Hyalopecten vityazi Kamenev, 2016
KAMENEV, G. M. 2016. Three new species of the genus Hyalopecten (Bivalvia: Pectinidae) from the abyssal and hadal zones of the North-western Pacific Ocean. Journal of the Marine Biological Association of the United Kingdom, Oct. 2016: 1-18, figs. 1-9. [p. 3, figs. 2, 3]
2016 Hyalopecten vityazi Kamenev, 2016
G. M. Kamenev, 2016, figures 2, 3.
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«Delectopecten sp.: Belyaev, 1966, p. 116; Belyaev, 1989, p. 216. Propeamussium sp.: Belyaev, 1966, p. 116; Belyaev, 1989, 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 welldeveloped, 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 welldeveloped. 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 1008. 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 a depth from 6090 (46°06′N 153°18′E) to 8100 m (44°55.5′N 152°24′E) (bottom temperature 1.65–2.158°, salinity 34.7‰ (Belyaev, 1989)); 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 1.788C at a depth 6328 m, salinity 34.7‰ (Belyaev, 1989)). 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. ETIMOLOGY
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, 1989). Hyalopecten vityazi was found in five (38.5%) out of the 13 trawl samples collected at depths only to 8100 m (Belyaev, 1989). 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, 1989). Unlike Hyalopecten hadalis, which was found in a wide depth range (1808–7000 m) from the bathyal to hadal zones (Knudsen, 1970; Dijkstra & Marshall, 2008), 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, 2015), 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.» GENNADY M. KAMENEV, 2016
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