Thyasira kharkovensis Kamenev 2023, sp. nov.

Thyasira kharkovensis sp. nov.

urn:lsid:zoobank.org:act: 5E4F1FDC-5A7D-47BA-9FE5-B1F700D80C60

Figs 6, 12–16, Table 4

Parathyasira sp. 1 – Kamenev 2018a: 234.

Diagnosis

Shell small (to 7.4 mm in length), obliquely pyriform, transparent, flattened, drawn out anteriorly. Sculpture of thin, commarginal riblets and weak undulations. Posterior folds weak. Posterior sulcus weak, as a flattened, narrow area. Escutcheon long, shallow. Auricle low. Lunule short, flat, weakly defined. Ligament partially visible externally, long. Prodissoconch large (to 200 µm), with 7 thin, lamellated folds, radiating from short, plicate ridge. Lateral body pouches large, extensively lobed. Foot long, distally bulbous; bulbous portion not divided into two parts; heel distinct.

Etymology

The species epithet honors the city of Kharkov, where I was born, grew up, and was educated.

Material examined

Holotype RUSSIA • Sea of Okhotsk, Kuril Basin floor; 48°02.512′ N, 149°59.911′ E –48°02.353′ N, 149°59.805′ E; depth 3350 m; 21 Jul. 2015; K.V. Minin leg.; Agassiz trawl, RV Akademik M.A. Lavrentyev, cruise no. 250, stn. 6-9; MIMB 43824.

Paratypes RUSSIA • 2 specs; same collection data as for holotype; MIMB 43825 • 2 specs; same collection data as for holotype; SMF 367802.

Other material

RUSSIA – Bering Sea • 6 specs; 55°13.2′ N, 167°29.07′ E –55°12′ N, 167°26.7′E; depth 3957 m; 31 Jul. 1990; L.I. Moskalev and S.V. Galkin leg.; Sigsbee trawl, RV Akademik Mstislav Keldysh, cruise no. 22, stn. 2309; IORAS OBF collection Cat. BIV00045 • 6 specs; same locality as for preceding; 55°36.1′ N, 167°23.04′ E –55°35′ N, 167°24.5′E; depth 4294 m; 5 Aug. 1990; L.I. Moskalev and S.V. Galkin leg.; Sigsbee trawl, RV Akademik Mstislav Keldysh, cruise no. 22, stn. 2316; IORAS OBF collection Cat. BIV00046 – Sea of Okhotsk • 1 spec.; 46°08.745′ N, 145°59.760′ E; depth 3304 m; 9 Jul. 2015; G.M. Kamenev leg.; box corer, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 1-3; MIMB 43826 • 2 specs; same locality as for preceding; 46°08.875′ N, 145°59.336′ E –46°08.839′ N, 145°59.165′ E; depth 3307 m; 10 Jul. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 1-8; MIMB 43827 • 3 specs; same locality as for preceding; 46°09.037′ N, 146°00.465′ E –46°09.020′ N, 146°00.269′ E; depth 3307 m; 10 Jul. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 1-9; MIMB 43828 • 7 specs; same locality as for preceding; 46°40.961′ N, 147°28.283′ E –46°40.877′ N, 147°28.499′ E; depth 3353– 3352 m; 13 Jul. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 2-7; MIMB 43829 • 2 specs; same locality as for preceding; 47°12.046′ N, 149°36.901′ E; depth 3366 m; 16 Jul. 2015; G.M. Kamenev leg.; box corer, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 4-4; MIMB 43830 • 5 specs; same locality as for preceding; 47°12.132′ N, 149°37.136′ E –47°11.991′ N, 149°36.990′ E; depth 3366 m; 17 Jul. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 4-9; MIMB 43831 • 4 specs; same locality as for preceding; 47°12.039′ N, 149°36.950′ E –47°12.177′ N, 149°36.726′ E; depth 3366 m; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 4–10; 17 Jul. 2015; MIMB 43832 • 1 spec.; same locality as for preceding; 48°03.258′ N, 150°00.581′ E –48°03.141′ N, 150°00.432′ E; depth 3347 m; 20 Jul. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 6-6; MIMB 43833 • 1 spec.; same locality as for preceding; 48°03.234′ N, 150°00.468′ E –48°03.078′ N, 150°00.351′ E; depth 3350–3351 m; 21 Jul. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 6-7; MIMB 43834 • 5 specs; same locality as for preceding; 48°02.512′ N, 149°59.911′ E –48°02.353′ N, 149°59.805′ E; depth 3350 m; 21 Jul. 2015; K.V. Minin leg.; Agassiz trawl, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 6-9; MIMB 43835 • 2 specs; same locality as for preceding; 46°56.952′ N, 151°04.975′ E; depth 3300 m; 21 Jul. 2015; G.M. Kamenev leg.; box corer, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 7-1; MIMB 43836 • 7 specs; same locality as for preceding; 46°56.556′ N, 151°05.013′ E –46°56.753′ N, 151°05.017′ E; depth 3299 m; 22 Jul. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 7-3; MIMB 43837 • 3 specs; same locality as for preceding; 46°57.466′ N, 151°05.068′ E –46°57.326′ N, 151°05.050′ E; depth 3300 m; 22 Jul. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 7–4; MIMB 43838 • 6 specs; same locality as for preceding; 45°36.792′ N, 146°22.589′ E –45°36.902′ N, 146°22.484′ E; depth 3210 m; 1 Aug. 2015; A. Brandt leg.; epibenthic sledge, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 11-6; MIMB 43839.

PACIFIC OCEAN – oceanic slope of the Kuril Islands • 2 specs; 46°16.082′ N, 152°02.060′ E; depth 3432 m; 27 Jul. 2015; G.M. Kamenev leg.; box corer, RV Akademik Mstislav Keldysh, cruise no. 71, stn. 9-1; MIMB 43840.

Description

SHELL. Small (to 7.4 mm in length and 7.6 mm in height), obliquely-ovate to obliquely pyriform, equivalve, subequilateral, grayish white, very thin, fragile, translucent, flattened (W/L=0.48 ± 0.04), slightly longer than high (H/L=0.98 ± 0.03), drawn out anteriorly; small quantities of ferruginous deposit may adhere to anterodorsal shell margin (Figs 12–13, Table 4). Periostracum thin, transparent, adherent. Sculpture of thin, commarginal riblets and weak, narrow, irregular undulations (Fig. 12F). Central part of shell with microscopic (to 3 µm), irregular, densely spaced granules (Fig. 14C–D). Beaks small, raised, pointed, prosogyrate, slightly anterior to midline (A/L=0.45 ±0.03). Anterodorsal shell margin straight, sloping rather steeply from beaks, smoothly transitioning to anterior margin. Anterior shell margin slightly convex, smoothly transitioning to ventral margin. Ventral margin strongly curved. Posterodorsal shell margin slightly convex, sloping rather steeply from beaks, forming weak, rounded angle at transition to posterior margin. Posterior shell margin straight or slightly convex, smoothly transitioning to ventral margin. Posterior folds weak. Posterior sulcus weak, shallow, as flattened, narrow area. Escutcheon long (EL/L=0.54 ±0.05), narrow, shallow, demarcated by low ridges (Figs 13B–C, E, 15A–D). Auricle weak, low, only slightly projecting (Figs 14E, 15C–D). Lunule relatively short (LL/L=0.34 ± 0.05), flat (Fig. 13B–D). Ligament opisthodetic, visible externally for more than half of its length, thin, almost as long as entire length of escutcheon, lying in shallow, almost straight, narrow groove in hinge plate (Fig. 14F–I). Prodissoconch large (length 189–200 µm), distinctly separated from dissoconch, ovate in outline, with 7 thin, lamellated folds (posterior fold sometimes bifurcated, central fold sometimes indistinct), radiating from a short, higher and wider, plicate ridge, located in mid-line of prodissoconch (Fig. 15E–F). Hinge plate thin, edentulous, sometimes with numerous, shallow, elongate pits (to 50 µm) beneath beak and ligamental groove (Fig. 14J). Muscle scars indistinct.

GROSS ANATOMY. Mantle thin, transparent; mantle edge free except at junction with gill axis (Fig. 16). Anterior adductor muscle large, strongly elongated, curved almost parallel to anterior shell margin. Posterior adductor muscle 3 × as short as anterior, oval. Ctenidium thin, wide, both demibranchs with fully reflected filaments (up to 60 filaments in specimen 5.2 mm in length); outer demibranch about half depth of inner demibranch (Fig. 16A, D–E). Labial palps relatively large (to 0.8 mm in length) (Fig. 16B). Lateral body pouches large, extensively lobed; numerous lobes thick, cloven or single; each pouch connecting to body by a wide neck (Fig. 16D–E). Kidneys extremely large, dorsoventrally elongated along entire posterodorsal shell margin, with numerous, pink or orange, large (to 100 µm in diameter), different-size granules well visible through transparent shell (Figs 14, 16D–H). Oesophagus short; stomach large, strongly elongated; combined style sac and midgut strongly curved, lies over stomach; hind gut forming anterior loop dorsal to style sac, running posteriorly dorsal to kidney and posterior adductor muscle. Food remains present in hindgut (Fig. 16G–H). Foot long, vermiform, distally bulbous, with muscular ring at junction with visceral mass. Bulbous tip not divided into two parts; surface with numerous, longitudinal, curved gathers; heel small (Fig. 16C, E). Anterior and posterior pedal retractors wide, long, well developed.

Variability

In small specimens (up to 5 mm in shell length), the shell is relatively low, angulate, with a strongly curved and anteriorly drawn-out ventral margin, and weakly defined lunule with valve margin junction raised; anterodorsal and posterodorsal margins are sloped more gently from beaks (Fig. 13G–H). In larger specimens, the shell shape and proportions do not vary.

Distribution and habitat

Kuril Basin floor (Sea of Okhotsk) (45°36.792′ N, 146°22.589′ E – 48°03.258′ N, 150°00.581′ E), 3210– 3366 m depth (bottom temperature (1 m above bottom) 1.85–1.88°C, salinity 34.6%, oxygen 1.73– 1.87 ml. l−1) (Kamenev 2018; Kamenev et al. 2022), Commander Basin floor (Bering Sea) (55°12′ N, 167°26.7′ E – 55°36.1′ N, 167°23.04′ E), 3957–4294 m depth, and oceanic slope of the Kuril Islands opposite the Bussol Strait (46°16.082′ N, 152°02.060′ E), 3432 m depth (Fig. 6).

Differential diagnosis

The new species described herein differs from the vast majority of species of Thyasira in having an obliquely-pyriform, anteriorly drawn-out, non-sulcate shell, with very narrow, indistinct posterior folds, large prodissoconch with thin, lamellated folds, strongly elongated, narrow adductor muscles, and a heel of foot. Thyasira kharkovensis sp. nov. is most similar in shell shape to Axinulus obliquus Okutani, 1968 (Fig. 7D–J) clearly differing in having a narrower escutcheon and thinner ligament, a shorter, indistinct and non-excavated lunule. Axinulus obliquus was described as Axinulus obliqua Okutani, 1968 following only examination of the morphology of the shell (Okutani 1968). The main character that distinguishes Axinulus from most genera in the family Thyasiridae is the presence of a single demibranch in the ctenidium (Oliver & Killeen 2002; Zelaya 2010; Oliver 2015; Kamenev 2020). I assumed that very probably the ctenidium of this species consists of two demibranchs, and after further anatomical investigation A. obliquus could be placed in the genus Thyasira (Kamenev 2020). Based on my assumption, A. obliquus was listed in the WoRMS Editorial Board (2022) under the name of Thyasira obliqua (Okutani, 1968). On the other hand, the WoRMS Editorial Board (2022) classifies the status of A. obliquus as a “taxon inquirendum”. Unfortunately, it is currently not possible to examine the gills of A. obliquus, because only dry material is deposited at the NSMT (Dr Hiroshi Saito, pers. comm.). Therefore, the species still falls under the category of a taxon of uncertain taxonomic validity.

Thyasira kharkovensis sp. nov. resembles T. obsoleta (Verrill & Bush, 1898), T. succisa (Jeffreys, 1876), and T. debilis (Thiele, 1912), from which it differs in having a more flattened, thin and fragile shell, a longer escutcheon, a lower auricle, a longer well-visible externally ligament, and a much larger prodissoconch (the prodissoconch length of T. obsoleta, T. succisa, and T. debilis does not exceed 165µm) (Payne & Allen 1991; Oliver & Killeen 2002; Zelaya 2009) with lamellated folds. In addition, the new species described herein lacks the pseudocardinal tubercle in the right valve described for T. succisa and T. debilis (Payne & Allen 1991; Oliver & Killeen 2002; Zelaya 2009). On the whole, because of its obliquely pyriform, non-sulcate shell with narrow, indistinct posterior folds T. kharkovensis sp. nov. does not correspond to the diagnosis of the genus Thyasira. Consequently, T. kharkovensis is assigned to Thyasira sensu lato.

Remarks

Thyasira kharkovensis sp. nov. was recorded in the Sea of Okhotsk and on the oceanic slope of the Kuril Islands at 9 of 10 stations at depths less than 4000 m. However, this species was not found any of the 24 stations conducted in the Kuril-Kamchatka Trench area by the KuramBio (2012), SokhoBio (2015) and KuramBio II (2016) expeditions at depths greater than 5000 m (Kamenev 2015, 2018 a, 2019). Probably, T. kharkovensis sp. nov. does not occur at depths greater than 5000 m, preferring the upper abyssal zone. Previous studies showed that the deep-sea benthic fauna of the Sea of Okhotsk is closely related to the western Pacific fauna. Many species from various groups of animals that were discovered on the Kuril Basin floor (Sea of Okhotsk) were also found at the Pacific side of the Kuril Islands (e.g., Alalykina 2018, Downey et al. 2018; Fukumori et al. 2018; Kamenev 2018a; Malyutina & Brandt 2018; Mironov et al. 2018; Ostermair et al. 2018). A number of studies showed that the deep-sea Pacific fauna mainly penetrates into the Sea of Okhotsk through the deep straits of the Kuril Islands, primarily through the wide and deepest Bussol Strait (depth of 2318 m) (Ushakov 1953; Savilov 1961; Kamenev 2018a; Mironov et al. 2018). Most of deep-sea bivalve species from the Sea of Okhotsk were found in the Pacific Ocean to depths of 4000 m (Kamenev 2018a; Kamenev et al. 2022). In the Pacific Ocean, at depths greater than 5000 m, only two out of 25 species found in the abyssal zone of the Sea of Okhotsk were recorded (Kamenev 2018 a, 2019). Perhaps, T. kharkovensis lives in the upper abyssal zone on the oceanic slopes of the Kuril Islands and the Kamchatka Peninsula. It also penetrated into the Bering Sea through the wide and deep Kamchatka Strait (depths of more than 4000 m) and was found on the floor of the Commander Basin located opposite the Kamchatka Strait. The Bering Sea has three deepwater basins (Commander, Bowers, and Aleutian basins) connected to one another. It is possible that T. kharkovensis lives on the floor of all the basins of the Bering Sea and has a continuous distributional range from the Asian to American continent along the Aleutian Islands, inhabiting the slope of the islands. For example, the deep-sea propeamussiid bivalve Catillopecten squamiformis (Bernard, 1978) has a similar geographic and vertical distribution in the northern Pacific (Kamenev 2018b).