Parvamussium pacificum Kamenev, 2017
KAMENEV, G. M. 2018. Four new species of the family Propeamussiidae (Mollusca: Bivalvia) from the abyssal zone of the northwestern Pacific, with notes on Catillopecten squamiformis (Bernard, 1978). Marine Biodiversity, 48: 647-676, figs. 1-16 (published online: 21 November 2017). [p. 650, figs. 2-5]
2017 Parvamussium pacificum Kamenev, 2017
G. M. Kamenev, 2018, figures 2-5.
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«Propeamussium meridionale: Okutani and Kawamura 2002, p. 12, figs. 4C, 5C (non Smith 1885).
Propeamussium sp.: Kamenev 2015, p. 191. Type material and locality: Holotype (MIMB 34143), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (40°35,40′N, 150°59,84′E – 40°34,27′N, 150°59,00′E), 5,398-5,389 m, epibenthic sledge, Coll. A. Brandt, 25-VIII-2012 (RV Sonne, cruise no. 223, stn. 9-12); paratypes (10) (MIMB 34144), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°58,35′N, 157°19,74′E – 43°58,62′ N, 157°18,15′E), 5,407-5,418 m, epibenthic sledge, Coll. A. Brandt, 30-VII-2012 (RV Sonne, cruise no. 223, stn. 1-11); paratypes (5) [Zoological Museum of the University of Hamburg (ZMH) 119349], abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°02,66′N, 152°59,46′E – 43°01,57′N, 152°58,59′E), 5,218-5,222 m, Agassiz trawl, Coll. G.M. Kamenev,18-VIII-2012 (RV Sonne, cruise no. 223, stn. 7-11); paratypes (3) (MIMB 34145), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°02,78′N, 152°59,30′E – 43°01,65′N, 152°58,45′E), 5,217-5,223 m, epibenthic sledge, Coll. A. Brandt, 17–18- VIII-2012 (RV Sonne, cruise no. 223, stn. 7-10); paratype (MIMB 34146), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (40°13,26′N, 148°06,24′E – 40°12,37′ N, 148°05,43′E), 5,348-5,350 m, epibenthic sledge, Coll. A. Brandt, 30-VIII-2012 (RV Sonne, cruise no. 223, stn. 11-9).
Other material examined: Four specimens (MIMB 34154) from holotype locality; 48 specimens (MIMB 34155), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°58,35′N, 157°19,74′E – 43°58,62′N, 157°18,15′E), 5,407-5,418 m, epibenthic sledge, 30-VII-2012 (RV Sonne, cruise no. 223, stn. 1-11); 59 specimens (MIMB 34156), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°58,26′N, 157°19,68′E – 43°58,33′N, 157°17,98′E), 5,417-5,423 m, epibenthic sledge, 30-VII-2012 (RV Sonne, cruise no. 223, stn. 1-10); 82 young specimens (MIMB 34157), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (46°13,60′N, 155°33,42′E – 46°14,93′N, 155°32,57′E), 4,860-4,866 m, epibenthic sledge, 2–3-VIII-2012 (RV Sonne, cruise no. 223, stn. 2-9); one young specimen (MIMB 34158), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (47°13,83′N, 154°41,88′E – 47°14,87′N, 154°43,18′E), 4,988-4,989 m, epibenthic sledge, 5-VIII-2012 (RV Sonne, cruise no. 223, stn. 3-9); 12 specimens (MIMB 34159), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°35,50′N,153°57,89′E – 43°34,30′N, 153°58,18′E), 5,378-5,376 m, epibenthic sledge, 11-VIII-2012 (RV Sonne, cruise no. 223, stn. 5-9); 1 specimen (MIMB 34160), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°35,50′N, 153°57,81′E – 43°34,20′N, 153°58,14′E), 5,379-5,375 m, epibenthic sledge, 11-VIII-2012 (RV Sonne, cruise no. 223, stn. 5-10); 1 young specimen (MIMB 34161), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (42°29,58′N, 154°00,04′E – 42°28,47′N, 153°59,67′E), 5,290-5,305 m, epibenthic sledge, 15-VIII-2012 (RV Sonne, cruise no. 223, stn. 6-11); 10 specimens (MIMB 34162), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°02,87′N, 152°59,45′E – 43°01,50′N, 152°58,35′E), 5,216-5,221 m, epibenthic sledge, 17-VIII-2012 (RV Sonne, cruise no. 223, stn. 7-9); 11 specimens (MIMB 34163), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (43°02,78′N, 152°59,30′E – 43°01,65′N, 152°58,45′E), 5,217-5,223 m, epibenthic sledge, 17–18-VIII-2012 (RV Sonne, cruise no. 223, stn. 7-10); 4 specimens (MIMB 34164), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (42°14,69′N, 151°44,05′E – 42°14,26′N, 151°42,49′E), 5,127 m, epibenthic sledge, 20-VIII-2012 (RV Sonne, cruise no. 223, stn. 8-9); 1 young specimen (MIMB 34165), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (40°35,49′N, 150°59,92′E – 40°34,25′N, 150°59,91′E), 5,399-5,398 m, epibenthic sledge, 23–24-VIII-2012 (RV Sonne, cruise no. 223, stn. 9-9); 1 young specimen (MIMB 34166), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (41°12,47′N, 150°05,64′E – 41°11,18′N, 150°05,61′E), 5,252-5,249 m, epibenthic sledge, 26-VIII-2012 (RV Sonne, cruise no. 223, stn. 10-9); 1 young specimen (MIMB 34167), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (40°13,10′N, 148°06,45′E – 40°12,10′N, 148°05,53′E), 5,351-5,348 m, epibenthic sledge, 31-VIII-2012 (RV Sonne, cruise no. 223, stn. 11-12); 1 specimen (MIMB 34168), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (39°43,47′N, 147°10,11′E – 39°42,54′N, 147°09,51′E), 5,229-5,227 m, Agassiz trawl, 1-IX-2012 (RV Sonne, cruise no. 223, stn. 12-5); 8 specimens (IO RAS), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (48°43,2′N, 160°55,9′E), 5,670-5,680 m, Sigsbee trawl, 27-VIII-1954 (RV Vityaz, cruise no. 19, stn. 3114); 1 specimen (IO RAS), abyssal plain adjacent to Japan Trench, Pacific Ocean (38°02′N, 146°33′), 5,495 m, Sigsbee trawl, 08-V-1957 (RV Vityaz, cruise no. 19, stn. 3575); 19 specimens (IO RAS), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (44°48′N, 156°33′E), 5,045-5,005 m, Galathea trawl, 15-VIII-1966 (RV Vityaz, cruise no. 39, stn. 5620); 2 specimens (IO RAS), abyssal plain adjacent to Kuril-Kamchatka Trench, Pacific Ocean (45°26′N, 154°59′E), 5,045-4,995 m, Sigsbee trawl, 19-VIII-1966 (RV Vityaz, cruise no. 39, stn. 5623); 3 specimens and 1 valve, abyssal plain adjacent to Japan Trench, Pacific Ocean (38°29.1′N, 145°43.0′E – 38°29,3′N, 145°41.1′E), 5,440 m, bim-trawl, 26-VI-1978 (RV Soyo-Maru). A total of 269 specimens and 1 valve.
Description: Shell small (to 10.4 mm in length), solid, thin, whitish, translucent, fragile, weakly inflated, subcircular, slightly drawn out anteriorly, slightly longer than high (H/L = 0.892 ± 0.004), equilateral (A/L = 0.498 ± 0.007), inequivalve (left valve slightly more convex than right). Auricles large (AL/L = 0.595 ± 0.011 for specimens with shell length greater than 4.0 mm), equal in length (AAL/AL = 0.500 ± 0.001), dissimilar in shape, anterior auricle slightly larger than posterior. Umbonal angle 120°.
Right valve with broad, flexible, marginal apron, pressed to the left valve and often broken off. Right valve disc with thin, closely spaced, commarginal, regularly arranged lamellate ribs. Outer prismatic layer consisting of commarginal arrays of high, polygonal prisms (20 μm in height) (to each array corresponds a commarginal, lamellate rib) (Fig. 4g). Between arrays of high prisms are bands of hexagonal, small prisms (5 μm in height) 6-7 prisms wide. Small prisms of outer prismatic layer with one or two minute, crisp, elongate outgrowths (Fig. 4h). Anterior auricle sharply demarcated from shell disc by a well-expressed byssal fasciole and a sharp suture. Surface auricle with widely spaced, prominent, strong, commarginal, lamellate ribs forming serration of auricle dorsal margin and very fine, closely spaced, antimarginal threads and granules between commarginal ribs; umbonal part of auricle smooth (Fig. 4d-f). Byssal notch shallow, broadly rounded. Byssal fasciole narrow, somewhat folded. Posterior auricle pointed; anterior and dorsal margins of anterior auricle forming rounded, acute angle. Posterior auricle demarcated from shell disc by a few (up to 5), well expressed, radial ribs with small noduloses at intersections with commarginal ribs, sculptured with widely spaced, high, commarginal ribs and very fine, closely spaced, antimarginal, discontinuous threads and granules between commarginal ribs forming sometimes from one to three weak radial ribs almost parallel to auricle dorsal margin (Figs. 2b, c). Left valve disc with reticulate sculpture of 22-27 irregularly spaced, well-developed, rounded, radial ribs commencing 1-1.5 mm below the beak and extending to valve margins and 12-15 widely spaced, high, overlying, commarginal, lamellate ridges with scaly intersections (Fig. 2j, k).Umbonal region with fine, closely spaced, thin, commarginal, lamellate ribs and very thin, short, dense, radial riblets. Posterior auricle pointed; anterior margin of anterior auricle passing vertically down (Fig. 2f). Anterior and posterior auricles weakly demarcated from disc by a shallow sulcus, sculptured with strong, widely spaced, high, commarginal, lamellate ribs forming serration of auricle dorsal margin and very fine, closely spaced, antimarginal, discontinuous threads and granules between commarginal ribs, and 1-3 distinct, radial ribs with scaly intersections (Fig. 4j, k). Interior of each valve usually with 13 (sometimes 14) radial ribs extending to marginal apron and one weak anterior and posterior auricular riblets, appearing first in individualswith shell length 2.9-3.0 mm; some specimens with one or two rudimentary interstitial riblets. Hinge line straight. Hinge plate roughened, strongly irregularly striated. Resilifer broadly triangular. Prodissoconch large (length 250-254 μm), smooth, Dshaped, convex, distinct, sharply separated from shell disc. Variability: In smaller specimens (shell length to 1.2 mm), compared to larger ones, the shell length is almost equal to the shell height (H/L = 0.882-0.936), and the beak is placed slightly anteriorly (A/L = 0.450-0.487). The left valve is Dshaped and slightly drawn out posteriorly. Auricles are not demarcated from the shell disc and the auricle dorsal margin is almost equal to the shell length (AL/L = 0.870-1.0). Left valve sculptured with regular, widely spaced, соmmarginal, lamellate ribs and more closely spaced, short, antimarginal riblets that are very short near the prodissoconch and located at intersection with more distinct commarginal ribs (Fig. 5f). As the shell grows (L > 1.5 mm), a weak byssal sinus first appears anteriorly on the left valve and then the auricles become separated from the valve disc. The commarginal ribs on the disc become higher, the distance between them increases, widely spaced radial ribs appear. In the right valve, in specimens with a shell length less than 1.5 mm, only the anterior auricle is demarcated from the disc which has a distinct byssal notch and suture, while the left auricle continuous with the valve disc (Fig. 5g). The auricles are almost smooth, with hardly visible irregular, commarginal wrinkles. The right valve disc in small specimens with thin, commarginal ribs and short, radial riblets crossing commarginal ribs. With increasing shell size, commarginal, lamellate ridges with very thin antimarginal threads located between them appear on the auricles. The shell of this species exhibits an allometric change, becoming relatively deeper and longer with increasing size. In adults, the variability of shell shape and proportions is insignificant (Table 1). Slight variations are found in the number of radial ribs on the left valve (22–25) and internal ribs (15–16, including 2 auricular ones).
Distribution and habitat (Fig. 6): This species was recorded at the abyssal plain adjacent to the Kuril-Kamchatka and Japan trenches (northwestern Pacific) at the latitudes from the middle of Honshu (Japan) (38°02′N, 146°33′) to the northern Kuril Islands (48°43,2′N, 160°55,9′E) at 4,860-5,680 m depth (bottom temperature (6-8 m above bottom) 1.5-1.6 °C, salinity 34.7‰, oxygen 7.71-7.72 ml/l).
Comparisons: To date, the genus Parvamussium contains 67 species (Bouchet et al. 2015). A comparison of Parvamussium pacificum sp. nov. with all species of the genus Parvamussium showed that the new species well differs from the majority of species of this genus by a combination of features such as the presence of subcircular shell with equallength auricles and central beaks, well-developed inner ribs, well-expressed reticulate sculpture of widely spaced, prominent, radial and commarginal ribs on the left valve, and a large prodissoconch no less than 250 μm long. In most species of this genus, the prodissoconch is no more than 200 μm long (Dijkstra and Gofas 2004; Dijkstra and Marshall 2008; Dijkstra et al. 2009). The new species is closest in terms of the shape, proportions, and sculpture of the shell to Parvamussium octodecimliratum (Melvill and Standen, 1907) and Parvamussium permirum (Dautzenberg, 1925).
Parvamussium pacificum sp. nov. is clearly distinguished from P. octodecimliratum by having about half the number of radial and commarginal ribs on the left valve. In P. pacificum sp. nov., the radial and commarginal ribs of the left valve are more widely spaced and, as a consequence, they form much wider reticulations at intersection than in P. octodecimliratum (Table 2). The new species also differs from P. octodecimliratum in pointed posterior auricles, a much smaller number of radial ribs on the auricles, and fewer internal ribs (Melvill and Standen 1907; Knudsen 1970; Schein 1989; Egorova 1999). Parvamussium pacificum sp. nov. is also well distinguished from P. permirum by having significantly fewer radial ribs on the left valve, a larger number of interior ribs, a separated and pointed posterior auricle of the left valve, and a significantly larger prodissoconch (Table 2) (Dautzenberg 1925; Schein 1989; Dijkstra and Gofas 2004; Oliver et al. 2016). Derivatio nominnis: The species name "pacificum" derives from the name of the Pacific Ocean, where this species was found and collected in large numbers.
Remarks
Okutani and Kawamura (2002) found at the abyssal plain (5,440 m) adjacent to the Japan Trench a species of the order Pectinida that was identified as Propeamussium meridionale (Smith, 1885). Until recently, that was the deepest record of this species, which in other regions was found at depths down to 4,981 m (Dijkstra and Maestrati 2008; Dijkstra and Marshall 2008). Comparison of all specimens of P. meridionale found by Okutani and Kawamura (2002) off the coast of Japan with numerous specimens of an unidentified species found by many expeditions of the IO RAS and the KuramBio expedition at the abyssal plain adjacent to the Kuril-Kamchatka Trench showed that they belong to the same species, which markedly differs from P. meridionale. A detailed examination of a large body of material showed that this species is distinguished from P. meridionale in the size, shape, and proportions of the shell, the absence of lateral gaps, the shape of auricles, the sculpture of the left valve and auricles, the presence of a byssal notch, the number and placement of inner ribs, and the prodissoconch size (Smith 1885; Knudsen 1970; Dijkstra 1995; Dijkstra and Marshall 2008; Dijkstra and Janssen 2013). Moreover, this species had the main morphological features of the shell consistent with the diagnostic characters of the genus Parvamussium (presence of byssal notch, prominent sculpture of valves, unequal auricles, appearance of internal riblets later in ontogeny than in the genus Propeamussium, and absence of lateral gaps) (Dijkstra 1995; Coan and Valentich-Scott 2012; Dijkstra 2013; Dijkstra and Janssen 2013). Therefore, I assigned this species to the genus Parvamussium. Its comparison with all species of the genus suggested that this species is new to science.
Parvamussium pacificum sp. nov. was found in the northwestern Pacific only at the abyssal plain at 4,860-5,680 m depth and is apparently an abyssal species. At the present moment, the lower boundary of its vertical distribution is the maximum depth of occurrence for genus Parvamussium species and it is the only species of the genus Parvamussium recorded at depths greater than 5,000 m. This species was found in large numbers (sometimes more than 50 indiv. in a sample) in all epibenthic sledge samples at all stations of the KuramBio expedition (Kamenev 2015). In addition, P. pacificum sp. nov. was found in many trawl samples collected by several IO RAS expeditions in the different regions of the northwestern Pacific. Thus, P. pacificum sp. nov. is a widely distributed and dominant species of bivalves at the abyssal plain of the Pacific Ocean adjacent to the Kuril-Kamchatka and Japan trenches. Considering that it was found down to the lower boundary of the abyssal zone, P. pacificum sp. nov. can possibly be collected in the hadal zone (depths greater than 6,000 m) of the Kuril-Kamchatka and Japan trenches using an epibenthic sledge, which is most efficient in sampling small bivalves with fragile shell (Kamenev 2015).» GENNADY M. KAMENEV, 2017
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