Parathyasira coani Kamenev 2023, sp. nov.

Parathyasira coani sp. nov.

urn:lsid:zoobank.org:act: B88594C5-63A1-4111-909B-D2359BB41965

Figs 2–6, Table 1

Parathyasira sp. 2 – Kamenev 2015: 191.

Parathyasira sp. – Kamenev 2019: 6.

Diagnosis

Shell medium in size (to 7.5 mm in length), obliquely-rhomboidal, slightly drawn out anteriorly. Sculpture of closely spaced commarginal riblets and weak undulations. Second posterior fold weak. Posterior sulcus weak. Escutcheon long, deep. Lunule long, flat, weakly defined. Ligament well visible externally, long. Prodissoconch large (to 250 µm) with 5 lamellated folds. Lateral body pouches large, extensively lobed. Foot distally bulbous; bulbous portion not divided into two distinct parts; heel absent.

Etymology

The species epithet honors Dr Eugene V. Coan, a well-known researcher of eastern Pacific bivalves who made an enormous contribution to the study of the bivalve fauna of the Pacific Ocean.

Material examined

Holotype PACIFIC OCEAN • abyssal plain adjacent to the southern part of the Kuril-Kamchatka Trench; 39°43.80′ N, 147°10.16′ E –39°42.49′ N, 147°09.37′ E; depth 5224– 5215 m; 1 Sep. 2012; A. Brandt leg.; epibenthic sledge, RV Sonne, cruise no. 223, stn. 12-4; MIMB 43813.

Paratypes PACIFIC OCEAN • 2 specs; same locality as for holotype; 40°13.26′ N, 148°06.24′ E –40°12.37′ N, 148°05.43′ E; depth 5348–5350 m; epibenthic sledge; 30 Aug. 2012; A. Brandt leg.; RV Sonne, cruise no. 223, stn. 11-9; SMF 367801 • 1 spec.; same collection data as for preceding; MIMB 43814 • 1 spec.; same locality as for holotype; 40°13.10′ N, 148°06.45′ E –40°12.10′ N, 148°06.45′ E; depth 5351– 5348 m; 31 Aug. 2012; A. Brandt leg.; epibenthic sledge, RV Sonne, cruise no. 223, stn. 11-12; MIMB 43815.

Other material

RUSSIA – oceanic slope of the Kamchatka Peninsula • 2 specs; 53°05.4′ N, 161°55.2′ E –53°07′ N, 161°56.1′ E; depth 4890–4984 m; 10 Aug. 1990; L.I. Moskalev and S.V. Galkin leg.; Sigsbee trawl; RV Akademik Mstislav Keldysh, cruise no. 22, stn. 2323; IORAS OBF collection Cat. BIV00044.

PACIFIC OCEAN – abyssal plain adjacent to the Kuril-Kamchatka Trench • 7 specs; 41°16′ N, 147°27.7′ E; depth 5210 m; 20 Aug. 1954; P.L. Bezrukov leg.; Okean grab (0.25 m 2); RV Vityaz, cruise no. 19, stn. 3102; IORAS OBF collection Cat. BIV00043 • 6 specs; same locality as for preceding; 40°13.26′ N, 148°06.24′E –40°12.37′ N, 148°05.43′ E; depth 5348–5350 m; 30 Aug. 2012; A. Brandt leg.; epibenthic sledge, RV Sonne, cruise no. 223, stn. 11-9; MIMB 43816 • 2 specs; same locality as for preceding; 40°13.33′ N, 148°06.48′ E –40°12.53′ N, 148°05.76′ E; depth 5348– 5347 m; 30 Aug. 2012; K.V. Minin leg.;Agassiz trawl, RV Sonne, cruise no. 223, stn. 11-10; MIMB 43817 • 5 specs; same locality as for preceding; 40°13.10′ N, 148°06.45′ E –40°12.10′ N, 148°05.53′ E; depth 5351– 5348 m; epibenthic sledge, RV Sonne, cruise no. 223, stn. 11-12; 31 Aug. 2012; A. Brandt leg.; MIMB 43818 • 13 specs; same locality as for preceding; 39°43.80′ N, 147°10.16′ E –39°42.49′ N, 147°09.37′ E; depth 5224– 5215 m; A. Brandt leg.; epibenthic sledge, RV Sonne, cruise no. 223, stn. 12-4; 1 Sep. 2012; MIMB 43819 • 1 spec.; same locality as for preceding; 39°43.47′ N, 147°10.11′ E –39°42.54′ N, 147°09.51′ E; depth 5229– 5217 m; 1 Sep. 2012; K.V. Minin leg.; Agassiz trawl, RV Sonne, cruise no. 223, stn. 12-5; MIMB 43820. – Kuril-Kamchatka Trench • 1 spec.; 45°56.587′ N, 152°54.251′ E –45°56.570′ N, 152°54.499′ E; depth 6204– 6202 m; 26 Aug. 2016; K.V. Minin leg.; Agassiz trawl, RV Sonne, cruise no. 250, stn. 29; MIMB 43821 • 1 spec.; same locality as for preceding; 45°56.821′ N, 152°51.185′ E –45°56.834′ N, 152°50.943′ E; depth 6168– 6164 m; 27 Aug. 2016; A. Brandt leg.; epibenthic sledge, RV Sonne, cruise no. 250, stn. 30; MIMB 43822.

Description

SHELL. Medium in size (to 7.5 mm in length and 7.3 mm in height). Obliquely-rhomboidal, equivalve, subequilateral, white, thick, inflated (W/L= 0.56 ±0.06), almost as long as high (H/L=1.01 ± 0.03), slightly drawn out anteriorly; patches of silty and ferruginous deposit adhering to anterodorsal and posterodorsal shell margins (Fig.2, Table1). Periostracum thin, colorless, adherent.Dissoconch sculptured with closely spaced, thin, smoothed, commarginal riblets and weak, wide, irregular undulations. Microsculpture of small, densely spaced, wrinkles (Fig. 3C–D). Beaks small, raised, pointed, prosogyrate, anterior to midline (A/L= 0.40 ±0.02) (Table 1). Anterodorsal shell margin long, slightly concave, sometimes straight, gently sloping from beaks, forming a rounded angle at transition to anterior margin. Anterior margin strongly curved, smoothly transitioning to ventral margin. Ventral margin strongly curved, sometimes slightly angulate. Posterodorsal margin long, convex, steeply sloping from beaks, forming distinct angle at transition to posterior margin. Posterior margin straight, sometimes slightly concave, smoothly transitioning to ventral margin. First posterior fold absent. Second posterior fold weak. Posterior sulcus weak and shallow. Escutcheon long (EL/L=0.62 ± 0.03), narrow, deep (Figs 2C– D, 4A–D, Table 1). Auricle absent. Lunule long (LL/L= 0.39 ±0.04), wide, flat, demarcated by weak, rounded ridges (Figs 2C–D, 4A–B, E, Table 1). Ligament opisthodetic, visible externally for more than a half of its total length, thick, evenly curved, long, three-fourths the length of escutcheon, lying in shallow, slightly curved, wide groove at surface of hinge plate (Figs 2C, 3F, J, 4A, C–D). Prodissoconch large (length 235–250 µm), ovate in outline, with 5 thin, lamellated folds, extending from high and sharp apex, located in midline of prodissoconch (Figs 3H, 4F). Hinge plate thick, edentulous, with numerous, shallow pits (to 20 µm) under beaks (Fig. 3F–G, J–K). Muscle scars indistinct.

GROSS ANATOMY. Mantle edge thick, no obvious proliferation of glandular tissue on its anterior inner edge. Mantle fused posteriorly forming small exhalant aperture below posterior adductor muscle (Fig. 5A–C). Anterior adductor muscle elongate (Fig. 5J). Posterior adductor muscle small, ovate, 2 × as short and as narrow as anterior adductor muscle. Ctenidium wide, consisting of two demibranchs with fully reflected filaments (up to 80 filaments in specimen 7.5 mm in length); outer demibranch approximately half the size of inner demibranch. Demibranchs covering greater part of lateral body pouches (Fig. 5A–B). Labial palps relatively large (to 0.9 mm length) (Fig. 5F). Lateral body pouches large, extensively lobed; each lobe is a short and thick process; each pouch connecting to body by a rounded neck (Fig. 5B, E). Kidneys large, dorsoventrally elongated along posterodorsal shell margin, with numerous, red-brown or yellow, small (to 30 µm in diameter), different-size granules (Fig. 5G). Alimentary system with short oesophagus leading to a relatively large, elongate stomach; combined style sac and midgut strongly curved; hind gut forming anterior, deep, narrow loop producing rounded distinct angle, passing through heart and running posteriorly dorsal to kidney and posterior adductor muscle, opening at ventral side of posterior adductor muscle (Fig. 5H–J). Foot long, vermiform, distally bulbous, with muscular ring at junction with visceral mass. Bulbous portion not divided into two distinct parts; surface with numerous wrinkles; heel absent (Fig. 5B, D). Anterior and posterior pedal retractors wide, short, well developed.

Variability

In small specimens (up to 3 mm in shell length) the shell is relatively low, angular, with a strongly curved and anteriorly drawn-out ventral margin; the anterodorsal and posterodorsal margins gently sloping from beaks; the anterodorsal margin is concave (Fig. 2J). The shell shape and proportions, the length of the lunule and escutcheon, the degree of bending of shell margins vary among larger specimens (Fig. 2A–I, Table 1). Some of large specimens have a shell rather elongated dorsoventrally (Fig. 2I).

Distribution and habitat

Oceanic slope of the Kamchatka Peninsula (53°05.4′ N, 161°55.2′ E – 53°07′ N, 161°56.1′ E), 4890– 4984 m depth, abyssal plain adjacent to the southern part of the Kuril-Kamchatka Trench (39°42.49′ N, 147°09.37′ E – 41°16′ N, 147°27.7′ E), 5210–5351 m depth (bottom temperature (6–8 m above bottom) 1.5–1.6 °C, salinity 34.7‰, oxygen 7.71–7.72 ml. l−1) (Brandt et al. 2015), and in the Kuril-Kamchatka Trench (45°56.570′ N, 152°51.185′ E – 45°56.834′ N, 152°50.943′ E), 6164–6204 m depth (Fig. 6).

Differential diagnosis

The genus Parathyasira currently includes 14 species (WoRMS Editorial Board 2022), among which 6 species (P. resupina Iredale, 1930, P. neozelanica Iredale, 1930, P. verconis (Cotton & Godfrey, 1938), P. granulosa (Monterosato, 1874), P. subcircularis (Payne & Allen, 1991), and P. bamberi P.G. Oliver, 2015) have a shell microsculpture consisting of calcareous spines arranged in dense radial rows (Oliver 2015). The new species described herein lacks this microsculpture. Parathyasira coani sp. nov. differs from most of other species of Parathyasira by having an anteriorly drawn-out shell, a flat, a nonexcavated lunule, and a large prodissoconch with sculpture of lamellated folds (Table 2).

The new species is most similar to Parathyasira biscayensis (Payne & Allen, 1991) described from two specimens found in the Bay of Biscay (Atlantic Ocean) (4720 m) (Payne & Allen 1991) (Fig. 7A– C). However, in contrast to P. biscayensis, P. coani sp. nov. has only about half as many filaments in the gills, compared to almost similar-sized specimens of the former species, up to 80 (specimen 7.5 mm in length) vs 150 (specimen 8.2 mm in length), respectively, and a longer ligament. Moreover, in P. biscayensis the lunule is medially elevated, while in P. coani the lunule is flat. Parathyasira coani is similar to Parathyasira fragilis Kamenev, 2020 from which it differs in having a thick, opaque and white shell, a long and thick ligament, the greater part of which is well visible externally, the foot with a bulbous distal portion not differentiated into two parts, and a different sculpture of the prodissoconch (Table 2). Parathyasira coani is also similar to Parathyasira dearborni (Nicol, 1965) in shell shape and ratios, as well as in the shape of the lunule and escutcheon, but differs by lacking microscopic irregular corrugations and pustules on the shell and a second siphonal opening, by having a larger (more than 2 ×) prodissoconch with a different sculpture (Table 2), and the lateral body pouches with a greater number of folds.

Remarks

To date, the benthic fauna of the Kuril-Kamchatka Trench is the most studied one, compared to other oceanic trenches (Belyaev 1989; Brandt et al. 2019, 2020), and the largest number of macrofaunal samples were collected with various sampling gears in its hadal zone. As a result of studying the entire material of bivalves collected in this trench (Kamenev 2019), P. coani sp. nov. was recorded only in two samples collected from the uppermost part of the trench slope at a depth of more than 6000 m. Outside of the Kuril-Kamchatka Trench, P. coani was found in many samples from the abyssal zone at depths less than 6000 m. Therefore, this species is probably a predominantly abyssal one and depths of slightly more than 6000 m are the lower limit of its vertical distribution. Apart from P. coani, seven bivalve species have a similar vertical distribution in the Kuril-Kamchatka Trench area (Kamenev 2019). As shown in the example of distribution of many macrofaunal species, depths of 6000–7000 m are a zone of transition between abyssal and hadal faunas and are the lower and upper boundaries of the vertical distribution of many abyssal and hadal macrofauna species, respectively (Belyaev 1989; Jamieson 2015; Kamenev 2019; Kamenev et al. 2022).