Bathylepeta wadatsumi, Chen & Tsuda & Ishitani, 2025

Chen, Chong, Tsuda, Miwako & Ishitani, Yoshiyuki, 2025, A new large-sized lepetid limpet from the abyssal northwestern Pacific is the deepest known patellogastropod, Zoosystematics and Evolution 101 (3), pp. 1249-1258 : 1249-1258

Bathylepeta wadatsumi sp. nov.

Figs 2 , 3 , 4

Type locality.

On shelf-like volcanic rock, on the western escarpment of a graben structure in the northwestern Pacific (32°37.0617'N, 144°6.5835'E, 5922 m deep), 500 km southeast of Tokyo, Japan (Figs 1 , 2 ).

Type material.

Holotype. ( NSMT-Mo 79627 ), female; SL 40.5 mm, SW 33.4 mm, SH 18.8 mm, preserved in 70 % ethanol. Taken by a suction sampler mounted on-board HOV Shinkai 6500 from the type locality .

Diagnosis.

A very large (at least up to 40.5 mm SL) Bathylepeta with about 80 clearly defined white radial streaks on the shell. When alive, ventral tissue generally pigmented reddish brown, oral shield greyish, oral lappets also reddish brown. Second lateral teeth very well-developed, each as large as the fused pair of first laterals. Basal plate rectangular, slightly overlapping. Marginal teeth also very well-developed, forming overhanging, spoon-like cusps larger in size than the laterals, edges smooth. Jaw strongly mineralised and reinforced. Genital papillae small, about 0.7 mm long when alive and contracted.

Description.

Shell (Fig. 3 ) large (holotype 40.5 mm SL), limpet-formed, thin, translucent, bluish-grey, slightly elastic, possibly reflecting high organic content. Shell height nearly half of shell length. Apex postcentral at just behind centre of shell, strongly corroded, likely due to abyssal habitat below calcium compensation depth. Entire shell with fine concentric growth lines, crossed by about 80 clearly defined white radial streaks that decrease in strength outwards (Fig. 3 C ). Aperture broad oval, rate of widening increases from the final one-fourth of shell growth (Fig. 3 D – E ). Shell margin not thickened, with sharp edge, margin smooth, uninterrupted. Protoconch corroded away in holotype.

Jaw plate (Fig. 4 A ) very sturdy, off-white, heavily mineralised, about 4 mm wide (mineralised part 2 mm wide). Cutting-edge U-shaped central part further reinforced with chitinous plates on inner side.

Radula (Fig. 4 B – H ) docoglossate, formula 2-2 - 0 - 2 - 2. Rachidian tooth lacking. Central element comprising two pairs of lateral teeth. First pair of lateral teeth fused together to form sharply pointed cusp with faint line at centre where fusion occur; bluntly pointed with overhanging, smooth cutting edges. Second laterals well-developed, each as large as fused pair of first laterals, with triangular, sharply pointed cusps with smooth cutting edge, pointed upwards at much higher angle compared to first laterals. Cusps of all laterals dark in colour, indicative of reinforcement by iron oxide (goethite), as for most other patellogastropod limpets. Basal plates underneath laterals whitish in colour (likely indicative of calcification), rectangular in outline, slightly imbricating with adjacent plates. Marginal teeth chitinous, translucent, strongly curved distally. Cusps of marginals well-developed, with broad, spoon-like overhanging morphology carrying rounded, smooth cutting edges. Marginal cusps much larger than those of laterals. Inner marginal (Fig. 4 H ) slightly wider than outer marginal (Fig. 4 G ), with broader shaft.

External anatomy (Fig. 5 ) when alive shows strong reddish-brown pigmentation when viewed ventrally, especially on inner fold of mantle edge; oral shield greyish in colour. Foot cream, lacking colouration. Mouth large, rounded with well-sized oral shield, containing heavily mineralised jaw. Oral lappets present on both sides of oral shield, large, reddish brown when alive. Pigmented eyes lacking. Cephalic tentacles simple, conical, elongate, tapering to a fine tip distally. Mantle edge thick, undulating, smooth, tentacles lacking. Mantle cavity shallow around foot, depressed in cephalic region. Genital papillae (Fig. 5 E ) present on right side, posterodorsal to right cephalic tentacle, located just right of anus, rather short at about 0.7 mm long when retracted and alive. Foot oval, fleshy, with undulating edge when contracted. Shell muscle horseshoe-shaped, occupying posterior two-thirds of body, split into numerous distinct muscular bundles.

Dorsal aspect of soft parts seen through mantle roof (Fig. 5 D ) largely occupied by very voluminous, dark-brown digestive gland. Intestine forms three loops, last two loops embedded in digestive gland. Final loop exits digestive gland from left side of mantle roof as rectum. Gonad (Fig. 5 A ) voluminous, positioned anteroventral of digestive gland and final loop of intestine. Gonad of holotype slightly damaged during collection, with some oocysts emerging; thus, holotype is female, assuming gonochorism like most other patellogastropod limpets. Ctenidium absent.

Etymology.

From ‘ Wadatsumi ’, god of the sea in Japanese mythology, alluding to its very deep habitat. It is also a reference to the fish-man character “ Large Monk ” Wadatsumi from Eiichiro Oda’s manga series " ONE PIECE " ( Oda 2011), whose enormous body size is reminiscent of the large size that B. wadatsumi sp. nov. reaches for a deep-water patellogastropod. Used as a noun in apposition.

Distribution.

Only known from the type locality at 5922 m deep, living on hard volcanic rock covered by a thin layer of sediment (Fig. 1 B ). A winding feeding trail is clearly visible posterior to the limpet, indicating it feeds on this sediment layer. Several individuals were sighted from the viewport of the submersible, but only one individual (the holotype) was successfully collected.

Remarks.

Bathylepeta wadatsumi sp. nov. is unique among the genus in having the radular basal plates slightly imbricating and overlapping with adjacent ones (Fig. 5 C ), but all other aspects of shell and radular morphology agree with the other two described Bathylepeta species; its placement in this genus is also supported by molecular data (Fig. 6 , see below). As such, we here emend the diagnosis for Bathylepeta to include slightly overlapping to non-overlapping basal plates. This feature thus separates B. wadatsumi sp. nov. easily from the other two described congeners.

Bathylepeta wadatsumi sp. nov. is most similar to B. linseae from the Weddell Sea in Antarctica in shell morphology, with both species carrying whitish radial rays on the shell surface that are missing in B. laevis ( Moskalev 1977; Schwabe 2006). However, the radial rays are more clearly defined and numerous in B. wadatsumi sp. nov. compared to B. linseae (about 80 vs 50). Due to the small number of specimens available, it is currently unclear how much intraspecific variation there is in these radial rays or if their formation is determined environmentally. These two species are also very different in other radular characters, where the second lateral and marginals are both strongly reduced in B. linseae but very well-developed in B. wadatsumi sp. nov. The marginals of B. linseae are hook-like and interpreted as function uncini ( Lindberg 1998; Schwabe 2006), whereas in B. wadatsumi sp. nov. they are broad and spoon-like. Radular features other than the imbricating basal plates can also easily differentiate B. wadatsumi sp. nov. from B. laevis , in that B. laevis has much smaller second laterals, its marginal teeth carry denticulate and serrated cusps, and its basal plate is widened posteriorly rather than rectangular ( Moskalev 1977). Furthermore, the shells of both B. wadatsumi sp. nov. and B. linseae are bluish-grey, whereas B. laevis is whitish-grey.

Figures

Figure 1. Map of the sampling locality (red star). On the western escarpment of a graben structure in the northwestern Pacific (32 ° 37.0617 ' N, 144 ° 6.5835 ' E, 5922 m deep), 500 km southeast of Tokyo, Japan. The inset shows the general location with Japan as a reference. Abbreviations: STL, Sofugan Tectonic Line; NMT, northern limit of Mariana Trough. Bathymetric data were retrieved from GEBCO 2023 (https://www.gebco.net/).

Figure 2. In situ imagery at the type locality of Bathylepeta wadatsumi sp. nov. A. General overview of the area with shelf-like volcanic rocks covered by a thin layer of sediment; B. Habitus of the holotype specimen with a clear feeding trail behind; C. is a close-up showing better details of the shell morphology. The shell length of the holotype is 40.5 mm.

Figure 3. Bathylepeta wadatsumi sp. nov., holotype (NSMT-Mo 79627). A. Dorsal view; B. Ventral view; C. Ventral view with the soft parts removed; D. Lateral view from the left; E. Lateral view from the right.

Figure 4. Bathylepeta wadatsumi sp. nov., jaw and radula of the holotype (NSMT-Mo 79627). A. Jaw viewed from the outside (above) and inside (below), the chitinous membrane is slightly damaged on the right side of the outside view; B. A section of the radula ribbon under a light microscope, showing dark tips of the lateral teeth indicative of iron oxide (goethite) mineralisation, with C. being a view from the side. D. The same radula section as C. but viewed under SEM. E. The same radula section as B. viewed under SEM. F. An isolated basal plate with the two lateral pairs attached, viewed from the front (top), back (middle), and side (bottom). G. An isolated feather-like outer marginal tooth. H. An isolated feather-like inner marginal tooth. Abbreviations: 1, first lateral tooth; 2, second lateral tooth; im, inner marginal; om, outer marginal; m, marginal teeth.

Figure 5. Bathylepeta wadatsumi sp. nov., external anatomy of the holotype (NSMT-Mo 79627). A. Soft parts viewed laterally from the left (note the gonad was slightly damaged during collection and some oocytes are emerging from the left side); B. the same viewed laterally from the right; C. ventral view; D. dorsal view. E. Close-up of the genital papillae. Abbreviations: a, anus; dg, digestive gland; f, foot; go, gonad; gp, genital papilla; i, intestine; j, jaw; me, mantle edge; mo, mouth; ol, oral lappet; re, rectum; sm, shell muscle; t, cephalic tentacle.

Figure 6. Molecular phylogeny of Lepetidae generated using Bayesian inference from 584 bp of the COI gene. Node values indicate Bayesian posterior probabilities (BPP); only those higher than 0.6 are shown.