Myonera aleutiana Machado & Sigwart 2025, sp. nov.

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