INTRODUCTION
Many species of the family Pectinidae inhabit the deep waters of the bathyal (200–3000 m) and abyssal (3000–6000 m) zones of the World Ocean (Schein, Reference Schein1989; Coan et al., Reference Coan, Scott and Bernard2000; Okutani, Reference Okutani2000; Dijkstra & Kilburn, Reference Dijkstra and Kilburn2001; Allen, Reference Allen2008; Dijkstra & Maestrati, Reference Dijkstra, Maestrati, Heros, Cowie and Bouchet2008; Dijkstra & Marshall, Reference Dijkstra and Marshall2008; Huber, Reference Huber2010; Dijkstra, Reference Dijkstra2013). Thirty-eight species of this family have been recorded in the deep Atlantic at depths greater than 500 m. Pectinidae, along with Thyasiridae, Cuspidariidae and Verticortiidae, are one of the richest families, in terms of the number of species, among the lamellibranchs in the deep Atlantic (Allen, Reference Allen2008). However, most of the deep-water pectinids inhabit the bathyal zone. A small number of species of the family Pectinidae, most of them belonging to the genus Hyalopecten Verrill, Reference Verrill1897, have been found in the abyssal zone of the World Ocean (Knudsen, Reference Knudsen1970; Coan et al., Reference Coan, Scott and Bernard2000; Okutani, Reference Okutani2000; Allen, Reference Allen2008; Coan & Valentich-Scott, Reference Coan and Valentich-Scott2012).
The genus Hyalopecten comprises 10 species, of which Hyalopecten frigidus (Jensen, Reference Jensen1904), Hyalopecten neoceanicus (Dall, Reference Dall1908), Hyalopecten profundicola (Okutani, Reference Okutani1962) and Hyalopecten pudicus (Smith, Reference Smith, Thomson and Murray1885) were recorded in the abyssal zone (Smith, Reference Smith, Thomson and Murray1885; Dall, Reference Dall1908; Okutani, Reference Okutani1962, Reference Okutani1975; Knudsen, Reference Knudsen1970; Schein, Reference Schein1989; Dijkstra & Gofas, Reference Dijkstra and Gofas2004; Dijkstra et al., Reference Dijkstra, Waren and Gudmundsson2009; Coan & Valentich-Scott, Reference Coan and Valentich-Scott2012), and Hyalopecten hadalis (Knudsen, Reference Knudsen1970) in the hadal zone (depths below 6000 m) (Knudsen, Reference Knudsen1970; Dijkstra & Marshall, Reference Dijkstra and Marshall2008). Until recently, H. hadalis and Delectopecten vancouverensis (Whiteaves, 1893) were the only pectinids found in the hadal zone that had been identified to species level (Filatova, Reference Filatova1961; Knudsen, Reference Knudsen1970; Belyaev, Reference Belyaev1989). Species of the genus Hyalopecten have been recorded in many regions of the World Ocean (Schein, Reference Schein1989; Egorova, Reference Egorova1999; Okutani, Reference Okutani2000; Dijkstra & Marshall, Reference Dijkstra and Marshall2008; Coan & Valentich-Scott, Reference Coan and Valentich-Scott2012). However, most Hyalopecten species are very rare in samples and were generally described from a minimal amount of material (1–2 specimens). An exception is H. frigidus and H. pudicus, which are widespread in the Arctic and Atlantic oceans and off Antarctica, with large number of specimens found in benthic samples and deposited in museum collections (Knudsen, Reference Knudsen1970, Reference Knudsen1985; Bouchet & Waren, Reference Bouchet and Waren1979; Schein, Reference Schein1989; Dell, Reference Dell1990; Dijkstra & Goud, Reference Dijkstra and Goud2002; Aldea & Troncoso, Reference Aldea and Troncoso2008).
Only one species of Hyalopecten, H. profundicola, has so far been recorded in the North-western Pacific. It was found off the shores of Japan at depths of 3150–3450 m (Okutani, Reference Okutani1962, Reference Okutani1975). Probably, this is a rare abyssal species, as in the course of intensive studies of the bathyal, abyssal and hadal zones of the Pacific Ocean Japanese researchers have found only four specimens in a narrow depth range off the shores of Japan. During the period from 1949 to 1990, numerous Russian (more than 20) expeditions explored the deep-water benthic fauna of the Sea of Japan, the Sea of Okhotsk, the Bering Sea and the North-western Pacific, including the abyssal plain and oceanic trenches (Monin, Reference Monin1983; Belyaev, Reference Belyaev1989). Starting in 2010, investigations of the deep-sea fauna of this extensive region have been continued by joint expeditions of Russian and German researchers (Malyutina & Brandt, Reference Malyutina and Brandt2013; Brandt & Malyutina, Reference Brandt and Malyutina2015). As a result of these studies, a rich bottom fauna of the abyssal and hadal zones of the North-western Pacific has been discovered and a large number of species of bivalves have been described (Filatova, Reference Filatova1958, Reference Filatova1960, Reference Filatova1971, Reference Filatova1976; Ivanova, Reference Ivanova1977; Filatova & Schileyko, Reference Filatova and Schileyko1984, Reference Filatova and Schileyko1985; Krylova, Reference Krylova1993, Reference Krylova1995, Reference Krylova, Kuznetsov and Zezina1997; Kamenev, Reference Kamenev2014, Reference Kamenev2015; Krylova et al., Reference Krylova, Kamenev, Vladychenskaya and Petrov2015). Representatives of the order Pectinida have also been found in deep-water basins of the Sea of Japan, the Sea of Okhotsk, the Bering Sea, and on the abyssal plains and in oceanic trenches of the North-western Pacific at depths to 8100 m. However, all specimens with the exception of D. vancouverensis were not identified to the species level (Filatova, Reference Filatova1961; Belyaev, Reference Belyaev1966, Reference Belyaev1989; Scarlato, Reference Scarlato1981; Kamenev, Reference Kamenev2013, Reference Kamenev2015).
A thorough examination of the entire extensive material of bivalves collected by different expeditions during the period from 1949 to 2015 in the abyssal and hadal zones of the Sea of Japan, Sea of Okhotsk, Bering Sea and the North-western Pacific has revealed three new species of Hyalopecten. Two species were found on the abyssal plain (depth 4550–5045 m) adjacent to the Kuril-Kamchatka and Aleutian trenches and one species was discovered in the hadal zone of the Kuril-Kamchatka and Aleutian trenches (depth 6090–8100 m). To date, the latter species is the most deeply distributed species of the order Pectinida. Therefore, the present paper focuses on the description of the new species on the basis of conchological characters and a detailed comparison with all species of the genus Hyalopecten.
MATERIALS AND METHODS
Material studied
The material of Hyalopecten species was collected by IO RAS expeditions from the abyssal plain of the Pacific Ocean adjacent to the Kuril-Kamchatka and Aleutian trenches and in the hadal zone of the Kuril-Kamchatka and Aleutian trenches (RV ‘Vityaz’, cruise no. 2, 7 August–20 October 1949; RV ‘Vityaz’, cruise no. 14, 2 May–5 July 1953; RV ‘Vityaz’, cruise no. 20, 28 April–14 June 1955; RV ‘Vityaz’, cruise no. 39, 7 July–13 September 1966). All material of these species was fixed and stored in 70% ethanol in the IO RAS.
The following were used for comparison purposes: the type material of Hyalopecten profundicola (Okutani, Reference Okutani1962) (holotype NMNS Mo 69702), photos of type and other materials of Hyalopecten neoceanicus (Dall, Reference Dall1908) (holotype USNM 110579, photos from USNM website (information provided with the permission of the National Museum of Natural History, Smithsonian Institution, 10th and Constitution Ave. N.W., Washington, DC 20560–0193 (http://www.nmnh.si.edu/)), Hyalopecten pudicus (Smith, Reference Smith, Thomson and Murray1885) (images of holotype (BMNH 1887.2.9.3280) from Marine Bivalve Shells of the British Isles website (Oliver et al., Reference Oliver, Holmes, Killeen and Turner2010), image numbers: NMW M011625–11628), Hyalopecten frigidus (Jensen, Reference Jensen1904) (images from Marine Bivalve Shells of the British Isles website (Oliver et al., Reference Oliver, Holmes, Killeen and Turner2010), image numbers: NMW M012784–12786, M012827, M012830), Hyalopecten mireilleae Dijkstra, Reference Dijkstra and Bouchet1995 (holotype, http://coldb.mnhn.fr/catalognumber/mnhn/im/2000-24350, Muséum national d'Histoire naturelle, Paris (France), Collection: Molluscs (IM), Specimen MNHN-IM-2000-24350), Hyalopecten undatus (Verrill & S. Smith in Verrill, Reference Verrill1885) (holotype USNM 44827, photos from USNM website (Information provided with the permission of the National Museum of Natural History, Smithsonian Institution, 10th and Constitution Ave. N.W., Washington, DC 20560–0193 (http://www.nmnh.si.edu/)); type material of different species of the families Propeamussiidae and Pectinidae deposited in CAS, LACM, NMNS, SBNHM.
Shell measurements
Figure 1 shows the shell morphology measurements. Shell length (L), height (H), anterior end length (A), auricles length (AL), anterior auricle length (AAL) and umbonal angle (UA) were measured for shells. The ratios of these parameters to shell length (A/L, H/L, AA/L, AAL/L, respectively) were determined. Shell measurements were made using a caliper and an ocular micrometer with an accuracy of 0.1 mm.
Fig. 1. Placement of shell measurements: L, shell length; H, height; A, anterior end length; AL, auricles length; AAL, anterior auricle length; UA, umbonal angle.
Methods
For scanning electron microscopy, shells were cleaned of traces of soft tissues and periostracum in 50% diluted commercial bleach, washed in distilled water, and dried. They were then mounted onto aluminium stubs using an adhesive tape and coated with carbon for examination with an EVO 40XVP and SIGMA 300VP (Carl Zeiss, Cambridge, UK). Shells of holotypes were only washed in distilled water and dried. Uncoated shells of holotypes were also used for examination with scanning electron microscopes.
Abbreviations
The following institutional abbreviations are used in the paper: BMNH – The Natural History Museum, London, UK; CAS – California Academy of Sciences, San Francisco; IMB – A.V. Zhirmunsky Institute of Marine Biology, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia; IO RAS – P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia; LACM – Natural History Museum of Los Angeles County, Los Angeles, USA; MIMB – Museum of the Institute of Marine Biology, Vladivostok, Russia; NMW – National Museums & Galleries of Wales, Cardiff, UK; MNHM – Muséum national d'Histoire naturelle, Paris, France; NMNS – National Museum of Nature and Science, Tsukuba, Japan; SBNHM – Santa Barbara of the Natural History Museum, Santa Barbara, USA; USNM – United States National Museum of Natural History, Smithsonian Institute, Washington, DC, USA; ZIN – Zoological Institute, Russian Academy of Sciences, St. Petersburg, Russia; ZMA – Zoological Museum, University of Amsterdam, Amsterdam, the Netherlands; ZMMU – Zoological Museum of Moscow State University, Moscow, Russia; ZMUC – Zoological Museum, University of Copenhagen, Denmark.
SYSTEMATICS
Order PECTINIDA Gray, 1854
Superfamily PECTINOIDEA Rafinesque, 1815
Family PECTINIDAE Rafinesque, 1815
Subfamily CAMPTONECTINAE Habe, 1977
Genus Hyalopecten Verrill, 1897
Type species (by original designation): Pecten undatus Verrill & S. Smith in Verrill, Reference Verrill1885
DIAGNOSIS
Shell up to 40 mm in height, oblique, thin, fragile, subequilateral, compressed to inflated, inequivalve, with left valve more convex than right. Valves with commarginal undulations or regular, lamellate ribs and radial ribs or riblets. Auricles unequal, posterior auricles often continuous with disc; anterior auricles distinct. Byssal notch rather deep; ctenolium present.
Hyalopecten vityazi sp. nov.
(Figures 2 & 3, Table 1)
Delectopecten sp.: Belyaev, Reference Belyaev1966, p. 116; Belyaev, Reference Belyaev1989, p. 216.
Fig. 2. Hyalopecten vityazi sp. nov., holotype (ZMMU Ld-3055), shell length 31.6 mm: (A) exterior view of right valve; (B) exterior view of left valve; (С) ctenolium; (D) prodissoconch; (E–H) auricles of left valve; (I, J) sculpture of left valve; (K–M) auricles of right valve; (N) sculpture of right valve. Scale bars: C–D, 500 µm; E–N, 1 mm.
Fig. 3. Hyalopecten vityazi sp. nov. A–E: Paratype (ZMMU Ld-3057), Kuril-Kamchatka Trench (44°55.5′N 152°24′E), 8100 m: (A) exterior view of left valve, valve length 27.0 mm; (B) interior view of auricles; (C) sculpture of valve; (D, E) anterior and posterior auricles. (F–M) Paratype (ZMMU Ld-3056), Aleutian Trench (52°25,8′N, 170°53,9′E), 7246 m: (F, H) exterior view of both valves, shell length 33.0 mm; (I, J) anterior and posterior auricles of right valve; (K, L) anterior and posterior auricles of left valve; (M) ctenolium; (N) ctenolium of damaged specimen from Kuril-Kamchatka Trench (46°N, 153°27′E), 7265–7295 m. (O–S) Scanning electron micrographs of fragment of right valve from Kuril-Kamchatka Trench (45°32′N 153°46′E), 6675–6710 m: (O, P) prodissoconch; (R, S): frontal and dorsal views of ctenolium. Scale bars: B–E, I–N, 1 mm; O, P, 100 µm; R, S, 500 µm.
Table 1. Hyalopecten vityazi sp. nov. shell measurements (mm) and indices.
L, shell length; H, height; A, anterior end length; AL, auricles length; AAL, anterior auricle length; “-” – no data.
Propeamussium sp.: Belyaev, Reference Belyaev1966, p. 116; Belyaev, Reference Belyaev1989, p. 216 (for the Aleutian and Kuril-Kamchatka trenches).
TYPE MATERIAL AND LOCALITY
Holotype: damaged specimen. (RV ‘Vityaz’, cruise 39, station 5609, Kuril-Kamchatka Trench; coordinates: 46°06′N 153°18′E; water depth: 6090–6235 m; Sigsbee trawl) (ZMMU Ld-3055); coll. L.A. Zenkevich, 23 July 1966.
Paratype: slightly damaged left valve. (RV ‘Vityaz’, cruise 2, station 162, Kuril-Kamchatka Trench; coordinates: 44°55.5′N 152°24′E; water depth: 8100 m, Sigsbee trawl) (ZMMU Ld-3057); coll. L.A. Zenkevich, 10 October 1949.
Paratype: damaged specimen. (RV ‘Vityaz’, cruise 20, station 3357, Aleutian Trench; coordinates: 52°25.8′N 170°53.9′E; water depth: 7246 m, Sigsbee trawl) (ZMMU Ld-3056); coll. V.P. Petelin, 7 June 1955.
OTHER MATERIAL EXAMINED
Two heavily damaged specimens, one left and one right valves (IO RAS) from holotype locality; one heavily damaged specimen, two heavily damaged right and one heavily damaged left valves (IO RAS) from paratype locality, Aleutian Trench; two heavily damaged specimens (IO RAS), Kuril-Kamchatka Trench (46°N 153°27′E), 7265–7295 m, Sigsbee trawl, coll. L.A. Zenkevich, 22 July 1966 (RV ‘Vityaz’, cruise 39, station 5608); four heavily damaged specimens and right valve (IO RAS), Kuril-Kamchatka Trench (45°32′N 153°46′E), 6675–6710 m, Sigsbee trawl, coll. L.A. Zenkevich, 6 August 1966 (RV ‘Vityaz’, cruise 39, station 5617); two heavily damaged specimens (IO RAS), Aleutian Trench (53°53.2′N 166°55.6′E), 6410–6757 m, Sigsbee trawl, coll. V.P. Petelin, 1 June 1955 (RV ‘Vityaz’, cruise 20, station 3340); two heavily damaged right and one left valves (IO RAS), Kuril-Kamchatka Trench (48°25′N 156°34′E), 6860 m, Sigsbee trawl, coll. L.A. Zenkevich, 1 June 1953 (RV ‘Vityaz’, cruise 14, station 2144). Total of 11 heavily damaged specimens and nine heavily damaged valves.
DIAGNOSIS
Shell large (to 40.0 mm in height), thin, slightly higher than longer, inequilateral. Umbonal angle 100°. Auricles small, unequal in length and shape. Right valve disc with well-developed, commarginal undulations and closely spaced, very thin, overlying radial riblets; anterior auricle demarcated from disc, with fine, lamellate, commarginal riblets overlying 4–9 coarse radial ribs; posterior auricle almost continuous with shell disc, with reticulate sculpture. Left valve disc with reticulate sculpture from commarginal, lamellate ridges and thin, more closely spaced, intercalated, radial ribs; anterior and posterior auricles weakly separated from shell disc, with reticulate sculpture. Byssal fasciole broad. Byssal notch moderately deep, rounded. Inactive and active ctenolium well-developed. Prodissoconch very large (length 300–340 µm), D-shaped.
DESCRIPTION
Shell large (to 40.0 mm in height), opaque, thin, fragile, weakly inflated, slightly higher than longer (H/L = 1.070–1.142), inequilateral (beaks slightly posterior to midline, A/L = 0.550–0.556), inequivalve (left valve more convex than right valve). Shell disc tear-shaped. Dorsal shell margin straight. Anterior margin rounded. Posterior margin straight or slightly convex, smoothly transitioning to rounded ventral shell margin. Auricles small (AL/L = 0.367–0.458), anterior auricle slightly larger and longer than posterior (AAL/AL = 0.529–0.587). Umbonal angle 100°.
Right valve disc with well-developed, numerous, rounded, variously high and broad, rather regularly spaced, commarginal undulations and closely spaced, very thin, rounded, overlying radial riblets slightly changing direction in between undulations and separated by interstices broader than riblets. Anterior auricle slightly twisted to the right, sharply demarcated from shell disc by a deep furrow with a long and well-developed ctenolium and a sharp suture (between furrow and shell disc). Surface of auricle with fine, lamellate, commarginal riblets overlying 4–9 coarse, radial ribs on dorsal half of auricle and forming barely visible noduloses at intersections with radial ribs and serration of auricle dorsal margin. Byssal fasciole broad. Byssal notch moderately deep, rounded. Active ctenolium short. Posterior auricle weakly separated and almost continuous with shell disc; sculptured with thin, closely spaced, commarginal, lamellate ribs and very weak, dense, radial riblets.
Left valve disc with reticulate sculpture from rather regularly spaced, numerous, commarginal, lamellate, slightly corrugated ridges with wide interspaces (0.8–0.9 mm) and low, thin, rounded, more closely spaced (0.3–0.4 mm) intercalated, radial ribs. Anterior and posterior auricles weakly separated from shell disc; surface with reticulate sculpture from closely spaced, thin, lamellate, commarginal and rounded, radial ribs.
Prodissoconch very large (length 300–340 µm), smooth, D-shaped, convex, distinct, sharply separated from shell disc.
VARIABILITY
The variability of shell form and proportions of Hyalopecten vityazi is difficult to assess, as all the shells and separate valves examined were damaged to a varying degree. Nevertheless, it can be noted that the degree of separation of the posterior auricle from the shell disc varies in this species. In most specimens from the Aleutian Trench, the posterior auricles are not separated and are almost continuous with the shell disc, and the posterior shell margin is straight, as opposed to specimens from the Kuril-Kamchatka Trench, in which the posterior auricles are more separated and the posterior shell margin is slightly convex.
DISTRIBUTION AND HABITAT (FIGURE 4)
This species was recorded in the Kuril-Kamchatka Trench at а depth from 6090 (46°06′N 153°18′E) to 8100 m (44°55.5′N 152°24′E) (bottom temperature 1.65–2.15°C, salinity 34.7‰ (Belyaev, Reference Belyaev1989)); in the Aleutian Trench, at a depth from 6410 (53°53.2′N, 166°55.6′E) to 7246 m (52°25.8′N, 170°53.9′E) (bottom temperature 1.78°C at a depth 6328 m, salinity 34.7‰ (Belyaev, Reference Belyaev1989)).
Fig. 4. Distribution of Hyalopecten species. ● Hyalopecten vityazi (○ type locality), ■ Hyalopecten abyssalis (□ type locality), ▴ Hyalopecten kurilensis.
COMPARISONS
Hyalopecten vityazi differs from all species of the genus Hyalopecten by its almost regular teardrop-shaped shell but with a slightly produced anterior part, the relatively small length of the auricles, the presence of well-developed undulations only on the right valve and prominent reticulate sculpture on the left valve, and the presence of a large prodissoconch (Figure 5, Table 2). This is the only species of the genus Hyalopecten that has undulations and reticulate sculpture on different shell valves. Hyalopecten mireilleae, Hyalopecten neoceanicus and Hyalopecten profundicola also possess variously expressed reticulate sculpture of the shell. However, these species are readily distinguished from the new species by having reticulate sculpture on both valves, a strongly produced anterior part of the shell, and large auricles. Moreover, H. mireilleae and H. profundicola lack undulations on both valves, while H. neoceanicus, on the contrary, in addition to reticulate sculpture, has undulations on both valves. In terms of shell size, form and proportions, H. vityazi is most similar to Hyalopecten frigidus, Hyalopeten hadalis and Hyalopecten pudicus; however, it markedly differs from them by having no well-developed undulations and by having reticulate sculpture on the left valve, as well as a less distinctly marked and non-pointed posterior auricles, a less deeper byssal notch and a much larger prodissoconch.
Fig. 5. Species of the genus Hyalopecten: (A) Hyalopecten neoceanicus, holotype (USNM 110579), right valve, shell length 12.0 mm (Photos from USNM website, ‘Information provided with the permission of the National Museum of Natural History, Smithsonian Institution, 10th and Constitution Ave. N.W., Washington, DC 20560–0193 (http://www.nmnh.si.edu/)’); (B) Hyalopecten strigillatus, syntype (USNM 94355), right valve, shell height 10.0 mm (from Dall, Reference Dall1889b); (C, D) Hyalopecten profundicola, holotype (NMNS Mo 69702), shell length 4.5 mm; (E, F) Hyalopecten arntzi, holotype (ZIN), shell length 10.2 mm (from Egorova, Reference Egorova1999); (H, I) Hyalopecten hadalis, holotype (ZMUC BIV-441), shell length 18.2 mm (from Knudsen, Reference Knudsen1970); (J, K) Hyalopecten pudicus, holotype (BMNH 1887.2.9.3280), shell length 18.5 mm (photos from Marine Bivalve Shells of the British Isles website (Oliver et al., Reference Oliver, Holmes, Killeen and Turner2010), image numbers: NMW M011625, 11628); (L), Hyalopecten tydemani, holotype (ZMA Moll. 3.89.009), left valve, shell length 7.5 mm (from Dijkstra, Reference Dijkstra1990); (M), Hyalopecten bavayi, holotype (ZMA Moll. 3.89.008), left valve, shell length 7.0 mm (from Dijkstra, Reference Dijkstra1990); (N), Hyalopecten mireilleae, holotype (MNHN-IM-2000–24350), left valve, shell length 11.0 mm (photos from Muséum national d'Histoire naturelle website http://coldb.mnhn.fr/catalognumber/mnhn/im/2000–24350); (O-R), Hyalopecten frigidus: (O, P), lectotype (ZMUC BIV-442) (Dijkstra et al., Reference Dijkstra, Waren and Gudmundsson2009), shell length 27.0 mm (from Jensen, Reference Jensen1904); (R), paralectotype (ZMUC), right valve (photos from Marine Bivalve Shells of the British Isles website (Oliver et al., Reference Oliver, Holmes, Killeen and Turner2010), image number NMW M012827), scale bar: 10 mm.
Table 2. Main differentiating characters of Hyalopecten species.
L, shell length; H, height; A, anterior end length; AL, auricles length; AAL, anterior auricle length.
ETYMOLOGY
The species name honours the famous RV ‘Vityaz’, on board of which the first deep-water expedition in 1949 discovered this species at a depth of 8100 m in the Kuril-Kamchatka Trench.
REMARKS
Over the whole period of studies (from 1949 to 1966) in the Kuril-Kamchatka Trench with a maximum depth of 9717 m, 26 trawl samples were taken at depths from 6080 to 9530 m, of which 13 samples were collected at depths greater than 8100 m (Belyaev, Reference Belyaev1989). Hyalopecten vityazi was found in five (38.5%) out of the 13 trawl samples collected at depths only to 8100 m (Belyaev, Reference Belyaev1989). In the Aleutian Trench (maximum depth 7822 m) during the period from 1955 to 1969, a total of eight trawl samples were collected in the depth range of 6296–7286 m. Hyalopecten vityazi was found in this trench to the maximum depth of study in two samples (25% of the total number of trawl samples) collected in the western part of the trench. Thus, despite the difficulties in sampling H. vityazi due to its large, thin and very fragile shell, which is readily broken into small pieces while collecting and washing on the ship's deck, this species was fairly often present in trawl samples at depths to 8100 m. Moreover, several specimens and empty valves were also found in most trawl samples with H. vityazi from each of the trenches. It seems likely that this species is fairly common to the hadal fauna of the Kuril-Kamchatka and Aleutian trenches and occurs there in large numbers at depths to 8100 m. Since H. vityazi was not found at depths below 8100 m despite the large number of samples from greater depths, it is not improbable that 8000–8500 m depths are the lower limit of its bathymetric distribution, as is the case for many other groups of animals found in the hadal zone (Belyaev, Reference Belyaev1989). Unlike Hyalopecten hadalis, which was found in a wide depth range (1808–7000 m) from the bathyal to hadal zones (Knudsen, Reference Knudsen1970; Dijkstra & Marshall, Reference Dijkstra and Marshall2008), H. vityazi is probably an endemic hadal species. This species was only found at depth greater than 6000 m despite the large number of samples collected in a wide depth range by various expeditions in the Kuril-Kamchatka and Aleutian trench area, as well as on the adjacent abyssal plain.
In the Kuril-Kamchatka Trench, H. vityazi was found along most of its length. However, in the Aleutian Trench, H. vityazi was only found at two stations in the western part of the trench adjoining the Kuril-Kamchatka Trench (Figure 4). The Aleutian Trench is one of the world's longest trenches (Jamieson, Reference Jamieson2015), extending latitudinally from the Asian to American continents. There may be some barriers in the trench that hamper the eastward distribution of H. vityazi towards the coast of North America.
Hyalopecten abyssalis sp. nov.
(Figures 6 & 7, Table 3)
Fig. 6. Hyalopecten abyssalis sp. nov., holotype (ZMMU Ld-3058), shell length 34.8 mm: (A–C) exterior view of both valves and interior view of left valve; (D–F) auricles of left valve; (H–J) auricles of right valve; (K, L) ctenolium; (M) ligament. Scale bars: 1 mm.
Fig. 7. Hyalopecten abyssalis sp. nov. (A–K) holotype (ZMMU Ld-3058): (A–D) sculpture of central, ventral, posterior, and anterior parts of right valve; (E–I) sculpture of central, ventral, posterior, and anterior parts of left valve; (J, K) prodissoconch. (L–M) Damaged left and right valves (IO RAS), abyssal plain adjacent to Aleutian Trench, Pacific Ocean (51°30.1′N 172°04.5′E), 5020 m, shell length 33.5 mm. Scale bars: A–I, 1 mm; J, K, 100 µm.
Table 3. Hyalopecten abyssalis sp. nov. shell measurements (mm) and indices.
L, shell length; H, height; A, anterior end length; AL, auricles length; AAL, anterior auricle length.
TYPE MATERIAL AND LOCALITY
Holotype: adult specimen (RV ‘Vityaz’, cruise 14, station 2116, abyssal plain adjacent to Kuril-Kamchatka Trench, North-western Pacific; coordinates: 45°16′N 156°13′E; water depth: 4550–4640 m, Sigsbee trawl) (ZMMU Ld-3058); coll. L.A. Zenkevich, 21 May 1953.
OTHER MATERIAL EXAMINED
Damaged specimen (IO RAS) from holotype locality; damaged left and right valves (IO RAS), abyssal plain adjacent to Aleutian Trench, Pacific Ocean (51°30.1′N 172°04.5′E), 5020 m, Sigsbee trawl, coll. V.P. Petelin, 8 July 1955 (RV ‘Vityaz’, cruise 20, station 3359).
DIAGNOSIS
Shell large (to 36.3 mm in height), thick, shell height and length almost equal, subequilateral. Umbonal angle 95°. Auricles small, unequal in length and shape. Right and left valve discs with well-developed, regular, commarginal undulations and well-developed, overlying radial ribs. Anterior auricle of right valve demarcated from disc, sculptured with fine, lamellate, closely spaced, commarginal riblets overlying three coarse radial ribs; posterior auricle continuous with shell disc, sculptured with thin, closely spaced, commarginal, lamellate ribs overlying rounded, radial riblets. Anterior auricle of left valve distinct from disc, with reticulate sculpture from widely spaced, lamellate, commarginal ribs and closer spaced, round, radial ribs; posterior auricle continuous with disc, sculptured with thin, closely spaced, lamellate, commarginal ribs and weak, round, radial ribs with noduloses at intersections. Byssal fasciole broad. Byssal notch deep, sharp. Inactive and active ctenolium well-developed. Prodissoconch large (length 245 µm), circular-shaped.
DESCRIPTION
Shell large (to 36.3 mm in height), opaque, thick, hard, weakly inflated, shell height and length almost equal (H/L = 1.043), subequilateral (beaks slightly posterior to midline, A/L = 0.511), inequivalve (left valve more convex than right valve). Shell disc subcircular. Dorsal shell margin straight. Anterior shell margin rounded. Posterior shell margin slightly convex, smoothly transitioning to rounded ventral shell margin. Auricles small (AL/L = 0.388), anterior auricle slightly larger and longer than posterior (AAL/AL = 0.511). Umbonal angle 95°.
Right and left valve discs with well-developed, regular, rounded, wide, commarginal undulations (distance between tops of undulations in disc centre 3.0–3.3 mm), becoming higher and broader towards ventral shell margin, and widely spaced, thin, well-developed, rounded, overlying radial ribs (interstices broader than ribs, 0.3–0.4 mm in disc centre), with numerous inserted radial riblets at ventral shell margin. Towards anterior and posterior shell margins undulations transforming into narrow ridges with noduloses at intersections with radial ribs.
Anterior auricle of right valve sharply demarcated from shell disc by a deep furrow with a long and strong ctenolium and a sharp suture (between furrow and shell disc). Surface of auricle with fine, lamellate, closely spaced, commarginal riblets overlying three coarse radial ribs on dorsal half of auricle and forming serration of auricle dorsal margin. Byssal fasciole broad. Byssal notch deep, sharp. Active ctenolium short, strong. Posterior auricle continuous with shell disc; sculptured with thin, closely spaced, commarginal, lamellate ribs overlying rounded, radial riblets and forming small noduloses at intersections with radial riblets.
Anterior auricle of left valve distinct from disc; anterior auricle margin passing vertically down. Surface of auricle with reticulate sculpture from widely spaced, lamellate, commarginal ribs and closer spaced, round, radial ribs sometimes with small noduloses at intersections. Posterior auricle continuous with disc, sculptured with thin, closely spaced, lamellate, commarginal ribs and weak, round, radial ribs with noduloses at intersections.
Prodissoconch large (length 245 µm), smooth, circular-shaped, convex, distinct, sharply separated from shell disc.
DISTRIBUTION AND HABITAT (FIGURE 4)
This species was recorded at the abyssal plain adjacent to the Kuril-Kamchatka and Aleutian trenches (North-western Pacific) at 4550–5020 m depth.
COMPARISONS
Hyalopecten abyssalis is most close in the shape, proportions and sculpture of the shell to Hyalopecten arntzi, Hyalopecten frigidus, Hyalopecten hadalis and Hyalopecten pudicus. However, it differs from them in having the shell almost rounded, markedly less elongate dorso-ventrally, posterior auricles continuous with the shell disc, and a sharp byssal notch (Figure 5, Table 2). Furthermore, H. abyssalis differs from H. arntzi, H. hadalis and H. pudicus in much smaller relative length of the auricles; and from H. frigidus with relatively small auricles, in having a more well-developed radial sculpture on both valves. This species is distinguished from Hyalopecten vityazi, which has a similar shape of the shell and small auricles, by the presence of commarginal undulations and the lack of reticulate sculpture on the left valve, by its posterior auricles continuous with the shell disc, a sharp and deeper byssal notch, and a smaller and circular-shaped prodissoconch.
ETYMOLOGY
The species epithet ‘abyssalis’ refers to the abyssal plain of the Pacific Ocean where the species was discovered.
REMARKS
Despite the large number of samples collected beginning in 1949 by many Russian expeditions at the abyssal plain of the North-western Pacific, Hyalopecten abyssalis and fragments of its shells were only found at two stations at the 4550–5020 m depth. This species may be assigned to the category of ‘rare’ species, forming very sparse populations at the abyssal plain of the North-western Pacific. A great part of the species of the abyssal fauna is represented in samples of various deep-sea expeditions by only very few specimens and many of the species were described from 1–2 specimens (Knudsen, Reference Knudsen1970; Okutani & Kawamura, Reference Okutani and Kawamura2002; Ellingsen et al., Reference Ellingsen, Brandt, Ebbe and Linse2007; Allen, Reference Allen2008; Kamenev, Reference Kamenev2015). On the other hand, it is not improbable that H. abyssalis, like many species of the genus Hyalopecten, prefers smaller depths in the bathyal and upper-abyssal zones of the North-western Pacific. Therefore, it can supposed that further thorough investigations at depths less than 4500–5000 m may uncover this species in larger numbers in the northern Pacific.
Hyalopecten kurilensis sp. nov.
(Figures 8 & 9, Table 4)
Fig. 8. Hyalopecten kurilensis sp. nov., holotype (ZMMU Ld-3059), shell length 9.4 mm: (A) exterior view of left valve; (B–D) auricles of left valve; (E) sculpture of left valve; (F) exterior view of right valve; (H–J) auricles of right valve; (K) ctenolium; (L) prodissoconch of both valves; (M) sculpture of right valve. Scale bars: B–E, H–J, M, 1 mm; K, L, 100 µm.
Fig. 9. Hyalopecten kurilensis sp. nov., holotype (ZMMU Ld-3059): (A, B) scanning electron micrographs of prodissoconch and ligament of left valve. Scale bars: 100 µm.
Table 4. Hyalopecten kurilensis sp. nov. shell measurements (mm) and indices.
L, shell length; H, height; A, anterior end length; AL, auricles length; AAL, anterior auricle length.
TYPE MATERIAL AND LOCALITY
Holotype: one adult specimen (RV ‘Vityaz’, cruise 39, station 5623, abyssal plain adjacent to Kuril-Kamchatka Trench, North-western Pacific; coordinates: 45°26′N 154°59′E; water depth: 4995–5045 m, Galathea trawl) (ZMMU Ld-3059); coll. L.A. Zenkevich, 19 August 1966.
OTHER MATERIAL EXAMINED
Damaged specimen (IO RAS) from holotype locality.
DIAGNOSIS
Shell small (to 9.7 mm in height), thick, slightly higher than longer, inequilateral. Umbonal angle 105°. Auricles large, unequal in size and shape. Right and left valve discs with 5–6 wide, commarginal undulations and reticulate sculpture from about 40 widely spaced, well-developed, radial ribs and more closed, commarginal, lamellate, overlying riblets. Anterior auricle of right valve demarcated from disc; sculptured with four distinct radial ribs and coarse, closely spaced, commarginal, overlying, lamellate ribs. Anterior auricle of left valve separated from disc by a shallow sulcus; sculptured with closely spaced, lamellate, commarginal ribs and a few, weak, wide radial ribs. Posterior auricles weakly separated and continuous with disc; sculptured with widely spaced, commarginal, lamellate ribs. Byssal fasciole broad. Byssal notch moderately deep, broadly rounded. Active and inactive ctenolium well-developed. Prodissoconch large (length 260 µm), D-shaped.
DESCRIPTION
Shell small (to 9.7 mm in height), opaque, thick, hard, inflated, slightly higher than longer (H/L = 1.032), inequilateral (beaks slightly posterior to midline, A/L = 0.521), inequivalve (left valve more convex than right valve). Shell disc oblique, slightly drawn out anteriorly. Dorsal shell margin straight. Anterior shell margin rounded. Posterior shell margin slightly convex, smoothly transitioning to rounded ventral shell margin. Auricles large (AL/L = 0.596), anterior auricle larger and longer than posterior (AAL/AL = 0.571). Umbonal angle 105°.
Right and left valve discs with wide, commarginal undulations (five in right and six in left valve) strongly smoothed near beak and almost indiscernible. Undulations in form of broad steps with a rounded edge widening (up to 1.7–1.8 mm) towards ventral shell margin. Valve discs with reticulate sculpture from about 40 widely spaced, well-developed, rounded, radial ribs (interstices broader than ribs, 0.2–0.5 mm in disc centre) and more closely spaced, commarginal, thin, lamellate, overlying riblets (with interspaces 0.2–0.3 mm) becoming denser in lower part of undulations.
Anterior auricle of right valve slightly twisted to the right, sharply demarcated from shell disc by a sharp, thin suture and a long, strong ctenolium. Surface of auricle with four distinct radial ribs on dorsal half of auricle and coarse, closely spaced, commarginal, overlying, lamellate ribs forming serration of auricle dorsal margin. Byssal fasciole broad. Byssal notch moderately deep, broadly rounded. Active ctenolium short. Anterior auricle of left valve separated from disc by a shallow sulcus; anterior auricle margin extending vertically down. Surface of auricle with closely spaced, lamellate, commarginal ribs, forming indistinct serration of auricle dorsal margin, and a few indistinct, weak, wide radial ribs. Posterior auricle of right and left valves weakly separated from shell disc; dorsal and posterior auricle margins producing a rounded acute angle; sculptured with widely spaced, commarginal, thin, lamellate ribs, forming indistinct serration of auricle dorsal margin.
Prodissoconch large (length 260 µm), smooth, D-shaped, convex, distinct, sharply separated from shell disc.
DISTRIBUTION AND HABITAT (FIGURE 4)
This species was recorded at the abyssal plain adjacent to the Kuril-Kamchatka Trench (North-western Pacific) at 4995–5045 m depth.
COMPARISONS
Hyalopecten kurilensis is most close to Hyalopecten arntzi, Hyalopecten neoceanicus and Hyalopecten profundicola in terms of the dimensions, shape and proportions of the shell, as well as the presence of cancellate sculpture (Figure 5, Table 2). However, unlike H. arntzi, H. kurilensis has a thick, inflated and opaque shell with much fewer and much wider commarginal undulations, a much smaller number of wider and more widely spaced radial ribs, and well expressed widely spaced lamellate, commarginal ribs. Hyalopecten kurilensis is distinguished from H. neoceanicus by having a less drawn-out anterior part of the shell, the beaks less displaced posteriorly, non-pointed and smaller auricles, a rounded byssal notch, and fewer rounded and more widely spaced radial ribs. Hyalopecten kurilensis differs from H. profundicola in having a shell less drawn-out anteriorly, less posteriorly displaced beaks, shorter auricles, commarginal undulations, and fewer wider and more widely spaced radial ribs.
ETYMOLOGY
The specific name ‘kurilensis’ derives from the name of the Kuril Islands, at the latitude of which it was found at the abyssal plain of the Pacific Ocean.
REMARKS
As was the case with Hyalopecten abyssalis, despite the relatively large number of benthic samples collected at the abyssal plain of the North-western Pacific, Hyalopecten kurilensis was only found at one station. Like H. abyssalis and Hyalopecten profundicola, this species may also fall under the category of ‘rare’ species, forming sparse populations on the abyssal plain of the North-western Pacific.
DISCUSSION
To date, four species of the genus Hyalopecten have been recorded in the abyssal and hadal zones of a relatively small region of the North-western Pacific Ocean (from Japan to the Commander and Aleutian Islands). This is almost half (44.4%) of all species of the genus Hyalopecten discovered in the abyssal and hadal zones of the World Ocean. The relatively high species richness of deep-water Hyalopecten species in this region of the North-western Pacific is not a unique phenomenon and reflects high species richness of both the entire abyssal and hadal fauna of the region and individual taxonomic groups (Zenkevich et al., Reference Zenkevich, Birstein and Belyaev1955; Belyaev, Reference Belyaev1989; Downey & Janussen, Reference Downey and Janussen2015; Elsner et al., Reference Elsner, Malyutina, Golovan, Brenke, Riehl and Brandt2015; Mironov et al., Reference Mironov, Minin and Dilman2015). Examination of the materials collected by just one KuramBio expedition on the abyssal plain (4861–5787 m) adjacent to the Kuril-Kamchatka Trench has revealed 55 species of bivalves belonging to 21 families (Kamenev, Reference Kamenev2015), which is much more diverse at the family level than the Atlantic deep-water bivalve fauna (Allen, Reference Allen2008). The high species richness of the abyssal and hadal faunas in the Kuril-Kamchatka Trench region may be connected with the fact that the region is one of the Pacific's most productive areas where favourable conditions are created for the feeding of bottom animals (Filatova, Reference Filatova1960, Reference Filatova1968; Sokolova, Reference Sokolova1976, Reference Sokolova1981; Belyaev & Mironov, Reference Belyaev and Mironov1977; Belyaev, Reference Belyaev1989). Remarkably, although different types of sampling gear were used (Agassiz trawl, epibenthic sledge, boxcorer, multicorer) to sample benthic animals in the same region of the abyssal plain where Hyalopecten abyssalis and Hyalopecten kurilensis were collected, the KuramBio expedition did not find any members of the genus Hyalopecten (Kamenev, Reference Kamenev2015). It can therefore be supposed that further studies of the abyssal zone of the North-western Pacific will markedly increase the species richness of the benthic fauna of this region by finding ‘rare’ species such as Hyalopecten species.
Filatova (Reference Filatova1961) and Belyaev (Reference Belyaev1966) have shown that, along with other bivalves, representatives of the order Pectinida also inhabit the hadal zone of the World Ocean. Delectopecten vancouverenis was found in Java (6820–6850 m) and Japan (6156–6207 m) trenches (Filatova, Reference Filatova1961; Belyaev, Reference Belyaev1966). Following preliminary identification by Z.A. Filatova of bivalve materials collected by RV ‘Vityaz’ expeditions in different oceanic trenches during 1949–1966, Belyaev (Reference Belyaev1966) reported that, presumably, various species of the order Pectinida were found at 6410–8100 m depths in the Aleutian, Kuril-Kamchatka, Bougainville and New Hebrides trenches of the Pacific Ocean. Later, Knudsen (Reference Knudsen1970) described H. hadalis from the Kermadec Trench, and Filatova (Reference Filatova1974) found Hyalopecten sp. resembling H. hadalis in the South Sandwich (5600–5670 m) and South Orkney (5450–5480 m) trenches of South Atlantic. Furthermore, Vinogradova (Reference Vinogradova1974) reported on the finding of pectinids in the Romanche Trench fault. Thus, various species of the family Pectinidae are widespread in the hadal zone of the World Ocean and further investigation of both existing and new material from oceanic trenches will contribute to the knowledge of the hadal fauna of the Pectinidae.
ACKNOWLEDGEMENTS
I am very grateful to Drs A. V. Gebruk, E. M. Krylova, T. N. Molodtsova and all collaborators of Laboratory of Ocean Bottom Fauna (IO RAS), as well as to Drs H. Saito (NMNS), L. T. Groves (LACM), E. Kools (CAS), P. Valentich-Scott (SBNHM) for arrangement of my work with the bivalve mollusc collections and great help during this work; to Drs E. M. Krylova, T. N. Molodtsova and A. S. Maiorova (IMB) for photographing some specimens of H. vityazi; to Drs E. M. Krylova, P. Valentich-Scott, K. A. Lutaenko (IMB), J. Goud (National Museum of Natural History, Leiden), B. A. Marshal (Museum of New Zealand Te Papa Tongarewa, Wellington) for sending copies of scientific papers necessary for this work; to Ms T. N. Koznova (IMB) for help with translating the manuscript into English; to Dr A. V. Gebruk and an anonymous reviewer for comments on the manuscript.
FINANCIAL SUPPORT
This research was supported by the Russian Foundation for Basic Research (grant no. 14-04-00872-a).
