Hyalopecten abyssalis 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. 6, figs. 6, 7]
2016 Hyalopecten abyssalis Kamenev, 2016
G. M. Kamenev, 2016, figures 2, 3.
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«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, circularshaped, 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. ETIMOLOGY
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, 1970; Okutani & Kawamura, 2002; Ellingsen et al., 2007; Allen, 2008; Kamenev, 2015). 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.» GENNADY M. KAMENEV, 2016
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