Parvamussium rainesi Dijkstra, 2012
DIJKSTRA, H. H. 2012. A new Recent species of Parvamussium (Bivalvia, Propeamussiidae) from Easter Island. Basteria, 76 (4-6): 133-138, figs- 1-12. [p. 136, figs. 1-6]
2012 Parvamussium rainesi Dijkstra, 2012
H. H. Dijkstra, 2012, figures 1-6
H. H. Dijkstra, 2012, figures 7-12
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«Parvamussium scitulum (E.A. Smith); Raines & Huber, 2012: 33, fig. 16A-E. Not Amussium scitulum E.A. Smith (1885: 312, pl. 23 figs 4, 4a-b).
Material examined.— Chile: Easter Island, off the western coastline near Tahai, 27˚07’20”S, 109˚26’30”W, 50-80 m, dead, dredged, leg. B.K. Raines, xii.2000 (lv, holotype, ZMAMoll. 365560; rv, figured paratype, ZMAMoll. 365561; 28 v, paratypes ZMAMoll. 365562; 10 v, paratypes, LACM 3229); off the southwestern coast near Orongo, 27˚11’58”S, 109˚29’28”W, 100-150 m, dead, dredged in fine sand, leg. B.K. Raines, xi.2002 (20 v, paratypes, LACM 3228).
Other material examined.— Mataveri Bay, 27˚08’44”S, 109˚26’46”W, 80-120 m, dead, dredged in sand and rubble, leg. B.K. Raines, xi.2002 (10 v, collection Mr B.K. Raines). Description. — Shell up to c. 6 mm high, fragile, semi-transparent, almost circular, inequivalve, nearly equilateral, left valve very weakly inflated, right valve strongly inflated, auricles unequal in size and shape, umbonal angle c. 90˚, internal riblets 10 plus 1-2 rudimentary or lacking, left valve brightly mottled, right valve whitish, dull or transparent glossy. Prodissoconch c. 200 μm long, smooth.
Left valve sculptured with numerous unevenly spaced radial riblets, increasing in number towards ventral margin, and delicate commarginal lamellae, intercostal and on the radial riblets, more closely spaced near the ventral margin than elsewhere. Both auricles with commarginal lamellae, more prominent and more widely spaced on the anterior than on the posterior auricle. Hinge line straight. Right valve with widely spaced commarginal lirae, somewhat more narrowly spaced in late growth stage than earlier. Anterior auricle with delicate, closely spaced commarginal lirae, posterior auricle with delicate closely spaced commarginal lamellae. Dimensions of type material: Holotype (NCB Naturalis) 5.2 mm in height, figured paratype (NCB Naturalis) 4.6 mm in height, paratypes NCB Naturalis 2 to 5.9 mm in height, paratypes NHMLAC 2 to 5.8 mm in height. Distribution. — Easter Island, 50-150 m, fine sand or sand and rubble. So far only single valves have been sampled. It is most likely that the present species is living somewhat deeper.
Comparison.— In a recent publication on the Mollusca from Easter Island and Salas y Gómez, Raines & Huber (2012: 33) incorrectly determined the present species as Parvamussium scitulum (E.A. Smith, 1885). It is an undescribed species. Parvamussium scitulum is recorded from the (sub)tropical Indo-West Pacific, living at 50-300 m (Dijkstra & Maestrati, 2008: 94). The true P. scitulum (Poppe, 2011: 42, pl. 1016 figs 5-8) differs morphologically from the present species in its larger size, reaching up to c. 10 mm in height (P. rainesi spec. nov. reaches up to c. 6 mm), in having very delicate, closely spaced radial lirae on the left valve (P. rainesi has widely spaced scaly radial riblets), and in having a smooth or very weakly and closely spaced, commarginally sculptured disc of the right valve (P. rainesi has prominent, widely spaced commarginal lirae). Similar species are compared in Table 1.
Parvamussium rainesi is somewhat similar in shape and sculpture to P. cristatellum (Dautzenberg & Bavay, 1912) (Poppe, 2011: 40, pl. 1015 figs 2-3), recorded from the tropical Indo-West Pacific, living in 194-510 m (Dijkstra & Maestrati 2008: 87), but differs in having better developed, prominent, radial and more closely spaced commarginal sculpture on the left valve (P. rainesi has fewer, less prominent radial riblets and more widely spaced, more delicate commarginal lamellae, which are lacking in some specimens). Moreover, the commarginal sculpture of the right valve of P. cristatellum is more closely spaced than in P. rainesi.
Parvamussium rainesi is morphologically closest to P. squalidulum Dijkstra, 1995, recorded from the southwestern Pacific, living in 260-523 m (Dijkstra, 1995: 32, figs 47-50; Dijkstra & Maestrati 2008: 94), but P. squalidulum differs mainly in reaching a much larger size, up to c. 14 mm in height (P. rainesi up to c. 6 mm), in having somewhat coarser and more strongly developed radial and commarginal sculpture on the left valve, in having a more weakly inflated right valve (P. rainesi has a strongly convex right valve, and in having more prominent commarginal lamellar sculpture on the anterior auricle of the right valve (P. rainesi has more closely spaced, more delicate lamellae) and in having more irregularly spaced internal riblets (riblets equally spaced and developed in P. rainesi). Etymology. — Named after Bret K. Raines, research associate at the Natural History Museum of Los Angeles County, who achieved his research trips to Easter Island in the years 1998, 2000 and 2002 to sample marine molluscs, including the present parvamussiid specimens, by diving and dredging.»
HENK H. DIJKSTRA, 2012
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