Three new species of Axinulus (Bivalvia: Thyasiridae) from the Japan and Kuril-Kamchatka trenches and abyssal zone of the northern Pacific Ocean

The Thyasiridae is one of the most species-rich families of bivalves in the deep-sea areas of the northern Pacific Ocean. Many thyasirid species form abundant populations in these regions and play an important role in the functioning of deep-sea benthic communities. However, most of these deep-sea thyasirid species have not been identified and many of them are new to science. Based on the material of bivalves collected by eight deep-sea expeditions in the northern Pacific Ocean during the period from 1954 to 2016, three new species of the genus Axinulus (Axinulus krylovae sp. nov., A. alatus sp. nov., and A. cristatus sp. nov.) are described from the Kuril-Kamchatka and Japan trenches, the Bering Sea, and other deep-water regions of the northern Pacific Ocean (3,200–9,583 m depth). The new species are distinguished due to a unique and complex sculpture of the prodissoconch, including tubercles and numerous thin folds of varying length and shape, as well as due to a thickening of the shell in the adductor scar areas, thus rendering the scars raised above the inner surface of the shell. Comparisons with all species of the genus Axinulus are provided.


INTRODUCTION
In recent years, international expeditions organized by the Russian Federation and Germany have sampled the deep-sea benthic fauna in an extensive region of the northwestern Pacific Ocean with depths greater than 3,000 m. The studies were focused on the composition and distribution of the benthic fauna in the deep-sea basins of the Seas of Japan and Okhotsk, the Kuril-Kamchatka Trench, and at the abyssal plain of the Pacific Ocean adjacent to the Kuril-Kamchatka Trench (Malyutina & Brandt, 2013;Malyutina, Chernyshev & Brandt, 2018;Brandt et al., 2019). Benthic macrofauna rich in species number and abundance was found in these deep-sea ecosystems, with bivalves being one of the dominant groups of animals (Brandt et al., , 2020Kamenev, 2013Kamenev, , 2014Kamenev, , 2015Kamenev, , 2018aKamenev, , 2018bKamenev, , 2018cKamenev, , 2019Kamenev et al., 2022). Among the bivalve fauna of all the studied deep-sea areas of the northwestern Pacific Ocean, the Thyasiridae was the most species-rich family and many thyasirids were the dominant species in terms of abundance in the benthic macrofaunal communities (Kamenev, 2013(Kamenev, , 2015(Kamenev, , 2018a(Kamenev, , 2019Kamenev et al., 2022). At least 14 thyasirid species were found at depths greater than 3,000 m in the abyssal and hadal zones of the northwestern Pacific Ocean. They have not been identified to the species level and are most likely new to science. Moreover, previous studies of the northwestern Pacific deep-sea fauna revealed many species of thyasirids in various oceanic trenches (Belyaev & Mironov, 1977;Belyaev, 1989;Allen, 2015), most of them remain so far unidentified to the species level (Belyaev, 1989). Thus, despite the great importance of thyasirids in the functioning of the deep-sea ecosystems of the northwestern Pacific Ocean, the abyssal and hadal thyasirid fauna in the region is almost not studied, except three species (Axinulus hadalis , Axinulus philippinensis Allen, 2015, and Thyasira kaireiae ), which were found in the Japanese and Philippine trenches Allen, 2015).
Several years ago, the author of this article started a study of unidentified deep-sea species of the Thyasiridae found in the northwestern Pacific Ocean. As a result of this work, six new species have been described from the abyssal and hadal zones of the Sea of Okhotsk and the Bering Sea, the Kuril-Kamchatka and Japan trenches, as well as the oceanic plain adjacent to these trenches (Kamenev, 2020(Kamenev, , 2023. The present article is a continuation of the study of northwestern Pacific deep-sea thyasirids and presents a description of three new species of the genus Axinulus that were found in the Japan and Kuril-Kamchatka trenches, as well as in the abyssal zone of other deep-sea regions of the northern Pacific Ocean.

Material studied
The material examined in this study was collected from 1954 to 1990 by expeditions of the P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow (IO RAS) in the hadal zone of the Kuril-Kamchatka Trench and from the Pacific abyssal plain adjacent to the Kuril-Kamchatka Trench (RV Vityaz, cruise no. 19, August 17-October 29, 1954;RV Vityaz, cruise no. 39, July 7-September 13, 1966), in the hadal zone of the Japan Trench (RV Vityaz, cruise no. 59, May 26-July 5, 1976), in the Gulf of Alaska (RV Vityaz, cruise no. 45, April 23-July 10, 1969), and from the bottom of the Commander Basin (Bering Sea) (RV Akademik Mstislav Keldysh, cruise no. 22, July 25-October 27, 1990), as well as by the German-Russian deep-sea expeditions KuramBio (RV Sonne, cruise no. 223, July 21-September 7, 2012) and KuramBio II (RV Sonne, cruise no. 250, August 16-September 29, 2016) from the Pacific abyssal plain adjacent to the Kuril-Kamchatka Trench and in the hadal zone of the Kuril-Kamchatka Trench; and by the Russian-German deep-sea expedition SokhoBio (RV Akademik M.A. Lavrentyev, cruise no. 71, July 6-August 6, 2015) in the abyssal zone of the Pacific slope of the Kuril Islands. The new species of Axinulus were found among the 72 samples of benthic fauna at depths of 3,200-9,583 m. The methods for the sample collecting and fixing have been in detail described previosly (Monin, 1983;Malyutina, Chernyshev & Brandt, 2018;Brandt et al., 2020;Kamenev, 2020Kamenev, , 2023. The samples obtained during the expeditions of the IO RAS were stored in the IO RAS Ocean Benthic Fauna collection. The samples collected by the KuramBio, KuramBio II, and SokhoBio expeditions were stored at the Museum (MIMB) of A.V. Zhirmunsky National Scientific Center of Marine Biology, Far Eastern Branch, Russian Academy of Sciences (NSCMB FEB RAS), Vladivostok, Russia. The type and other materials of the new species were deposited at the MIMB, IO RAS, and Senckenberg Museum Frankfurt, Germany (SMF).

METHODS
Shell length (L), height (H), anterior end length (A), and shell width (W) were measured with the ocular micrometer at the accuracy level of 0.1 mm (Fig. 1). The ratios H/L, A/L, and W/L were determined.
The methods for the sample preparation and the shell morphology study using the scanning microscopy as well as the gross anatomy analysis of the preserved alive-taken specimens have been described by Kamenev (2020Kamenev ( , 2023. The microscopic studies of the new species described here were carried out at the Far Eastern Center of Electron Microscopy of the NSCMB FEB RAS. To describe the shell morphology and body anatomy of the new species belonging to the genus Axinulus, the terms proposed by Payne & Allen (1991) and Oliver & Killeen (2002) were used.

Nomenclatural acts
The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org: pub:0AE8C4E1-0F13-478E-81E9-287980139049. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central SCIE and CLOCKSS. Diagnosis (after Kamenev, 2020): Shell small (<10 mm), equilateral or subequilateral, ovate to ovate-rhomboidal, slightly higher than long; margins entire, without any posterior sinus, but sometimes with weak angulation of posterior margin; posterior sulcus absent or weak. Lunule variably expressed, indistinct to deep; escutcheon and auricle sometimes present. Hinge plate edentulous, sometimes with a small swelling. Ligament sunken, sometimes visible externally. Ctenidium of a single demibranch, foot vermiform, lateral body pouches without or with distinct lobes.
Gross anatomy: Mantle thin; margins thickened and unfused except limited interconnection at the posterior ventral margin with a small exhalant aperture below the posterior adductor muscle (Fig. 5A). Anterior adductor muscle elongated, two times longer than posterior adductor, slightly curved parallel to anterodorsal shell margin, dorsal part narrower than ventral part. Posterior adductor muscle small, oval. Ctenidium thin, narrow, consisting of a single inner demibranch with fully reflected filaments (up to 50 filaments in large specimens more than 4.7 mm in length). Demibranch not covering lateral body pouches and consisting of both ascending and descending lamellae; ascending lamellae slightly shorter than descending lamellae. Labial palps small (to 150 µm), triangular, located at proximal end of longer (more than 1 mm) oral grooves extending to antero-ventral corner of gill (Figs. 5B-5F). Lateral pouches large (Figs. 5B, 5C, 5J, 5M and 5N), dorsoventrally elongated, oval in outline, with undulated margins and a few small, slightly projecting lobes along different margins; each pouch connecting to body by a wide neck (Figs. 5D-5F). Kidneys large, elongated, occupying a posterodorsal position between posterior adductor muscle and heart, containing numerous, bright orange, large (to 80 µm in diameter), different-size granules (Figs. 5D, 5J and 5H). Gonad occupying inner side of lateral pouches. Sexes are separate. Eggs oval or polygonal (up to 120-130 µm in length after fixation) (Fig. 5L). Alimentary system with short esophagus leading to a relatively large, elongate stomach; combined style sac and strongly curved midgut forming a deep and narrow loop between neck of lateral pouches and kidney; hind gut forming an anterior, deep, and wide loop dorsal to style sac, running posteriorly dorsal to kidney and posterior adductor muscle, opening at ventral side of posterior adductor muscle (Figs. 5I and 5P). Foot long, vermiform, depending upon its state of contraction, may either form coil within mantle cavity or form a relatively short, curved steam with bulbous tip; surface of bulbous portion with densely spaced papillae (Figs. 5D, 5K and 5N). Heel absent. Anterior and posterior pedal retractors short, narrow (Figs. 5D and 5F).
Variability: The shell shape and proportions, the degree of slope of anterodorsal and posterodorsal shell margins and curving of all shell margins vary significantly among different sized specimens (Figs. 2-4 and Table 2). Some specimens have a shell rather elongated dorsoventrally and ovate in outline. In some specimens, the shell is more rounded and angulate, the anterodorsal and posterodorsal margins are sloped more gently from beaks, the anterodorsal margin is more convex and the posterodorsal margin is almost straight, the ventral margin is more curved and slightly drawn out anteriorly. Moreover, in thicker-shelled specimens the hinge plate is wider and the inner thickenings of the shell in the аdductor muscle scar areas are much thicker and more expressed compared with specimens with a thinner shell (Figs Comparisons: Axinulus krylovae sp. nov. strongly differs from all species of the genus Axinulus in having expressed thickenings of the shell in the adductor muscle scar areas, which are visible on the outer surface of the anterior and posterior parts of the shell as wide radial rays extending from the beaks (Table 3). Due to the shell thickening, the muscle scars are raised to various degrees above the inner shell surface, depending on the thickness of the shell itself. Also, Axinulus krylovae sp. nov. is clearly distinguished from all the other species by a unique sculpture of the prodissoconch (Table 3). Moreover, the new species well differs from almost all species of the genus Axinulus in having a conspicuous radial sculpture of the shell. Judging by photos of the type material and descriptions of species, A. brevis, A. croulensis, and A. philippinensis also have a radial sculpture on the shell surface, but it is less distinct and was described as fine radial lines or a peculiar radial texture (Oliver & Killeen, 2002;Allen, 2015). In contrast to most species of the genus, the new species has a larger shell. Only A. oliveri and A. roseus also have a large shell, but  unlike Axinulus krylovae sp. nov., their shells lack the radial sculpture and inner thickenings in the adductor muscle scar areas and are characterized by a well defined escutcheon (Kamenev, 2020). In addition, the above species have extensively lobed lateral pouches.
Derivation of name: The new species was named in honor of the well-known malacologist E.M. Krylova, who made a significant contribution to the study of the world fauna of the family Vesicomyiidae and the superfamily Cuspidarioidea and has always provided invaluable help in my research on the deep-sea bivalve fauna of the northern Pacific.
Remarks: Axinulus krylovae sp. nov. has more or less expressed inner thickenings of the shell, depending on the shell thickness, in the area of adductor muscle scars. As a result, the muscle scars are raised above the inner shell surface. The thickened shell area is, as a rule, more pronounced posteriorly than anteriorly. The presence of elevated adductor scars is a characteristic and main distinguishing feature of species of the genus Genaxinus Iredale, 1930 from other genera of the family Thyasiridae (Hedley, 1907;Iredale, 1930;Oliver & Killeen, 2002;Oliver & Levin, 2006;Oliver, 2015). Nevertheless, I think the new species should be assigned to the genus Axinulus but not to Genaxinus. In species of Genaxinus, only muscle scars themselves are raised. The new species has the thickened anterior and posterior parts of shell, which are larger in area than the muscle scars proper, and these thickened areas bear muscle scars. The degree of inner shell thickening significantly varies, thus influencing the degree of elevation of muscle scars above the inner shell surface. Unlike the species described herein, in species of the genus Genaxinus, muscle scars bear prominent concentric growth marks that were formed during migration of the muscle during growth. In the Genaxinus species, the muscle scars are white color, project from the inner shell surface, and are well visible through the shell from outside (Payne & Allen, 1991;Oliver & Killeen, 2002). The thickenings of the shell in muscle scar areas analogous to those of Axinulus krylovae sp. nov. are also characteristic of other species of the Thyasiridae (e.g., A. croulinensis, A. alleni, Mendicula ultima (Payne & Allen, 1991)) (Payne & Allen, 1991;Oliver & Killeen, 2002). In these species, inner shell thickening is also more expressed in the posterior area of the shell. Most probably, thorough studies of small thyasirids using scanning electron microscopy will reveal analogous inner shell thickenings in some other species as well.
The new species described here has a wide geographic and vertical distribution in the northwestern Pacific Ocean. It was found in large numbers in many samples from this vast region. This allowed a study of both the age and individual variability of its shell morphology. On the whole, the shell shape and proportions vary greatly, irrespective of the sampling area and depth. Specimens found in different areas and at different depths were of different size and had both the rounded and dorsoventrally elongated shell shape. A thorough examination of the shell morphology of specimens from various parts of the species range showed that they have the same main morphological characters (shell sculpture, hinge plate morphology, inner shell thickenings in muscle scar areas, prodissoconch sculpture). I found no unique morphological and anatomical features characteristic of a group of specimens. Moreover, specimens with a transitional shape of the shell between elongated oval and almost rounded were found in samples. Therefore, I think that specimens with different shell shape belong to the same species. It should be noted that many species of the Thyasiridae, among them species of Axinulus, exhibit a large age and individual variability of the shell shape and proportions (Payne & Allen, 1991;Oliver & Killeen, 2002;Allen, 2015;Kamenev, 2020).
Axinulus krylovae sp. nov. was found in the Pacific Ocean in a wide depth range (about 5,000 m). An earlier study recorded three species of bivalves with a vertical distribution range of 4,000 to 7,000 m among the hadal fauna of the Kuril-Kamchatka Trench. In addition, a number of eurybathic species from various taxonomic groups have a vertical distribution range greater than 5,000-7,000 m (Kamenev, 2019). By all appearances, A. krylovae sp. nov. also belongs to the group of eurybathic species inhabiting a wide depth range in the abyssal and hadal zones of the Pacific Ocean. Other material examined: 89 live specimens (Table 4).
Diagnosis: Shell small (to 2.9 mm in length), oval to ovate-polygonal, slightly drawn out anterior, with two slightly curved, radiating, whitish, elongate-triangular, opaque rays extend from beaks to anteroventral and posteroventral margins. Sculpture of closely spaced, commarginal riblets. Micro-sculpture of densely spaced pits. Posterior folds and sulcus absent. Escutcheon long, narrow, shallow. Auricle long, low. Lunule as a weak crest, slightly raised, short, lanceolate. Ligament sunken, not visible externally, short. Prodissoconch medium in size (length 161-174 µm); initial part with small area of densely spaced, short, curved folds and wrinkles and two series of commarginal, long, thin folds extending as wings from this area. Adductor muscle scars distinct, elongated triangular in outline; posterior scar raised above inner shell surface due to thickening of shell. Lateral body pouches small, simple, without projecting lobes.
Description. Shell small (to 2.9 mm in length and 3.1 mm in height), inflated (W/L = 0.618 ± 0.024), higher than long (H/L = 1.105 ± 0.026), slightly drawn out anteriorly, median area divided by a weak change in angulation; oval to ovate-polygonal, sometimes slightly pyriform, subequilateral, white, thin, translucent, with two slightly curved, radiating, whitish, elongate-triangular, opaque rays extending from beaks to anteroventral and posteroventral margins, respectively, more marked in posterior shell area, formed by internal thickening of shell associated with anterior and posterior adductor muscles; patches of silty deposit adhering to anterior and posterior shell margins ( Fig. 6 and Table 5). Periostracum very thin, colorless, translucent, adherent. Dissoconch sculptured with thin, closely spaced, commarginal riblets. Micro-sculpture of small, densely spaced pits. Beaks small, prosogyrate, curved slightly inwards, anterior to midline (A/L = 0.369 ± 0.033) (Fig. 7 and Table 5). Anterodorsal shell margin long, convex, steeply sloping from beaks, forming a rounded angle at transition to anterior margin. Anterior shell margin curved, smoothly transitioning to ventral margin. Ventral margin strongly curved, slightly angulate. Posterodorsal shell margin long, convex, steeply sloping from beaks, forming a broadly rounded angle at transition to posterior margin. Posterior shell margin slightly curved, smoothly transitioning to ventral margin. Posterior folds absent but posterior shell area a little flattened. Escutcheon long, almost as long as entire length of posterodorsal shell margin, narrow, shallow, demarcated by low ridges. Auricle long, almost as long as entire length of escutcheon, weak, low, only slightly projecting (Figs. 7B, 7C and 7G). Lunule as a weak crest, slightly raised, short, about half length of anterodorsal shell margin, narrow, lanceolate, weakly defined by low and thin ribs (Fig. 7D). Ligament opisthodetic, sunken, not visible externally, short, about one third of entire length of the escutcheon, thick, almost straight, lying in deep, wide groove in hinge plate (Figs. 7L-7N, 7P-7R). Prodissoconch medium in size (length 161-174 µm), distinctly separated from dissoconch, ovate in outline. Initial part of prodissoconch with densely spaced, short, curved folds and wrinkles interspersed with pits, forming a rounded zone. This zone giving rise to two series of commarginal, long, thin, low folds (about 20 folds in each series), extending as wings in different directions; remaining surface of prodissoconch smooth (Fig. 8). Hinge plate thin, edentulous, with almost straight ligamental groove (Figs. 7L and 7P). Adductor muscle scars distinct, long, elongated triangular in outline, extending into umbonal cavity, bearing thin, radial striation. Posterior adductor scar narrow, straight, raised above inner shell surface due to thickening of shell in this area. Anterior adductor scar wider than posterior one, curved, not raised above inner shell surface (Figs. 7K, 7M, 7O and 7Q). Anatomy: Mantle thin; margins thickened, unfused except long posterior suture with two exhalant apertures below the posterior adductor (Fig. 9A). Anterior adductor muscle elongated, two times longer than posterior adductor, slightly curved, dorsal part narrower than ventral part. Posterior adductor muscle small, oval (Figs. 9B and 9C). Ctenidium thin, narrow, consisting of a single inner demibranch with fully reflected filaments (up to 30 filaments in largest specimen 2.9 mm in shell length). Demibranch not covering lateral body pouches and consisting of both ascending and descending lamellae; ascending lamellae slightly shorter than descending lamellae. Labial palps small, as extensions of the short (about 300 µm), oral grooves (Fig. 9B). Lateral body pouches small (Figs. 9C and 9F), dorsoventrally elongated, outline oval, with undulated margins, without projecting lobes; each pouch connecting to body by a wide neck (Fig. 9D). Kidneys large, dorsoventrally elongated along posterodorsal shell margin, without granules (Figs. 7C-7G). Alimentary system with short oesoghagus leading to an elongate stomach; combined style sac and midgut strongly curved, forming a deep and narrow loop between neck of lateral pouches and kidney; hind gut forming an anterior, deep, and wide loop dorsal to style sac, running posteriorly dorsal to kidney and posterior adductor muscle, opening at ventral side of posterior adductor muscle (Figs. 7E and 7H). Foot long, vermiform, distally bulbous, with a muscular ring at the junction with the visceral mass. Bulbous portion of foot not divided into two distinct parts; surface with densely spaced papillae; heel absent. Anterior and posterior pedal retractors short, narrow (Figs. 7B, 7C and 7F).
Variability: The shell shape and proportions vary little among different-sized specimens (Figs. 6G-6M and Table 5). The shell shape is oval to ovate. Some specimens have a more  angular and relatively shortened shell. In small specimens (less than 2 mm in shell length), the shell is more elongated dorsoventrally, with a more anteriorly drown-out ventral margin.  (Payne & Allen, 1991). Axinulus alatus sp. nov. is similar in shell shape and proportions to some species of the genus Adontorhina Berry, 1947, but it is readily distinguished from them by the absence of irregular, minute granules on the hinge plate (Scott, 1986;Kamenev, 1996Kamenev, , 2013Coan, Scott & Bernard, 2000;Güller & Zelaya, 2011;Coan & Valentich-Scott, 2012;Valentich-Scott, Coan & Zelaya, 2020).
Derivation of name: The species epithet meaning "winged" indicates the similarity of two series of long, commarginal folds of the prodissoconch to the extended wings of a flying bird.
Remarks: Axinulus alatus sp. nov. has a wide range in the northern Pacific Ocean and was found off the Asian and American continents in the lower abyssal zone in the depth range of 3,000-5,000 m. The species was not found among extensive material of bivalves collected by the numerous deep-sea expeditions at depths greater than 5,000 m at the abyssal plain adjacent to the Kuril-Kamchatka and Japan trenches (Okutani, 1974(Okutani, , 2000Okutani & Kawamura, 2002;Kamenev, 2015Kamenev, , 2018a, as well as in the hadal zone of the Aleutian, Kuril-Kamchatka, and Japan trenches (Okutani, 1974(Okutani, , 2003Fujikura et al., 1999Fujikura et al., , 2002Okutani & Fujiwara, 2005;Kamenev, 2019Kamenev, , 2020Kamenev, , 2023. In addition, this species was not recorded for the benthic fauna of the Kuril Basin (the Sea of Okhotsk) a little less than 3,300 m deep (Kamenev, 2018a), though it was found on the oceanic slope of the Kuril Islands opposite the deepest Bussol Strait connecting the Sea of Okhotsk and the Pacific Ocean. The deep-sea ecosystem of the Kuril Basin is characterized by low oxygen concentrations near the bottom, largely influencing the composition and quantitative distribution of benthic fauna (Kamenev et al., 2022). It is possible that oxygen deficiency exerts a negative influence, not allowing the species to live in the Kuril Basin. In the Commander Basin (Bering Sea), in contrast to the Kuril Basin, A. alatus sp. nov. forms high-density populations. Probably, A. alatus sp. nov. is a low abyssal species, preferring depths of 3,000-5,000 m.
( Fig. 10  Diagnosis: Shell medium in size (to 3.6 mm in length), pyriform. Sculpture of closely spaced, commarginal ribs and narrow, radial rays consisting of fin, short, concentric wrinkles. Posterior folds and sulcus absent. Escutcheon and auricle absent. Lunule as a weak crest, raised, long, wide, weakly defined. Ligament sunken, long. Prodissoconch small (length 127 µm), with 24 thin, almost straight folds of different length, extending from high crest, located in mid-line of prodissoconch. Adductor muscle scars distinct, elongated triangular in outline, not raised above inner surface of shell. Inner shell surface with distinct, thin, radial striae.
Description: Shell medium in size (to 3.6 mm in length and 3.8 mm in height), pyriform, equivalve, subequilateral, white, thick, inflated, slightly higher than long (H/L = 1.043) (Fig. 10 Distribution: This species is known only from the holotype locality in the Kuril-Kamchatka Trench at a depth of 6,168-6,164 m. Comparisons: Axinulus cristatus sp. nov. is readily distinguished from all species of the genus Axinulus in having a radial sculpture in the form of rays consisting of closely spaced, short, fin, wrinkles and distinct radial lines on the inner shell surface, as well as by its unique sculpture of the prodissococh (Table 3). In addition, the new species differs from other species of the genus Axinulus in having a distinct spoon-shaped tubercle on the hinge plate in the left valve. However, the unusual shape of the tubercle on the hinge plate may be an individual abnormality of the examined specimen rather than being a specific character.
Derivation of name: The species name means "crested" and refers to a high crest on the surface of the prodissoconch.
Remarks: Unfortunately, only a single specimen of A. cristatus sp. nov. was found after examining all the material collected by many expeditions in the northwestern Pacific Ocean. Therefore, the gross anatomy of the body of this species is not studied, because the body has partially dissolved during preparation of the specimen for examination in a scanning microscope. In the specimen described here, the gill had only one demibranch; therefore, I assigned the species to the genus Axinulus. Since the shell of the species has a number of unique morphological characters that readily separate it from the congeners, I described it as new to science, despite the minimal material at my disposal. Due to the difficulty of collecting bottom animals from great depths, many deep-sea species are often represented by a single specimen in the benthic fauna collections made by different deep-sea expeditions (Kamenev, 2015(Kamenev, , 2019 and are thus described from merely 1-2 specimens (e.g., Knudsen, 1970;Payne & Allen, 1991).

DISCUSSION
Until recently, the genus Axinulus was represented only by six species (Zelaya, 2010;Allen, 2015;Oliver, 2015;Kamenev, 2020). Of these, five species were found in the Atlantic Ocean (Payne & Allen, 1991;Allen, 2008;Zelaya, 2010), while in the Pacific Ocean only one species of the genus, A. philippinensis, was recorded and described from the bottom of the Philippine Trench (Allen, 2015). In recent years, a study of the extensive material of bivalves collected by many deep-sea expeditions mainly in the northwestern Pacific Ocean, another five species of the genus Axinulus were recorded for the Pacific Ocean (Kamenev, 2020). Thus, the number of species in the genus has increased almost twice, and at present the fauna of Axinulus in the Pacific Ocean has more species, compared to the Atlantic Ocean.
Out of all the species found in the Atlantic Ocean, only A. subequatorius was recorded exclusively in the abyssal zone at depths of more than 3,000 m. The other species have a wider vertical distribution range and were recorded in the subtidal and bathyal zones at depths less than 3,000 m. In contrast to Atlantic species, all Pacific species of the genus Axinulus were found exclusively at depths greater than 3,000 m. Moreover, almost all of them, except A. alatus sp. nov., were recorded in the hadal zone of different oceanic trenches at depths of more than 6,000 m (Allen, 2015;Kamenev, 2020). Thus, unlike the Atlantic Ocean, species of the genus Axinulus are typical members of the abyssal and hadal benthic fauna in the northern Pacific Ocean. In addition, the species A. roseus, A. krylovae sp. nov., A. philippinensis, and A. alatus sp. nov. were found in large numbers in samples collected from the floor of the deepest basins of the Kuril-Kamchatka, Japan, and Philippine trenches, and the Bering Sea, respectively (Allen, 2015;Kamenev, 2020;Kamenev et al., 2022), where they probably play a substantial role in the functioning of benthic deep-sea ecosystems in these northwestern Pacific regions.
Presumably, species of the genus Axinulus were found in another seven oceanic trenches, apart from the Kuril-Kamchatka, Japan, and Philippine trenches (Belyaev, 1989). Therefore, further study will most likely increase the species richness of the genus Axinulus.
Unlike many species of the genus Thyasira, many species of the genus Axinulus have a uniquely sculptured prodissoconch, which is one of the important diagnostic characters of thyasirids (Zelaya, 2009(Zelaya, , 2010Kamenev, 2020Kamenev, , 2023. The prodissoconch sculpture of A. brevis, A. alleni, A. subequatorius, and A. philippinensis has not been reported. Likewise, the prodissoconch of A. croulensis, which is smooth and lacks sculpture, was not examined with a scanning microscope (Oliver & Killeen, 2002). Perhaps, the prodissoconch of all the above species has an external sculpture that can be distinguished only at high magnification using scanning microscopy. It cannot be ruled out that further examination of the prodissoconch of these species will reveal more morphological features, allowing a more accurate diagnosis of largely morphologically similar species of the genus Axinulus.