragment and ~ 9 tube fragments.

Description.

(based on holotype NHMUK ANEA 2025.3262) Holotype 35 mm length for more than 25 chaetigers, widest at post-branchial region 2 mm, thereafter gradually tapering to abdomen (1 mm width) (Fig. 3 A). Thorax with 16 or 17 chaetigers (lower thorax damaged so exact number of chaetigers uncertain). Neurochaetae as small acicular spines on segments II – V and uncini on remaining> 21 chaetigers.

Prostomium with well-defined anterior and posterior sections separated by a pair of deep transverse nuchal slits meeting mid-dorsally (Fig. 3 D). Anterior part of prostomium whole, without any distinct lobes, and with a slightly raised lip. No eyespots or pigmented glandular bands present. No speckled pigment on anterior part of prostomium. Segment I continued ventrally forming lower margin of mouth with no crenulations on the ventral side (Fig. 3 B).

Buccal tentacles in holotype missing, only one large stump remaining.

Lateral wings of anterior body between prostomium and segment V slightly arched, peak approx. segment IV (Figs 3 E, 4 B).

Segment I collar-like, laterally and ventrally encompassing head region. Branchiae emerging together on dorsal branchial ridge at level of segment II, arranged in two basally fused groups of four, three branchiae in front and one situated slightly behind (towards the anterior) (Figs 4 A, 5 A), the latter being the longest pair. Inner- and anteriormost branchia of each group completely separate, not joined by membrane. Branchiae in cross-section circular to slightly flattened smooth with central groove, gently tapering to filiform tips. Post-branchial dorsal membrane inconspicuous (Fig. 5 A).

Post-branchial hooks absent. Segmentation visible dorsally in post-branchial area. No visible nephridial papillae.

Notochaetae from segment IV, neurochaetae from segment II (Figs 4 B, 5 B).

Capillary notochaetae starting from segment IV present in 12 or 13 thoracic chaetigers (lower thorax damaged exact number of chaetigers uncertain). Chaetiger 3 (segment IV) with few fine notochaetal capillaries and chaetiger 4 (segment V) with more abundant fine notochaeta arising from small slightly projecting notopodia (Fig. 5 B). Short cylindrical notopodia with thicker capillaries evident from chaetiger 5. Notochaetae arranged in double rows, those of anterior rows shorter.

Rudimental abdominal notopodia missing. No small, rounded projections, evident in notopodial positions, no cilia observed.

Neurochaetae as small acicular spines with lanceolate tips on segments II – V. Neuropodial uncini from chaetiger 5 (segment VI) (Fig. 5 B), present in 12 thoracic uncinigers and abdominal chaetigers. Holotype incomplete with ~ 15 abdominal uncinigers.

Thoracic uncini emerging subdistally on short flaps from chaetigers 5 to approximately chaetiger 11 (Figs 3 F, 5 C). Abdominal uncini mostly missing in holotype but arranged on narrow lappets (Fig. 5 E).

Uncini of thoracic uncinigers with three teeth (two small, one larger) in one vertical row over rostral tooth, subrostral process and basal prow (Figs 3 G, 5 D).

Thoracic uncini in a single line with ~ 70 uncini (Fig. 5 C).

Abdominal uncini in a single line with ~ 16 uncini (Fig. 5 E).

Pygidium missing in holotype.

Methyl blue staining pattern. Use of methyl blue in holotype reveals weak staining of prostomium. Posterior section of prostomium with speckled staining (Fig. 3 B). Strong staining transversely on segments I – IV (Fig. 3 A). Branchiae very lightly speckled. No clear lateral staining. Strong staining of ~ 13 or 14 ventral shields, staining strong in anterior section of ventral shield, light staining of posterior section of shield. This pattern is more defined towards the posterior of the worm. Shields do not cover entire ventral surface of the segment, stopping short at the uncini (Fig. 3 B).

Tube. Missing in holotype.

Variations.

Variation in other specimens up to 48 mm length for more than 60 chaetigers, widest at post-branchial region 3 mm, thereafter gradually tapering to abdomen (1 mm width) and pygidium. Thorax with 16 chaetigers; neurochaetae as small acicular spines on segments II – V and uncini on remaining> 56 chaetigers.

One large buccal tentacle (≤ 15 mm), edges of tentacle undulate in small folds (Fig. 3 C). Four smaller tentacles (2 pairs) ridged and smooth measuring ~ 1 mm length.

Branchiae ~ 1 / 5 the length of longest buccal tentacle.

Thirteen thoracic uncini from chaetiger 5–17. Some specimens with ≤ 49 abdominal uncinigers.

Uncini lappets decreasing in size until pygidium, minute at the end. First abdominal neuropodia widely separated, more closely spaced towards the pygidium, last seven neuropodia very close together.

Uncini of abdominal uncinigers with numerous teeth over rostral tooth, subrostral process and basal prow (Fig. 5 F).

Terminal crenulated anus, bounded by four, small indistinct lobes. No anal cirri (Fig. 5 G).

Methyl blue staining pattern: staining of post-branchial region laterally (mid-dorsal region not stained) until chaetiger 8. Staining of 13 ventral shields. Abdominal staining light. No staining of dorsal side of neuropodial lappets.

Tube: fine-grained sediment tube with some foraminifera encrusted. The tube is lined with a thin, clear membrane. Length of tube ≤ 30 cm (Fig. 6 B).

Ecology.

Found in association with the actiniarians Actinauge abyssorum Carlgren, 1934 and Amphianthus bathybium Hertwig, 1882, and the ascidian Culeolus sp. Herdman, 1881 (Fig. 6 A, B, C). The worm tubes are grasped by the pedal disc of the Actiniaria (Fig. 6 A, C).

Distribution.

Porcupine Abyssal Plain, northeast Atlantic, 4843–4850 m.

Etymology.

The new species is named for the second author’s son, Nathaniel Serpell-Stevens.

Remarks.

Melinnopsis nathanieli sp. nov. is morphologically very similar to M. chadwicki and M. gardelli from the eastern Australian margin. Melinnopsis nathanieli sp. nov. differs from these two species in having lateral wings that are slightly arched rather than highly arched, and possessing fewer abdominal uncini: only 16 in a row in M. nathanieli sp. nov. versus 30 in both M. gardelli and M. chadwicki. Melinnopsis gardelli also exhibits a ‘ conspicuous stained band on the dorsal area when stained with methyl blue’ which was not observed in M. nathanieli sp. nov. Two other species of Melinnopsis have been described from the Atlantic (Table 4), M. angolensis and the type species, M. atlantica. Melinnopsis nathanieli sp. nov. differs from M. angolensis in lacking a dorsal serrated membrane on segment V (present in M. angolensis) and the possession of three teeth above the rostral tooth in the thoracic uncini (two teeth in M. angolensis).

Melinnopsis nathanieli sp. nov. differs from M. atlantica in lacking a dorsal membrane between segments IV and V (present in M. atlantica) and possessing a flexible tube composed of fine sediment with few attached foraminifera (cylindrical tube rigid and composed of greyish sediment with many foraminifera attached in M. atlantica). Furthermore, the depth distribution differs between the two species, M. atlantica was recovered from 1700 fms (3109 m), whereas M. nathanieli sp. nov. was collected from 4843–4850 m.

Since its description in 1885 there has only been a single additional report of M. atlantica (OBIS 2025), a specimen collected from the Golfe de Gascogne / Bay of Biscay, northeast Atlantic in 1973 at 1180 m (Olu et al. 2025). It is unclear whether this specimen is M. atlantica or potentially another new species. We suggest that due to the differences in depth and geographical location, it is likely to be a different species and M. atlantica is not distributed in the northeast Atlantic

Unfortunately, our holotype specimen was incomplete with the end of the abdomen missing. Most of the specimens collected in 2023 and 2024, which were fixed in ethanol and suitable for genetic analysis, were in poor condition, often missing the posterior end. We selected the holotype based on condition of the specimen and availability of genetic data.