Myonera aleutiana Machado & Sigwart, 2025

(SOSA), Senckenberg Ocean Species Alliance, Andrade, Luiz F., Boyko, Christopher B., Brandt, Angelika, Buge, Barbara, Dávila Jiménez, Yasmín, Henseler, Mats, Hernández Alcántara, Pablo, Jóźwiak, Piotr, Knauber, Henry, Marcondes Machado, Fabrizio, Martínez-Muñoz, Carlos A., Momtazi, Farzaneh, Nakadera, Yumi, Qiu, Jian-Wen, Riehl, Torben, Rouse, Greg W., Sigwart, Julia D., Sirenko, Boris, Souza-Filho, Jesser F., Steger, Jan, Stępień, Anna, Tilic, Ekin, Trautwein, Bianca, Vončina, Katarzyna, Williams, Jason D. & Zhang, Junlong, 2025, Ocean Species Discoveries 13 – 27 — Taxonomic contributions to the diversity of Polychaeta, Mollusca and Crustacea, Biodiversity Data Journal 13, pp. e 160349-e 160349 : e160349-

Myonera aleutiana Machado & Sigwart sp. nov.

Materials

Type status: Holotype. Occurrence: catalogNumber: SMF 374321 ; recordNumber: AB 4950 D 1; recordedBy: Die Expedition AleutBio (Aleutian Trench Biodiversity Studies); individualCount: 1; lifeStage: adult; preparations: whole animal (ETOH); occurrenceID: B2767C84-6115-5F0D-B11B-5A833BF687CF; Taxon: scientificName: Myonera aleutiana Machado & Sigwart ; kingdom: Animalia; phylum: Mollusca; class: Bivalvia; order: Anomalodesmata ; family: Cuspidariidae ; genus: Myonera ; specificEpithet: aleutiana ; taxonRank: species; scientificNameAuthorship: Machado & Sigwart; nomenclaturalCode: ICZN; Location: higherGeography: Pacific Ocean; waterBody: North Pacific; islandGroup: Aleutian Islands, off Alaska, United States of America; locality: Aleutian Islands, off Alaska, United States of America ; verbatimDepth: 5170 m; verbatimLatitude: 51°41.711'N; verbatimLongitude: 166°28.024'W; Identification: identifiedBy: Fabrizio Marcondes Machado; Event: samplingProtocol: box corer?; eventDate: 20 / 8 / 2022; habitat: abyssal sediment; Record Level: institutionCode: SMF; basisOfRecord: PreservedSpecimen

Type status: Paratype. Occurrence: catalogNumber: SMF 373797 ; recordNumber: Mol 558 / AB 3563; recordedBy: Die Expedition AleutBio (Aleutian Trench Biodiversity Studies); individualCount: 1; lifeStage: adult; preparations: whole animal (ETOH); occurrenceID: FFDD565F-A761-5896-8F1E-C8D888A8FFE6; Taxon: scientificName: Myonera aleutiana Machado & Sigwart ; kingdom: Animalia; phylum: Mollusca; class: Bivalvia; order: Anomalodesmata ; family: Cuspidariidae ; genus: Myonera ; specificEpithet: aleutiana ; taxonRank: species; scientificNameAuthorship: Machado & Sigwart; nomenclaturalCode: ICZN; Location: higherGeography: Pacific Ocean; waterBody: North Pacific; islandGroup: Aleutian Islands, off Alaska, United States of America; locality: Aleutian Islands, off Alaska, United States of America ; verbatimDepth: 5280 m; verbatimLatitude: 50°38.253'N; verbatimLongitude: 169°45.744'W; Identification: identifiedBy: Fabrizio Marcondes Machado; Event: samplingProtocol: box corer?; eventDate: 8 / 8 / 2022; habitat: abyssal sediment; Record Level: institutionCode: SMF; basisOfRecord: PreservedSpecimen

Description

Holotype

Dimensions: Length: 7.4 mm; Height: 4.1 mm; Width: 3.1 mm (Fig. 18 A – C).

Paratype

Dimensions: Length: 6.8 mm; Height: 3.5 mm; Width: 2.5 mm (Fig. 18 D – R). This individual presents a single and short byssal thread (bt), visible both through the shell's transparency and in tomographic images.

Shell small (up to 7.4 mm in length), extremely fragile, translucent to olive, inflated, inequilateral, elongate, rostrate, slightly inequivalve; posterior dorsal margin almost straight, anterior dorsal margin wider and rounded, dorsal and posteroventral margins of right valve (rv) overlapping left (lv) (Fig. 18 A – E); umbones inflated and rounded, slightly prosogyrate; prodissoconch I barely visible, prodissoconch II (pII) (1040 ± 20 μm, n = 2) elliptical, smooth, well preserved in the two individuals analysed (Fig. 18 C). External surface with 16-18 equidistant concentric foliaceous lamellae (fl), covering the entire shell, extending to the umbo (u) and complete to the rostral ridge (Fig. 18 A – E); countless small pustules (pu) present between the lamellae. Prominent rostral ridge (rr) present, with a secondary ridge (sr) parallel to the posterior dorsal margin, usually visible (Fig. 18 D). Hinge edentulous. Resilifer and lithodesma (lit) elongate, deflected, posteriorly pointed.

Anatomy:

Mantle margin with two fused points, anteriorly forming short pedal gape and posteriorly forming siphonal apertures; posteroventrally, mantle margin (mm) fusion (Fig. 18 J-Q) probably involving inner and middle folds (Type B) ( Yonge 1982); absence of fourth pallial aperture. No evidence of arenophilic glands (= radial mantle glands), likely due to the resolution limitations of the micro-CT scan, as these glands are commonly found in cuspidariids ( Morton and Machado 2019).

Siphons separated, inter-siphonal septum (iss) present; inhalant (is) large and contracted (Fig. 18 H), probably funnel-shaped; both surrounded by small finger-shaped tentacles at base, four in inhalant and three in exhalant siphon (es), encased in a thin and muscular siphonal sheath (ssh) (Fig. 18 G-I). Inhalant lateral sinus present (il). Inhalant sphincter (sph) in the base of the inhalant siphon, which is dorsally attached to the posterior septal retractor muscle (prm) (Fig. 18 F, K). The inhalant sphincter plays a crucial role in regulating water flow and the intake of prey into the infra-septal chamber (isc) and appears to be present exclusively in members of Cuspidariidae ( Machado et al. 2016) . Carnivorous bivalves with highly modified inhalant siphons, often regarded as active predators (e. g. Allogramma formosa (Jeffreys, 1882) or Poromya granulata (Nyst & Westendorp, 1839 )) , do not possess an inhalant sphincter.

Septum horizontal, pierced by four pores (sp) is present dividing the mantle cavity into infra (ventral / isc) and supraseptal (dorsal / ssc) chambers (Fig. 18 F, G, L-O). It is suspended in the mantle cavity by posterior and bifurcated anterior septal retractor muscles (psm / asm), attached to the shell valves just above their respective adductor muscles and lie close to the pedal retractor muscles (prm / arm). Lateral septal retractor muscles (lsm) present being more concentrated posteriorly. Extra lateral septal muscle, absent. Left and right halves of the septum are united by the septal membrane (sm), which separates anteriorly to create the siphonal gape through which the foot can protrude (Fig. 18 O).

Labial palps small and reduced, anterior and posterior palps present (Fig. 18 F). Posterior palps (plp) flattened and located along the postero-lateral margins of the funnel-shaped mouth (m). Anterior palps (alp) greatly reduced and unattached to the posterior ventral surface of the anterior adductor muscle (aam). Unlike most Cuspidariidae , M. aleutiana sp. nov. does not have the anterior palp attached to the anterior adductor muscle, likely due to the effects of fixation ( Yonge 1928, Machado et al. 2016).

Posterior and anterior adductor muscles present (pam / aam), isomyarian; with posterior and anterior pedal retractor muscle (prm / arm); posterior pedal and anterior septal retractor muscles (prm / asm) bifurcated and dorsally attach to the shell close to the posterior adductor muscle (Fig. 18 G, Q).

Foot with small, unpronounced heel (Fig. 18 F, L – O); presence of a single byssal thread (bt) (Fig. 18 O). The presence of byssus threads in Anomalodesmata is rare, having been documented in only 14 species to date ( Yonge 1952, Allen and Turner 1974, Morgan and Allen 1976, Krylova 1993, Leal 2008, Simone and Cunha 2008, Machado et al. 2016, Senckenberg Ocean Species Alliance (SOSA) et al. 2024), including M. aleutiana sp. nov. Amongst the Cuspidariidae , so far, only Cardiomya cleryana (A. d'Orbigny, 1846) has had its long and single byssus thread documented through illustrations and video recordings ( Machado et al. 2016). The new species is, therefore, the first known representative of Myonera to exhibit this feature. The potential functions of the byssus thread in deep-sea carnivorous bivalves have never been tested, but it likely aids in anchoring within the sediment, as lie-in-wait predators require stability to effectively capture their prey. A small pebble can be observed attached to the distal portion of the byssus thread in the new species, reinforcing the anchorage hypothesis (Fig. 18 O).

Digestive system with funnel-shaped mouth (m), opening into a short and muscular oesophagus (e) that enters into the anterodorsal portion of the stomach (s) (Fig. 18 F, P, Q); stomach Type II ( Purchon 1987, Purchon 1990) with a muscular wall (sw), well developed, rounded, with gastric shield (gs), prey inside (p) and associated with a crystalline style sac (css) not conjoined with the mid-gut; surrounded dorsally, anteriorly and laterally by female gonads (ov) and the digestive gland; digestive diverticula (dd) with large gastric caecum (gc) (Fig. 18 F, G, N-P).

Pericardium / heart + kidney (h + k) area reduced and poorly visible (Fig. 18 G, L). Haemocoel spaces (hs?) probably present in the anterior portion of the visceral mass (Fig. 18 F, N).

Probably dioecious, ovary (ov) well visible (> 40 mature oocytes counted, ~ 100 μm in diameter); only mature oocytes observed i. e. larger and usually free in the ovarian lumen (Fig. 18 F, M). Testis not found / visible. Although the measurements of prodissoconch I are unavailable, other evidence — such as the small body dimensions (miniaturisation) and the possibility that mature oocytes are encapsulated oocytes (post-fertilisation) — could support the hypothesis of brooding care in Myonera aleutiana sp. nov. The brooding care has already been described for other anomalodesmatans, such as members of the Spheniopsidae ( Morton et al. 2016 a, Morton et al. 2016 b) and hypothesised for the verticordioideans Trigonulina ornata d'Orbigny, 1853 and Lyonsiella illaesa Machado & Sigwart, 2024 ( Morton et al. 2019, Senckenberg Ocean Species Alliance (SOSA) et al. 2024).

Cerebro-pleural ganglia (ceg) present and visceral ganglia not visible. Pedal ganglia (pg) present, with no statocysts associated.

Type material

Holotype ( SMF 374321), one whole individual with soft parts. Paratype ( SMF 373797), one whole individual with soft parts.

Material examined

Only known from the type material.

Type locality

North Pacific Ocean, Aleutian Islands, off Alaska, United States of America, 51°41.711'N, 166°28.024'W, 5170 m depth.

Diagnosis

Shell translucent to olive, thin, fragile, inflated, rostrate; posterior dorsal margin almost straight; presence of two rostral ridges; dorsal and posteroventral margins of right valve overlapping left; outer surface with 16-18 equidistant concentric foliaceous lamellae covering the entire shell with countless small pustules present between them; anterior septal retractor muscle bifurcated; extra lateral septal muscles absent; reduced kidney and pericardial cavity; presence of a single and short byssus thread; probably dioecious.

Etymology

The specific epithet aleutiana refers to the species type locality, the Aleutian Islands.

Distribution

Known only from the Aleutian Islands, off Alaska, North Pacific. Bathymetric range: 5170–5280 m, a new record for the genus previously recorded at 4400 m depth in the Tasman Sea, Southwest Pacific (see Knudsen (1970)).

Taxon discussion

The genus Myonera presents significant challenges in distinguishing between some of its species, as well as from certain species of Cuspidaria Nardo, 1840 and Rengea Kuroda & Habe, 1971 , due to the overlap of anatomical and shell features. The new species was, therefore, compared with all morphologically similar species from these three genera through a detailed examination of the holotypes and / or original illustrations (Fig. 19 ). In general, Myonera and Rengea are distinguished by the presence of radial and concentric elements forming the outer sculpture in the former and only the presence of strong concentric plications in the latter. Additionally, Rengea have umbones depressed and more elongated shells. In Cuspidaria , the presence of a posterior lateral tooth in the right valve is the key feature that differentiates it from the other two genera – both edentulous.

This is the second time that a new species of bivalve has been described in detail (including shell and internal tissues) without the use of any invasive tool.

Myonera aleutiana new species differs from the other Pacific species, as M. pailoloana (Dall, Bartsch & Rehder, 1938) , M. tasmanica ( Knudsen, 1970) and M. lischkei (E. A. Smith, 1891) by the presence of fewer (16–18) and more spaced concentric lamellae, with fine pustules interspersed between them, a straighter rostrum and anterior and posterior dorsal margins of the right valve overlapping left. Furthermore, the new species possesses two rostral ridges and differences in shell outline (see Fig. 19 ). This same set of features was also used to differentiate Myonera aleutiana sp. nov. from the Atlantic congeners Myonera lamellifera (Dall, 1881) , M. limatula (Dall, 1881) , M. sulcifera (Jeffreys, 1882) , M. pretiosa A. E. Verrill & K. J. Bush, 1898 , M. paucistriata Dall, 1886 (type species), M. alleni Poutiers, 1995 , M. kaiwa Oliveira & Absalão, 2009 and M. atlasiana Utrilla, Rueda & C. Salas, 2020 . It is worth noting that the new species was thoroughly compared with M. alleni , not only through its probable syntype ( USNM 151906), but also with the original drawings by Allen and Morgan (1981), figs. 35 and 36. Although the new species is edentulous, a comparison with some similar Cuspidaria species was necessary, since the fragility of the shells may prevent access to detailed information about the hinge teeth for future studies. In comparison with these non-congeneric species, besides the absence of teeth on the right and left valves, the new species exhibits a more elongated shell with a wider anterodorsal margin, larger, rounder and more developed umbones, as well as two clearly visible rostral ridges. Finally, M. aleutiana was also compared with Rengea murrayi , as both species are sympatric in Aleutian Islands and were initially misidentified as the same species during the preliminary screening of the material.

In general, little is known about the anatomy of Myonera species, except for some soft parts details of M. alleni , M garreti and M. tasmanica ( Allen and Morgan 1981, Knudsen 1970). Myonera aleutiana sp. nov. is, therefore, the first species of the genus to be described in detail, using the combination of over 2,000 tomographic slices / images. Even so, features, such as a muscular septum perforated by four pairs of pores, presence of lateral septal muscles, reduced labial palps, configuration of 4 + 3 siphonal tentacles, presence of crystalline style sac and absence of projections on the inter-tentacular margin of the exhalant siphon (es), are not, in themselves, sufficient to distinguish the new species from other Myonera or from their confamilial relatives. It is worth noting, however, that the presence of an anterior septal retractor muscle bifurcated, a large number of eggs in the ovary (> 40), a significantly reduced kidney area and pericardial cavity and a single, short byssus thread have only been observed in M. aleutiana to date. Furthermore, the presence of a single byssus thread may also be debatable, as it could: (i) be broken during sampling procedures; (ii) dissolve in fixation solutions such as formalin and EtOH used in scientific collections or (iii) simply be ejected by the animal throughout its life. In addition to the shell and internal tissue characteristics described here, the new species also exhibits a distinct bathymetric range and geographic distribution compared to its congeners.

Finally, it is worth noting that the small individual ( 1.5 mm in length) collected by Wiklund et al. (2017) at 4082 m in the polymetallic nodule province of the eastern Pacific may represent juveniles of M. aleutiana or Rengea murrayi . Genetic data will be used in a future study by the authors to compare specimens from the AleutBio expedition with the available data at http://data.nhm.ac.uk/object/45033e06-fb54-49d5-b632-767e63c1cfd3 for the unique specimen archived in NHMUK under the number 20170037.

Methods

The two well-preserved specimens analysed here were collected in the eastern part of the Aleutian Trench, Alaska by the SO 293 AleutBio expedition, using an epibenthic sledge (EBS). The new species individuals are part of a larger collection with more than 1,200 lots and 3,500 individuals of Mollusca collected between 2500 and 7500 m depth. Both were described using only non-invasive techniques / tools, such as photos by stereomicroscope (NIKON) and tomographic images using the WERTH micro-CT scanner TomoScope ® XS Plus. Only the paratype was scanned; for this purpose, it was previously immersed in a contrast solution containing 0.3 % phosphotungstic acid (PTA at a concentration of 99.995 %), with 3 % dimethyl sulphoxide ( DMSO) in 95 % ethanol by 10 days (adapted from Machado et al. (2018)). Scanning parameters were as follows: source voltage = 150 kV, current = 125 µA, exposure time = 666 ms, ignore images = 0, image quality = 7, voxel size = 3.56 µm, number of images per revolution = 2500, CTsensor = Kurzer _ AAI _ Size _ 10 _ XL, filter = no, drift correction = on, time of acquisition = 3 h 15 min. The 2125 tomographic slices / images produced were reconstructed and analysed using the softwares WinWerth ® RAW Viewer, Viewer and 3 D Viewer available in https://www.werth.de/en/downloads.html. In addition, all the volumetric datasets are on-line available at the Harvard’s Dataverse under the link https://doi.org/10.7910/DVN/O2QQIM.

Specimen data for this description were (in parts) gathered via the Discovery Laboratory infrastructure of the SENCKENBERG OCEAN SPECIES ALLIANCE.

Repository: Type material is held at the Senckenberg Research Institute and Natural History Museum Frankfurt ( SMF). In addition to the holotype and paratype ( SMF), other museum lots were also analysed for comparison with the new species, most of them with images available on the websites of the respective institutions: Museum of Comparative Zoology - Harvard, ( MCZ, https://mczbase.mcz.harvard.edu/SpecimenSearch.cfm?collection_id=1), Smithsonian National Museum of Natural History ( USNM, https://collections.nmnh.si.edu/search/iz/), Muséum National D’Histoire Naturelle ( MNHN, https://science.mnhn.fr/institution/mnhn/collection/im/item/search).

Figures

Figure 18. Myonera aleutiana Machado & Sigwart, sp. nov. shell and anatomy. A – C Photomicrography of holotype (SMF 373402), right and left valves plus a dorsal view, respectively; D – R Paratype (SMF 374320), right and left views, respectively; F – R Selected X-ray slices / images showing the arrangement and details of the pallial cavity organs and visceral mass; F – G Tomographic sagittal sections of different parts of the specimen; purple, yellow, red and green squares indicate the approximate areas of the transversal sections; H – R Tomographic transversal sections, from posterior to anterior orientation. Abbreviations: ap, anterior labial palp; aam, anterior adductor muscle; arm, anterior pedal retractor muscle; asm, anterior septal retractor muscle; b, bubble (artifact); bs, broken shell area; bt, byssal thread; ceg, cerebro-pleural (= circum-esophagic glanglia); css, crystalline style sac; dd, digestive diverticula; es, exhalant siphon; est, exhalant siphonal tentacles; f, foot; fl, foliaceus lamellae; gc, gastric caecum; gs, gastric shield; h + k, heart + kidney; hg, hind gut; hs?, haemocoel spaces; il, Inhalant lateral sinus; is, inhalant siphon; isc, infra-septal chamber; iss, inter-siphonal septum; ist, inhalant siphonal tentacles; lit, lithodesma; m, mouth; mm, mantle margin (ventral); o, oesophagus; ov, ovary; p, prey (s) inside stomach; pp, posterior labial palps; pu, pustules; pII, prodissoconch II; pam, posterior adductor muscle; prm, posterior pedal retractor muscle; r, rostrum; rr, radial ridge; rv, right valve; s, septum; sp, septal pores; sm, septal membrane; sr, secondary ridge; st, stomach; sph, sphincter of inhalant siphon; ssc, supra-septal chamber; ssh, siphonal sheath; u, umbones. Scale bars: 1.5 mm (A – C), 1 mm (D – R).

Figure 19. Myonera aleutiana Machado & Sigwart, sp. nov. comparison with other similar species from Atlantic and Pacific waters, including information on their occurrence, bathymetry, type numbers or original illustrations. External view of the shells of Myonera lamellifera, right view; M. limatula, right view; M. pailoloana, left view; M. alleni, right view (holotype not found); M. aleutiana sp. nov., right view; M. tasmanica, right view (see Knudsen (1970): figs. 110–111); M. sulcifera, right view; M. pretiosa, left view; M. atlasiana, right view (see Utrilla et al. (2020): fig. 6); M. kaiwa, left view (after Oliveira and Absalão (2009)); M. paucistriata, right view; M. lischkei, left view; Cuspidaria atlantica, right view; C. lamellosa, right and left views, respectively and Rengea murrayi, right view. Abbreviations: MNHN, Muséum national d'Histoire naturelle, France; MNCN, Museo Nacional de Ciencias Naturales, Spain; MCZ, Museum of Comparative Zoology, US; NHMD, Natural History Museum of Denmark; NHMUK, Natural History Museum, London, UK; NMW, National Museum Wales, UK; SMF, Senckenberg Museum of Frankfurt, Germany; USNM, Smithsonian National Museum of Natural History, US.