Symmetriapelta radiata sp. nov.

(Figs 9–11)

LSID: urn:lsid:zoobank.org:act: 187A29B5-FE50-40F1-8288- F5FD68A2C33A

Diagnosis: A Symmetriapelta with greenish white periostracum, radial ribs of variable strength on the shell, and carrying strong serrations on the three innermost lateral teeth.

Etymology: ‘ Radiata ’, Latin (feminine adjective in the nominative singular), meaning ‘radiated’. Refers to the numerous radial ribs on the shell spreading out from the apex.

Type locality: La Scala hydrothermal vent field, Woodlark Basin, 9°47.944 ʹ S, 155°03.161 ʹ E, 3388 m deep, R/V L’Atalante CHUBACARC cruise, ROV Victor 6000 dive 738, 28 May 2019. Taken from I. nautilei assemblages (Fig. 2A). Type material: Holotype, MNHN-IM-2019-30341 (Fig. 9A) . Paratype #1, SMF 372662 (Figs 9B, 11) . Paratype #2, NSMT-Mo 79465 (Fig. 9C) . Paratype #3, MNHN-IM-2019-30342 (Fig. 9D) . Paratype #4, SMF 372663 . Paratype #5, NSMT-Mo 79466 . Paratype #6, MNHN-IM-2019-30343, COI barcode GenBank accession OK635421 . Paratype #7, SMF 372664, COI barcode GenBank accession OK635439 . Paratype #8, NSMT-Mo 79467, COI barcode GenBank accession OK635440 . Paratype #9, MNHN-IM-2019-30344, COI barcode GenBank accession OK635441 . Paratype #10, SMF 372665, COI barcode GenBank accession OK635442 . Paratype #11, NSMT-Mo 79468, COI barcode GenBank accession OK635464 . Paratype #12, MNHN-IM-2019-30345, COI barcode GenBank accession OK635465 . Paratype lot #13 (growth series of five specimens), MNHN-IM-2019-30346. Paratype lot #14 (growth series of five specimens), SMF 372666, COI barcode GenBank accession OK635465 . Paratype lot #15 (growth series of five specimens), NSMT-Mo 79469. All specimens from the same collecting event at the type locality and preserved in 80% ethanol. Paratypes #7–12 were used for barcoding, with only the head and shell left. For shell measurements of the types, see Table 1.

Description: Shell (Fig. 9) limpet-formed, with apex positioned slightly left of mid-line in adults (SL ≤ 6.8 mm). Protoconch (Fig. 10A) diameter 210 μm, sculptured by a sparse, irregular, reticulated network of ridges on first two-thirds, fading into completely smooth final one-third. First 0.5 whorl of juvenile teleoconch smooth, with only fine growth lines, loosely coiling; rapidly developing radial ribs thereafter, together with rapid expansion of aperture to transition into limpet-shaped adult shell. Aperture oval, nearly symmetrical in adults. Radial ribs finely nodulous; strength variable across individuals; in most individuals, all ribs of equal strength (Fig. 9A, C) but some individuals exhibit a pattern where three to seven weaker ribs alternate with one much stronger rib (Fig. 9B, E). Number of ribs between ~100 and 250 in adults; individuals with stronger ribs having fewer of them. Interior of shell white, with silky sheen. Shell microstructure (Fig. 10B) with two clearly discernible layers above myostracum: outer one very thin, homogeneous; inner one much thicker, complex crossed lamellar. Shell pores densely packed in inner complex crossed lamellar layer, not penetrating homogeneous layer. Periostracum variable shades of greenish white, thin, not overhanging shell edge.

Radula (Fig. 10C, D) rhipidoglossate, formula ~30 + 4 + 1 + 4 + ~30. Rachidian solid, well supported with two distinct lateral ridges on shaft, base rapidly expanding laterally almost at a right angle. Rachidian cusp narrow, heart-shaped, tapering distally to a sharp point; cutting edge smooth. Two inner laterals with sigmoidal, interlocking shafts carrying additional protrusion or node; cusps sickle-like, triangular; inner cutting edge carrying a few strong, coarse denticles; outer cutting edge smooth. Third lateral with similar cusp but shaft straight, without additional protrusion. Outermost lateral much broader than others, with shaft similar to third lateral; cusp carrying strong, coarse denticles on both sides. Marginals with long shafts. Innermost three marginals moderate in size, with rake-like cusps divided into ~15 denticles, transitioning outwards to next set of larger marginals, with more finely serrated cusps divided into ~20–25 denticles. These finely serrated outer marginals decrease in strength outwards, with the last ones being smaller than the innermost marginals.

Soft parts (Fig. 11) nearly identical to those of Symmetriapelta becki described above, except for epipodial tentacles being slightly denser, numbering ~70 (~35 on either side). Operculum lacking.

Distribution: Known only from the La Scala vent field, Woodlark Basin.

Remarks: Symmetriapelta radiata is highly variable in the strength of shell radial ribs; although in most individuals the strength is consistent across the entire shell (Fig. 9A, C, D), some alternate between three to five weaker ribs and one much stronger rib (Fig. 9B, E). As such, the shell sculpture cannot be used reliably to distinguish Symmetriapelta radiata from Symmetriapelta wareni (Chen and Sigwart 2023) and Symmetriapelta becki . However, the radula of Symmetriapelta radiata is clearly distinct from those two species in that the inner side of all four laterals carries a few coarse, strong serrations. In Symmetriapelta wareni, only the fourth lateral carries very fine serrations on the inner side, whereas the other three laterals have smooth cutting edges (Chen and Sigwart 2023), and in Symmetriapelta becki all four laterals typically carry very fine serrations, with the three innermost laterals sometimes lacking serration altogether. Furthermore, Symmetriapelta radiata has denser epipodial tentacles (~35 on either side) compared with Symmetriapelta becki (~30) and Symmetriapelta wareni (~26).