Symmetriapelta becki, Chen & Poitrimol & Matabos, 2025

Symmetriapelta becki View in CoL sp. nov.

( Figs 6–8 View Figure 6 View Figure 7 View Figure 8 )

‘ Mediapex ’ sp. – Zhong et al. 2022: table 1.

LSID: urn:lsid:zoobank.org:act:D75D67CD-63B8-4FA5-978E-2A8085075EA6

Diagnosis: A Symmetriapelta with all radial ribs being of equal strength and carrying only very fine to no serrations on the three innermost lateral teeth.

Etymology: Named in honour of the late German malacologist Lothar A. Beck (Philipps-University of Marburg) who studied vent limpets from North Fiji and Lau Basins in the 1990s ( Chen and Sigwart 2023). Beck had evidently already noticed the presence of two distinct forms of Symmetriapelta in North Fiji and Lau Basins, because he separated a vial containing specimens of Symmetriapelta becki labelled ‘fine’ (i.e. fine sculpture), whereas those containing Symmetriapelta wareni were marked ‘stark’ (i.e. stronger sculpture). Although at the time Beck apparently did not consider these two forms to be distinctive at the species level, we nevertheless dedicate the specific epithet of Symmetriapelta becki in recognition of his observations and work.

Type locality: Mangatolo hydrothermal vent field , northern Lau Basin, 15°24.876 ʹ S, 174°39.208 ʹ W, 2031 m deep, R/V L’Atalante CHUBACARC cruise, ROV Victor 6000 dive 726, 17 April 2019. Taken from Alviniconcha assemblages ( Fig. 2C View Figure 2 ). Type material: Holotype, MNHN-IM-2019-30332 ( Fig. 6A View Figure 6 ). GoogleMaps Paratype #1, SMF 372657 About SMF ( Figs 6B View Figure 6 , 8 View Figure 8 ) GoogleMaps . Paratype #2, NSMT-Mo 79461 GoogleMaps . Paratype #3, MNHN-IM-2019-30333 ( Fig.6C View Figure 6 ) GoogleMaps . Paratype #4, SMF 372658 About SMF GoogleMaps . Paratype #5, MNHN-IM-2019-30334 GoogleMaps . Paratype #6, NSMT-Mo 79462 ( Fig. 6D View Figure 6 ), COI barcode GenBank accession OK635421 View Materials GoogleMaps . Paratype #7, SMF 372659 About SMF , COI barcode GenBank accession OK635447 View Materials GoogleMaps . Paratype #8, MNHN-IM-2019-30335 , COI barcode GenBank accession OK635448 View Materials . GoogleMaps All specimens from the same collecting event at the type locality and preserved in 80% ethanol. Paratypes #6–8 were used for barcoding, with only the head and shell left. For shell measurements of the types, see Table 1 View Table 1 .

Other material examined: Kulo Lasi hydrothermal vent field , Futuna Arc, 14°56.538 ʹ S, 177°15.007 ʹ W, 1414 m deep, R/V L’Atalante CHUBACARC cruise, ROV Victor 6000 dive 729, 24 April 2019. Living on the tubes of siboglinid tubeworms ( Fig. 2B View Figure 2 ) GoogleMaps . One specimen, MNHN-IM-2019-30336 ( Fig. 6F View Figure 6 ), shell curved . One specimen, MNHN-IM-2019-30337 ( Fig. 6G View Figure 6 ), intermediate shell form . One specimen, MNHN-IM-2019-30338 , used for barcoding, with only head and shell left, COI barcode GenBank accession OK635460 View Materials . One specimen, MNHN-IM-2019-30339 , used for barcoding, with only head and shell left, COI barcode GenBank accession OK635461 View Materials . One specimen, SMF 372660 About SMF , used for barcoding, with only head and shell left, COI barcode GenBank accession OK635462 View Materials . One specimen, NSMT-Mo 79463 , used for barcoding, with only head and shell left, COI barcode GenBank accession OK635462 View Materials . A series of five specimens, MNHN-IM-2019-30340. A series of five specimens, SMF 372661 About SMF . A series of five specimens, NSMT-Mo 79464. All specimens preserved in 80% ethanol.

Niuatahi Seamount hydrothermal vent field , Tonga-Tofua Arc ( Fig. 6E View Figure 6 ), 15°23.870 ʹ S, 174°0.610 ʹ W, 1603 m deep, collected by ROV Quest dive #437 during R/V Sonne cruise SO263 ‘TONGARIFT’, June 2018. Ten specimens preserved in 70% ethanol, SIO-BIC M19504 . GoogleMaps

White Lady hydrothermal vent field, North Fiji Basin, 16°59 ʹ S, 173°55 ʹ E, 2000 m deep, collected by the human-occupied vehicle ( HOV) Nautile dive # PL12 during R / V Nadir ‘STARMER II’ cruise, 7 July 1989. Two specimens fixed in 10% formalin and transferred to 70% ethanol, MNHN-IM-2014-7987 GoogleMaps .

Description: Shell ( Fig. 6 View Figure 6 ) limpet-shaped, with apex slightly left of mid-line in adults (SL ≤ 6.1 mm). Protoconch ( Fig. 7A, B View Figure 7 ) diameter 200–210 μm; sculpture of sparse, irregular, reticulated network of ridges on first half, rapidly fading into completely smooth second half. First half teleoconch whorl of juveniles smooth, lacking any sculpture except fine growth lines, loosely coiling. This rapidly transitions to limpet-shaped adult shell carrying numerous radial ribs by rapid expansion of oval aperture, more or less symmetrical in adults. Side profile and shape of aperture conform to substrate shape, being nearly flat in specimens from Ifemeria shells or chimneys ( Fig. 6A–E View Figure 6 ) but strongly curved in specimens collected from siboglinid worm tubes ( Fig. 6F, G View Figure 6 ). Radial ribs finely nodulous, always of equal strength across entire shell but strength variable across individuals; number of ribs ranges between ~100 and 250 in adults; those with stronger ribs also carry fewer of them. Interior of shell white, with silky sheen. Shell microstructure ( Fig. 7D View Figure 7 ) consists of two layers dorsal to myostracum, with a very thin homogeneous layer on top of a much thicker complex crossed lamellar layer. Numerous shell pores are present in the complex crossed lamellar layer, opening on ventral side wide but rapidly tapering, ending immediately before reaching homogeneous layer. Shell pores therefore do not penetrate outer homogeneous layer. Periostracum variable shades of greenish to yellowish brown, thin, terminates at shell edge rather than overhanging over it.

Radula ( Fig. 7F, G View Figure 7 ) rhipidoglossate, formula ~25 + 4 + 1 + 4 + ~25. Rachidian solid, well supported with two distinct lateral ridges on shaft, cusp heart-shaped, tapering distally to a sharp point; cutting edge completely smooth. Three inner laterals with sigmoidal, interlocking shafts, cusps sickle-like, triangular; cutting edge varies from smooth to carrying very fine serrations. Shafts of two innermost laterals always carry additional protrusion or node, which is sometimes also present on third lateral; strength of node on third lateral variable even among rows within each radula ribbon. Outermost lateral much broader than others, with straighter shaft lacking protrusion. Cusps of outermost lateral carry very fine serrations on inner edge, whereas outer edge carries much stronger, coarse denticles. Marginals with long shafts. Size of first few innermost marginals moderate, with rake-like cusps carrying ~15 rather strong denticles. Next set of marginals larger, also with rake-like cusps but serrated into 20–25 finer denticles. These gradually decrease in size outwards, with outermost marginals being much narrower and smaller in comparison to innermost marginals. Across all marginals, lowermost denticle strongest on both sides, with lowest denticle on outer cutting edge being the strongest by far. Outer marginals reduced in width, with hook-like cusps serrated into denticles.

Soft parts are shown in Figure 8 View Figure 8 . Head lacking eyes; snout with thick musculature around mouth. Cephalic tentacles triangular, thick at base, rapidly tapering to blunt ends in preserved specimens, extending slightly beyond snout. Neck short. Foot elongated oval; opening of anterior food gland moderate in size, slit-like in appearance. Epipodium well developed as continuous ridge, densely lined with ~60 epipodial tentacles (~30 on either side); often larger primary tentacles alternate with smaller secondary tentacles. Mantle edge lacking large papillae but carrying numerous micropapillae. Shell muscle horseshoe-shaped, with muscles of similar size and strength on two sides connected via a narrow muscular ridge at the posterior edge. Mantle cavity of moderate size extending to about half the length of the shell muscle, occupied anterodorsally by moderate-sized bipectinate ctenidium carrying ~30 filaments. Anterior edge of ctenidium emerges on right side of mantle cavity immediately posterior to right cephalic tentacle. Heart monotocardian, with elongated pericardium lying dorsally on pallial roof posterior of ctenidium; auricle positioned anterior to ventricle. Visceral mass rather small, occupied dorsally by digestive gland, ventrally by gonad (animal gonochoristic). Rectum emerges on right side of mantle cavity, running along right pallial margin, ending in anus positioned on right side of mantle cavity on pallial roof. Operculum absent.

Distribution: Hydrothermal vents in North Fiji and Lau Basins and the Futuna Arc.

Remarks: Symmetriapelta becki co-occurs with its congener Symmetriapelta wareni in North Fiji and Lau Basin vents ( Chen and Sigwart 2023), and the two species can be distinguished based on the shell sculpture. Although the strength of radial ribs in Symmetriapelta wareni is rather variable, the sculpture is always composed of three to five weaker ribs alternating with one much stronger rib ( Chen and Sigwart 2023). In Symmetriapelta becki , however, all radial ribs are of the same strength. The radial ribs in Symmetriapelta becki are consistent in strength, and they are typically weaker than in Symmetriapelta wareni ; some specimens of Symmetriapelta wareni can carry broad, scaly ribs, whereas this is never seen in Symmetriapelta becki . Furthermore, the radula of Symmetriapelta becki carries many more marginal teeth (~25) than that of Symmetriapelta wareni (~18). Another interesting difference is seen in the distribution of epipodial tentacles, whereby in Symmetriapelta wareni the two posterior-most tentacles are positioned somewhat distant from the rest (although this is not clear from all figures in the description, see Discussion below) but this is not evident in Symmetriapelta becki .

The protoconch ( Fig. 7C View Figure 7 ) and shell microstructure ( Fig. 7E View Figure 7 ) of Symmetriapelta wareni were not figured or described in its original description ( Chen and Sigwart 2023). We imaged them using newly collected material from Tui Malila vent field, Lau Basin (10 specimens, preserved in 80% ethanol, MNHN-IM-2019-30331, Tui Malila vent field, Lau Basin, 21°59.355 ʹ S, 176°34.098 ʹ W, 1886 m deep, R/V L’Atalante CHUBACARC cruise, ROV Victor 6000 dive 722, 4 April 2019, taken from I. nautilei assemblages) and describe them as follows. The protoconch of Symmetriapelta wareni is 200 μm in diameter and exhibits sparsely distributed, irregular reticulated sculpture formed by cross-cutting low ridges on the first two-thirds of the protoconch, whereas the last one-third is smooth. This differs from Symmetriapelta becki , in which only slightly less than half of the protoconch is sculptured ( Fig. 7A View Figure 7 ). The shell microstructure of Symmetriapelta wareni is virtually identical to that of Symmetriapelta becki , with a very thin homogeneous layer outside a much thicker complex crossed lamellar layer, and with numerous shell pores penetrating only the complex crossed lamellar layer.

For morphological distinction from Symmetriapelta radiata , see Remarks section of that species below.