Bathykermadeca thanatos Jimi & Fujiwara & Kajihara 2018, sp. nov.

Bathykermadeca thanatos sp. nov.

(New Japanese name: geikotsu-shinigami-urokomushi)

(Figs 1–4)

Material examined. Holotype (NSMT-Pol H-682): 47 mm long, 4 mm wide, sex unknown, 20 chaetigers, complete; obtained from whale bones sunken off the Sakishima Islands (about 150 km away from the islands), at a depth of 4974 m; collected by YF. Two paratypes (NSMT-Pol P-683; JAMSTEC No. 1120033073): 28–29 mm long, 10 mm wide, sex unknown, 16–20 chaetigers, one (NSMT-Pol P-683) is complete, another (JAMSTEC No. 1120033073) is incomplete (head dissected for morphological study); obtained together with the holotype.

Comparative material examined. ZMUC POL-1669 (holotype), USNM 51977 (paratype) of B. hadalis (Kirkegaard, 1956). USNM 96245 (holotype) of B. turnerae Pettibone, 1985.

Description. Holotype (NSMT-Pol H-682) complete. Body 47 mm long, 4 mm wide, with 21 segments, flat. Living colour white, with dorsal transverse red bands on each segment (Fig. 2A). Nine pairs of elytrophores, on segments 2, 4, 5, 7, 9, 11, 13, 15, 17. Dorsal tubercles on cirrigerous segments inconspicuous. Elytra white (all elytrae dropped off while collecting, but preserved in same glass vial) (Figs 1B, 1C, 2A). Prostomium posteriorly red in live specimens, pinkish in ethanol, deeply bilobed, about as wide as long, with frontal filaments. Median antenna with short cylindrical ceratophore, inserted at median notch of prostomium; style about 250 µm, as long as width of prostomium, 2/7 as long as tentacular cirri (Fig. 2B). Eyes absent. Two pairs of rounded areas remain in center of the prostomium. Palps as long as tentacular cirri, whitish, smooth. Tentacular cirri with tentaculophores lateral to prostomium; tentaculophores achaetous but with acicula, latter slightly emerging from dorsal side of tentaculophore. Dorsal and ventral tentacular cirri of equal lengths, whitish.

Parapodia biramous, notopodia shoter than neuropodia (Fig. 3A). Notochaetae numerous (about 30 chaetae in each parapodium), short to long, with two rows of spines along one side, tapering to blunt tips; stouter than neurochaetae (Figs 3B, 4A, 4B). Neurochaetae of one type, numerous, short to long, with two rows of spines along one side (Figs 3C, 4C, 4D), tips pointed; one black robust acicula in each ramum, not penetrating epidermis; acicular lobe present, digitate (Fig. 3A). Cirrigerous segments (Fig. 2A) with long smooth dorsal cirri, about 1.5 times length of notochaetae. Ventral cirri short, tapered, smooth, attached on middle part of parapodia (Figs 2C, 3A). On segment 2, ventral cirri elongated (reaching to half of notochaetae) and projected from base of neuropodia. Parapodia in posterior segments (17–21) modified and compressed. Segment 19 without dorsal cirri. Pygidium cylindrical in shape, achaetous; anal cirri conical, extending behind tip of neuropodia in segment 20 (Fig. 2C). Nephridial papillae present on segments 12–15, elongated. Anus situated dorsally on segment 19.

Pharynx dissected in one of paratypes (JAMSTEC No. 1120033073), with 7 pairs of papilllae and 2 pairs of jaws; papillae rectangle in shape; each jaw with main fang; jaws growing inwardly, with smooth margins (Fig. 2D).

Etymology. In Greek mythology, Thanatos is the god of death, who comes beside dying people to take their souls away. The habitat of this new species living on and around whale carcass evokes this mythology.

Distribution. Only known on and around whalebones, south-east off Miyakojima Island, the Nansei-Shoto Trench, at a depth of 4974 m.

Remarks. Bathykermadeca thanatos sp. nov. can be distinguished from other members of the genus by the following features: i) there is only one type of neurochaetae, ii) the teeth lack serration and grow inwardly, iii) median antenna extend beyond the tip of frontal filament, iv) the nephridial papillae are present in segments 12–15, and v) there are about 50 notochaetae in each parapodium (Table 1). The last character, the number of notochaetae, was not mentioned in previous descriptions for B. hadalis and B. turneae (Kirkegaard 1956; Pettibone 1976, 1985), but newly confirmed in this study. Bathykermadeca thanatos most resembles B. hadalis; however, the nephridial papillae of B. hadalis are present in segments 12–17, 3–6 (mostly 5) notochaetae are present in each parapodium, and the teeth of jaws grow outward (Kirkegaard 1956; Pettibone 1976). Bathykermadeca turnerae differs from the new species by having two types of neurochaetae, serrated teeth, and median antenna that does not extend beyond the tip of the frontal filaments.

Bathykermadeca thanatos sp. nov. represents the first member of the genus recorded from whale-fall community. At the site, specimens of B. thanatos sp. nov. were found on and around the bones (Fig. 1C), but none was found swimming. On the other hand, Jamieson (2015) reported some deep-sea polynoids that have a potential to swim temporally. If B. thanatos sp. nov. can also swim, it is likely to be an opportunistic predator, living in the surrounding benthos, feeding on the other community members that live on whale falls. At the moment, however, whether B. thanatos sp. nov. is opportunistic or specialist to whale-fall community remains uncertain. Further research based on in-situ observation using ROVs, not only around whale-bone carcasses but also in other environments, would clarify the natural behavior and habitat of the species.

a Counted for three parapodia from anterior, middle, and posterior portions of the body; B. hadalis (n = 2), B. turnerae (n = 1), B. thanatos (n = 3).

b Confirmed in one of paratypes (USNM 51977), not in the holotype (ZMUC POL-1669).