Symmetromphalus mithril, Chen & Poitrimol & Matabos, 2025

Symmetromphalus mithril View in CoL sp. nov.

( Figs 3–5 View Figure 3 View Figure 4 View Figure 5 )

Symmetromphalus aff. hageni Woodlark – Poitrimol et al. 2022: table 4.

LSID: urn:lsid:zoobank.org:act:8A58ADAB-4B10-4563-84E3-8EE1E83D352E

Diagnosis: A Symmetromphalus with silvery white periostracum, protoconch size 220–230 μm, teleoconch lacking broad and raised radial ribs, and radula with 10–12 pairs of marginal teeth.

Etymology: ‘ Mithril ’, meaning ‘grey glitter’ in Sindarin ( Tolkien 1954), a fictional silvery metal. Named in allusion to the silvery colouration and sheen of the periostracum in comparison to other congeners, used as a noun in apposition.

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 Ifemeria nautilei assemblages ( Fig. 2A View Figure 2 ). Type material: Holotype (female), MNHN-IM-2019-30328 ( Fig. 3A View Figure 3 ) GoogleMaps . Paratype #1 (male), SMF 372653 About SMF ( Figs 3B View Figure 3 , 5A View Figure 5 ) GoogleMaps . Paratype #2 (female), NSMT-Mo 79457 ( Fig. 5B View Figure 5 ). GoogleMaps Paratype #3 (female), MNHN-IM-2019-30329 GoogleMaps . Paratype #4 (female), SMF 372654 About SMF ( Fig. 3C View Figure 3 ) GoogleMaps . Paratype #5 (male), NSMT-Mo 79458 ( Fig. 3D View Figure 3 ) GoogleMaps . Paratype #6 (female), MNHN-IM-2019-30330 , COI barcode GenBank accession OK635567 View Materials GoogleMaps . Paratype #7 (female), SMF 372655 About SMF , COI barcode GenBank accession OK635568 View Materials GoogleMaps . Paratype #8 (male), NSMT-Mo 79459 , COI barcode GenBank accession OK635569 View Materials GoogleMaps . Paratype #9 (male), SMF 372656 About SMF , COI barcode GenBank accession OK635570 View Materials GoogleMaps . Paratype #10 (female), NSMT-Mo 79460 , COI barcode GenBank accession OK635571 View Materials . GoogleMaps All specimens were from the same collecting event at the type locality and preserved in 80% ethanol. Paratypes #6–10 were used for barcoding, with only the head and shell left. For shell measurements of the types, see Table 1 View Table 1 .

Description: Shell ( Figs 3A–D View Figure 3 , 4A View Figure 4 ) medium-sized for genus (SL ≤ 12.7 mm), coiled as juveniles but rapidly expanding to become limpet-shaped as adults. Aperture oval, wavy, irregularly shaped, conforming to substrate morphology. Protoconch ( Fig. 4B View Figure 4 ) diameter 220–230 μm, first half with irregular net-like sculpture, gradually fading into completely smooth second half. First whorl of juvenile teleoconch smooth, tightly coiled. At ~ 1 mm shell length, strong spiral (appears radial in adults), nodulous ribs appear; coinciding with rapid expansion of the aperture shifting to limpet-form. Number of ribs initially ~40 but increases to ~ 200 in adults. Apex approximately at mid-line as adults. Interior of shell white, with silky sheen. Shell muscle scar horseshoe-shaped, only weakly marked. Shell microstructure consisting of two layers dorsal to myostracum, with a thin simple prismatic layer outside a thick complex crossed lamellar layer. Periostracum thin, semi-transparent, bluish-white, with a silvery, silky sheen, slightly overhanging at shell edge. Numerous shell pores penetrate both layers of shell but not periostracum ( Fig. 4C, D View Figure 4 ).

Operculum ( Fig. 4E View Figure 4 ) present, thin, film-like, nearly transparent, vestigial in adults. Early volutions multispiral, final volution rapidly expanding, resulting in distinctly semi-circular shape in adults.

Radula ( Fig. 4F, G View Figure 4 ) rhipidoglossate, formula 10–12 + 4 + 1 + 4 + 10–12. Rachidian with triangular, well-reinforced base; narrow, triangular, overhanging cusp completely smooth. Three inner laterals interlocked at base, slightly increasing in size from inner to outer; cusps triangular, overhanging, smooth. Outermost lateral much larger, inner cutting edge smooth but outer cutting edge finely serrated, with lowermost serration much stronger than others. Marginals all similar in morphology, with long shafts ending in rounded, comb-like cusps finely serrated all over; lowermost serration on outer edge much larger, denticle-like.

Soft parts are shown in Figures 3 View Figure 3 and 5 View Figure 5 . Head large, eyes lacking. Gonochoristic, sexually dimorphic. In females, cephalic tentacles short, of equal size but in males left cephalic tentacle greatly enlarged to function as penis, lappet-like when expanded but rolled up as seminal groove, with tubule-like structure present at distal tip. Snout lacking oral disc, with two prominent, ventrally grooved oral lappets. Neck long, laterally compressed; in males, left edge enlarged to form deep seminal groove. Foot circular in outline; anterior pedal gland opening narrow, slit-shaped. Epipodial ridge bearing 8–10 symmetrically distributed epipodial tentacles on posterior two-thirds of foot. Shell muscle horseshoe-shaped, broad anteriorly but narrowing rapidly towards posterior edge, where two arms are connected by a narrow muscular ridge; slightly asymmetrical, with right arm slightly shorter but broader than left. Mantle edge apparently smooth to the naked eye but carries numerous fine papillae. Mantle cavity deep, extending to approximately half the length of the shell muscle. Bipectinate ctenidium (gill) very large, extending beyond pallial edge to reach anterior edge of head. Heart monotocardian; pericardium elongated, with auricle directly anterior to ventricle. Visceral mass small, dorsally occupied by testis or ovary depending on sex, with prostate gland or oviduct directly anterior to gonad on pallial roof. Digestive gland and kidney visible to left of gonad, right of pericardium. Rectum emerges to right side of gill, ending in anus at right side of pallial roof. Rest of digestive system agrees well with condition of genus described previously ( Beck 1992).

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

Remarks: Symmetromphalus mithril is the fourth species described in its genus, which has been found only in southwestern Pacific hydrothermal vent habitats. The new species can be distinguished from the other three based on a combination of shell and radular characters, as follows ( McLean 1990, Beck 1992, Chen and Sigwart 2023). The protoconch diameter of 220–230 μm in Symmetromphalus mithril is similar to that of Symmetromphalus regularis (220 μm) but is larger than that of Symmetromphalus hageni (160–180 μm) and Symmetromphalus mcleani (180 μm). In Symmetromphalus mithril , the periostracum is thin and always bluish white with a silvery sheen across the entire size range, as opposed to the thick, tan periostracum in Symmetromphalus regularis . Although the periostracum is also thin and can range from white to yellowish brown in Symmetromphalus hageni and Symmetromphalus mcleani , this generally changes with growth, and in adults they are typically yellowish brown. Nevertheless, periostracum coloration might vary according to the environment, and future findings of Symmetromphalus mithril from other localities might broaden the range of variability of this character in this species. The shell sculpture of Symmetromphalus mithril consists of only dense, fine radial ribs, like Symmetromphalus regularis and Symmetromphalus hageni , lacking the broad radial ribs seen in Symmetromphalus mcleani . The radula of Symmetromphalus mithril is similar to Symmetromphalus mcleani in that the marginal teeth are reduced to only 10–12 pairs, as opposed to 13–15 pairs in Symmetromphalus regularis and Symmetromphalus hageni .