Macellicephala clarionensis, Bonifácio & Menot, 2019

Bonifácio, Paulo & Menot, Lénaïck, 2019, New genera and species from the Equatorial Pacific provide phylogenetic insights into deep-sea Polynoidae (Annelida), Zoological Journal of the Linnean Society 185, pp. 555-635 : 602-604

MACELLICEPHALA CLARIONENSIS SP. NOV.

( FIG. 14A–H; TABLES 1, 2, 4)

Type material: Holotype, MNHN-IA-TYPE 1828 ( IFR633-1 ), complete, length 4.48 mm, width 0.85 mm, 18 segments (including tentacular segment), Equatorial Eastern Pacific Ocean , Clarion-Clipperton Fracture Zone, APEI#3 seamount, station 212, collected 24 April 2015, ROV Kiel 6000, biobox, start 18°32.83′N, 128°44.88′W, end 18°32.57′N, 128°44.93′W, 1853– 1713 m depth . Paratype 1, MNHN-IA-TYPE 1829 ( IFR633- 2 ), complete, length 4.03 mm, width 0.74 mm, 18 segments (including tentacular segment), Equatorial Eastern Pacific Ocean , Clarion-Clipperton Fracture Zone, APEI#3 seamount, station 212, collected 24 April 2015, ROV Kiel 6000, biobox, start 18°32.83′N, 128°44.88′W, end 18°32.57′N, 128°44.93′W, 1853– 1713 m depth . Paratype 2, NHMUK 2018.25350 ( IFR633-3 ), complete, length 3.67 mm, width 0.80 mm, 18 segments (including tentacular segment), Equatorial Eastern Pacific Ocean , Clarion-Clipperton Fracture Zone , APEI#3 seamount, station 212, collected 24 April 2015, ROV Kiel 6000, biobox, start 18°32.83′N, 128°44.88′W, end 18°32.57′N, 128°44.93′W, 1853– 1713 m depth .

Description (based on holotype and paratypes): Holotype complete, 4.48 mm long and 0.85 mm wide for 18 segments (including tentacular segment), dorsoventrally flattened, not tapering posteriorly; live specimen slightly translucent, bluish; ethanolpreserved specimen pale; nephridial papillae whitish. Body surface smooth.

Prostomium bilobed, about as wide as long, lobes well pronounced, anteriorly truncated; frontal filaments present, very small, inserted at innermost margin of the lobes; median notch between prostomial lobes very narrow and deep ( Fig. 14A); eyes absent. Median antenna present, lateral antennae absent; ceratophore of median antenna large, cylindrical, short (much shorter than anterior margin of prostomial lobes), inserted medially on prostomium, near median notch, style missing. Palps smooth, tapering, short (reaching to segment 5; Fig. 14A, B). Trilobed facial tubercle present, median tubercle not pronounced ( Fig. 14B).

Tentacular segment with short lobe, inserted laterally and slightly ventral to prostomium; without acicula or chaetae; tentaculophores large, cylindrical, equal sized ( Fig. 14A); tentacular styles missing in holotype; in paratype (MNHN-IA-TYPE 1829) dorsal tentacular style missing, ventral tentacular style smooth, tapering, short (reaching to segment 4). Pharynx not everted in holotype; dissected in paratype (NHMUK 2018.25350), with pharyngeal papillae not possible to count; two pairs of jaws, each with main fang, margin smooth ( Fig. 14C). Second segment with elytrophores, subbiramous parapodia, chaetae and ventral cirri.

Nine pairs of prominent, bulbous elytrophores present on segments 2, 4, 5, 7, 9, 11, 13, 15 and 17 (all elytra missing).

Cirrigerous segment with prominent, bulbous cirrophore ( Fig. 14D), inserted subdistally on notopodia; style smooth, tapering, long (longer than tip of neuroacicular lobe); dorsal tubercles distinct, subconical to oval.

Ventral cirri smooth, tapering, present from segment 2 to last segment; inserted basally on neuropodia of segment 2, style long (longer than tip of neuroacicular lobe); in subsequent segments inserted medially on neuropodia, styles very short (shorter than tip of neuroacicular lobe).

Parapodia subbiramous; notopodia shorter than neuropodia ( Fig. 14D). Notopodia subtriangular, tapering into long acicular lobe, tip of notoacicula not penetrating epidermis. Neuropodia large, rectangular to subtriangular, tapering into long acicular lobe, tip of neuroacicula not penetrating epidermis; post-chaetal lobe slightly fleshy. Notochaetae moderate in number (19–25 observed), short to long, distally slightly curved, with very faint spinous rows on convex side, with pointed to blunt tips ( Fig. 14E); notochaetae stouter than neurochaetae. Neurochaetae very numerous (45–60 observed), short to very long, very narrow, distally flattened to concave, with very faint spinous rows of spines on both margins ( Fig. 14F, H), with minute bidentate hooked tips ( Fig. 14G).

Nephridial papillae on segments 10, 11 and 12, enlarged, rounded. Last segment very reduced; notoacicula and neuroacicula about the same size. Pygidium bulbous, not enclosed by last segment; with dorsal anus. Anal cirri lost, scars not seen.

Morphological variation: All specimens show great similarities in the shape of the prostomium, form of the chaetae and size of the neuro- and notoacicula on the last segment. However, the paratype has slightly longer frontal filaments.

Remarks: Macellicephala clarionensis sp. nov. is very close to Macellicephala parvafauces sp. nov., with both having truncated prostomial lobes and very faint spinous rows on the notochaetae. However, in Macellicephala clarionensis sp. nov. the median notch between the prostomial lobes is very narrow, the median facial tubercle is not so pronounced, and the neurochaetae present minute bidentate tips, whereas Macellicephala parvafauces sp. nov. has a moderately wide median notch, a well-developed median facial tubercle and neurochaetae with hooked tips. Macellicephala clarionensis sp. nov. is the only species within Macellicephala having minute bidentate tips ( Table 4). The average K2P distance between the two new species was high (26.9% for COI and 25.8% for 16S).

Etymology: The term ‘ clarionensis ’ refers to the area of distribution; APEI#3 is crossed by the Clarion Fracture. During the SO239 cruise, the area was sampled for the first time ever.

Genetic data: DNA sequencing for this species was successful for COI, 16S and 18S. Specimens shared ≥ 99.3% of genetic material in 16S and 100% of genetic material in COI or 18S. The average K2P distance for intraspecific variation was 0.0% for COI and 0.6% for 16S.

Distribution: Only three specimens were sampled and all at a single station within the Clarion-Clipperton Fracture Zone in APEI#3 area (type locality).

Ecological notes: The specimens were found in the water sieved from the ROV biobox at station 212, which contained several species of sponges (hexactinellid), alcyonaceans, antipatharians and pennatulacean corals, in addition to ophiuroids. Furthermore, spicules were found inserted in the body of the worms, suggesting an artefact of sieving or a commensal life with sponges. More studies in the area are needed to find out which species is the host.