Parathyasira pauli, Kamenev, 2023

Kamenev, Gennady M., 2023, Three new deep-sea species of Thyasiridae (Mollusca: Bivalvia) from the northwestern Pacific Ocean, European Journal of Taxonomy 856, pp. 87-119 : 101-106

Parathyasira pauli sp. nov.

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Figs 6 , 8–11 , Table 3

Diagnosis

Shell medium in size (to 8.0 mm in length), rhomboidal, inflated, slightly drawn out anteriorly. Sculpture of closely spaced, commarginal ribs and conspicuous undulations. Shell surface microscopically pitted. Second posterior fold weak. Posterior sulcus indistinct. Escutcheon long, deep. Lunule long, strongly excavated, well defined. Ligament well visible externally, long. Prodissoconch large (to 260 µm), with 6 lamellated folds, extending from high and sharp apex. Lateral body pouches large, extensively lobed. Foot distally bulbous; bulbous tip not differentiated into two parts; heel absent.

Etymology

The species epithet honors Dr Paul Valentich-Scott, a well-known researcher of bivalves, who made an enormous contribution to the study of the bivalve fauna of the Pacific Ocean. His scientific works and personal assistance have been invaluably helpful in my studies of bivalves of the northwestern Pacific Ocean.

Material examined

Holotype PACIFIC OCEAN – Japan Trench • 37°41.5′ N, 143°54.3′ E; depth 7370 m; 28 Jun. 1969; Z.A. Filatova leg.; Sigsbee trawl; RV Vityaz, cruise no. 45, stn. 6151; IORAS OBF collection Cat. BIV00039 .

Paratypes PACIFIC OCEAN • 1 spec.; same locality as for holotype; 37°38.9′ N, 143°58.3′ E; depth 7370– 7350 m; 21–22 Jun. 1976; F.A. Pasternak leg.; Galathea trawl; RV Vityaz, cruise no. 59, stn. 7500; IORAS OBF collection Cat. BIV00040 • 1 spec.; same collection data as for preceding; MIMB 43823 .

Other Material

PACIFIC OCEAN – Japan Trench • 1 spec.; 36°44′ N, 143°19′ E; depth 7540 m; 23 Jun. 1976; F.A. Pasternak leg.; Galathea trawl; RV Vityaz, cruise no. 59, stn. 7503; IORAS OBF collection Cat. BIV00042 • 11 specs; same locality as for preceding; 38°38′ N, 144°06′ E; depth 7500 m; 27 Jun. 1976; F.A. Pasternak leg.; Okean grab (0.25 m 2); RV Vityaz, cruise no. 59, stn. 7511; IORAS OBF collection Cat. BIV00041 .

Description

SHELL. Medium in size (to 8.0 mm in length and 8.4 mm in height). Rhomboidal, equivalve, subequilateral, white, thick, inflated (W/L=0.51 ± 0.07), nearly as long as high (H/L=0.99 ± 0.03), slightly drawn out anteriorly; patches of silty and ferruginous deposit adhering to anterior and posterior shell margins ( Fig. 8 , Table 3). Periostracum thin, colorless, adherent. Dissoconch sculptured with closely spaced, thin, commarginal ribs and conspicuous, irregular undulations, more conspicuous near ventral margin ( Figs 8C , 9A–B ). Micro-sculpture of small (to 3 µm), shallow, closely spaced pits ( Fig. 9C–D ). Beaks small, raised, pointed, prosogyrate, anterior to midline (A/L=0.41 ± 0.02) ( Fig. 9B , Table 3).Anterodorsal shell margin long, straight, sloping rather steeply from beaks. Anterior shell margin curved, smoothly transitioning to ventral margin. Ventral margin strongly curved ( Fig. 8H–K ). Posterodorsal shell margin long, slightly convex, sloping steeply from beaks, forming distinct angle at transition to posterior margin. Posterior margin slightly concave, sometimes straight, smoothly transitioning to ventral margin. First posterior fold absent. Second posterior fold weak. Posterior sulcus weak, narrow and shallow. Escutcheon long (EL/L=0.60 ± 0.04 ( Table 3), narrow, deep, demarcated by rounded ridge ( Figs 8D, G , 10D ). Auricle absent. Lunule long (LL/L=0.38 ± 0.06), wide, well defined, strongly excavated, demarcated by rounded, strong ridges ( Figs 8D–F , 10A–C ). Ligament opisthodetic, partly sunken in shallow, slightly curved groove close to margin of hinge plate, well visible externally along its almost entire length, thick, long about half the length of escutcheon ( Figs 8D–E , 9F–I ). Prodissoconch large (length 252–260 µm), well distinct, ovate in outline, convex, with 6 thin, narrow, lamellated folds, extending from high and sharp apex, located in mid-line of prodissoconch ( Figs 9J , 10G–H ). Hinge plate thick, edentulous, with small flattened peg beneath beak in each valve, and sometimes numerous, shallow, elongate pits (to 40 µm) beneath beak and ligamental groove ( Fig. 9F–G ). Muscle scars indistinct.

GROSS ANATOMY. Mantle thin; edges unfused except limited interconnection at posterior ventral margin with small exhalant aperture below posterior adductor. Anterior adductor muscle elongate ( Fig. 11 ). Posterior adductor muscle small, 2 × as small as anterior one, oval. Ctenidium thin, wide, composed of both inner and outer demibranchs with fully reflected filaments (up to 45 filaments in specimen 4.3 mm in length); outer demibranch is less than half size of inner demibranch. Labial palps relatively large (to 0.5 mm in length in specimen 4.3 mm long) with small zone of sorting ridges close to junction with ctenidium ( Fig.11C ). Lateral body pouches large, extensively lobed; lobes short and thick, with rounded tips; each pouch connecting to body by large, rounded neck at base of stomach ( Fig. 11B, E ). Kidneys large, situated below hind gut, without granules ( Fig. 11E–G ). Oesophagus short; stomach large, strongly elongated; combined style sac and midgut strongly curved, lying over the stomach; hind gut running posteriorly dorsal to kidney and posterior adductor muscle ( Fig. 11G ). Foot long, vermiform, distally bulbous, with muscular ring at junction with visceral mass. Bulbous tip not divided into two distinct parts; surface with numerous warts; heel absent ( Fig. 11D ). Anterior and posterior pedal retractors wide, short, well developed.

Variability

In small specimens (up to 4–5 mm in shell length) the shell is low, angulate with a strongly curved and anteriorly drawn-out ventral margin; the anterodorsal and posterodorsal margins are sloped gently from beaks; the anterodorsal margin is concave ( Figs 8K , 9A, E, H ). In larger specimens, the degree of obliqueness and relative length of the anterodorsal and posterodorsal shell margins vary ( Fig. 8H–J ).

Distribution and habitat

The Japan Trench (36°44′ N, 143°19′ E – 38°38′ N, 144°06′ E), 7350–7540 m depth ( Fig. 6 ).

Differential diagnosis

As was the case of P. coani sp. nov., this species differs from most species of Parathyasira in having an obliquely-rhomboidal shell with a long anterodorsal margin and a large prodissoconch with sculpture of lamellated folds ( Table 2 ), and in lacking the dissoconch of calcareous spines. Parathyasira pauli sp. nov. is most similar to P. coani from which it differs in having a deeply excavated lunule, shorter ligament, small flattened peg beneath beak at the hinge plate. In addition, the prodissoconch of P. pauli has six lamellated folds versus five lamellated folds in P. coani The new species is also similar to P. dearborni , from which it differs in having a strongly excavated lunule, a less distinct posterior sulcus, a twice as large prodissoconch with a different number and position of lamellated folds and in lacking corrugations and pustules in the dissoconch and a second siphonal opening ( Table 2 ). Furthermore, P. pauli is similar to P. biscayensis ( Fig. 7A–C ) but differs from it in having a deeply excavated lunule. Parathyasira pauli differs from a large species (to 12.3 mm in length), Parathyasira kaireiae Okutani, Fujikura & Kojima, 1999 , which was also found in the Japan Trench (depth of 5343–6390 m) ( Okutani et al. 1999; Fujikura et al. 2002; Okutani & Fujikura 2002), in having a smaller and rhomboidal shell with a weak posterior sulcus and excavated lunule ( Table 2 ).

Figures

Fig. 6. The location of stations where the new species of Thyasiridae were found. Parathyasira coani sp. nov. (red spots); Parathyasira pauli sp. nov. (yellow spots); Thyasira kharkovensis sp. nov. (green spots). (Map data © 2022 Google).

Fig. 8. Parathyasira pauli sp. nov. A–G. Holotype (IORAS OBF collection Cat. BIV00039), exterior and dorsal views of both valves, shell length 7.6 mm. H. Paratype (MIMB 43823), shell length 7.0 mm. I. Paratype (IORAS OBF collection Cat. BIV00040), shell length 6.5 mm. J–K. Variability of shell shape (IORAS OBF collection Cat. BIV00041). J. Shell length 4.9 mm. K. Shell length 3.8 mm. Scale bars: A–B, D–K= 1 mm; C=500 µm.

Fig. 9. Scanning electron micrographs of Parathyasira pauli sp. nov. (IORAS OBF collection Cat. BIV00041). A. Exterior view of right valve. B. Sculpture of beak region. C–D. Sculpture of central shell part. E. Interior view of left valve. F. Hinge plate of left valve. G. Pits under beak of left valve. H. Interior view of right valve.I. Hinge plate of right valve.J. Prodissoconch.Abbreviation: see Material and methods. Scale bars: A, E, H =1 mm; B–C, F–G, I–J =100 µm; D =10 µm.

Fig. 10. Scanning electron micrographs of Parathyasira pauli sp. nov. (IORAS OBF collection Cat. BIV00041). A–B. Dorsal and oblique dorsal views of both valves. C. Lunule. D. Escutcheon. E. Prodissoconchs. F. Interior view of left valve of young specimen, shell length 1.3 mm. G–H. Prodissoconchs of different specimens. Scale bars=100 µm.

Fig. 11. Parathyasira pauli sp. nov. (IORAS OBF collection Cat. BIV00041). A. Gross anatomy after removal of right valve and mantle, shell length 4.3 mm. B. Gross anatomy after removal of right valve, mantle, ctenidium, and anterior adductor muscle, shell length 4.3 mm. C. Labial palps. D. Bulbous portion of foot. E. Gross anatomy after further removal of right lateral body pouch. F–G. Digestive system. Abbreviations: see Material and methods. Scale bars: A–B, E–F =1 mm; C–D, G =500 µm.

Fig. 7. A–C. Parathyasira biscayensis Payne & Allen, 1991. D–J. Axinulus obliquus Okutani, 1968. A. Holotype (MNHN-IM-38099), shell length 8.2 mm (photos by P. Maestrati (MNHN)). B–C. Paratype (MNHN-IM-38100), shell length 7.3 mm (photos by P. Maestrati (MNHN)). D–G. Holotype (NSMT Mo 69719), shell length 19.0 mm. H–I. Exterior and dorsal views of small specimen from the locality of the paratype (NSMT Mo 69720), shell length 10.0 mm (photos by Dr H. Saito (NSMT)). J. Dorsal view of paratype (NSMT Mo 69720), shell length 16.5 mm (photo by Dr H. Saito (NSMT)). Scale bars =1 mm.