Propeamussium arabicum Dijkstra & Janssen, 2013

Dijkstra, H. H. & Maestrati, P., 2015, Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the southwestern Indian Ocean, African Invertebrates 56 (3), pp. 585-585 : 588-590

Propeamussium arabicum Dijkstra & Janssen, 2013

Fig. 1A, B

Propeamussium arabicum Dijkstra & Janssen, 2013: 184 , figs 1–2, 7–9. TYpe localitY: Gulf of Aden

(12°21.5'N 44°33.0'E), - 506 m, dead, grey-greenish sandy mud, 04.iii.1995 (Meteor 31, stn 105.4). Description: Shell small, up to 15 mm in height, fragile, transparent, almost circular, somewhat higher than wide, inequivalve, equilateral, left valve slightlY more inflated than right valve, auricles slightly unequal, umbonal angle 110°–115°, colour of left valve creamy or reddish, right valve whitish. Prodissoconch 200 µm high. Left valve sculptured with numerous delicate, regularly spaced, antimarginal lirae, crossed by irregularly arranged, minutely undulated growth lines, commencing 1 mm below umbo and extending to above central part of disc, central part and periphery smooth. Anterior auricle somewhat larger than posterior with fine commarginal lirae, weaker on posterior. Dorsal margin weakly lamellated near anterior margin. Right valve sculptured with fine, regularlY close-set commarginal lirae, more widelY spaced near ventral margin. Auricles smooth, unequal in form and size. No byssal fasciole or byssal notch. Ctenolium absent. Hinge line straight. Interior ribs of both valves prominent (7 with 2 auricular) commencing in early growth stage and extending to the submarginal area. No rudimentary interstitial riblets.

Type material examined: Holotype (lv) SMF 341743, 108 paratypes (lv & rv) MBUR (14 v), RMNH (40 v), SMF (341744-47, 341748-50). The additional valves of the type and other stations are not belonging to the type series ( Dijkstra & Janssen 2013: 184).

Other material examined: NW MADAGASCAR: Between Majunga and Cap Saint-André (15°34'S 45°44'E), - 287–450 m, live, campaign Miriky, stn CP3263, 10.vii.2009 ( MNHN) ; in front of the Narendry Bay (14°29'S 47°26'E), - 332–379 m, live, campaign Miriky, stn CP3289, 14.vii.2009 ( MNHN) ; in front of the Narendry Bay (14°29'S 47°26'E), - 409–425 m, live, campaign Miriky, stn CP3290, 14.vii.2009 ( MNHN) ; in front of the Narendry Bay (14°27'S 47°25'E), - 347–408 m, live, campaign Miriky, stn CP3291, 14.vii.2009 ( MNHN) ; in front of the Narendry Bay (14°30'S 47°26'E), - 268–408 m, live, campaign Miriky, stn CP3293, 14.vii.2009 ( MNHN) ; in front of the Narendry Bay (14°29'S 47°27'E), - 263–331 m, live, campaign Miriky, stn CP3294, 14.vii.2009 ( MNHN) . MOZAMBIQUE CHANNEL: Zambeze transect (19°34'S 36°45'E), - 825–827 m, live, campaign Mainbaza, stn CC3155, 13.iv.2009 ( MNHN) . SOUTH AFRICA: Zululand : Off Matigulu River mouth (29°22.3'S 31°57.7'E), - 400 m, dead, mud, NMDP stn ZR12, dredged RV Meiring Naudé, 16.vi.1989 ( NMSA E8820 ) .

Distribution and habitat: Gulf of Aden, dead at - 321– 917 m. Now also extended southwards to northwestern Madagascar and the Mozambique Channel (new record). Present specimens are on soft substrata of mud and sand at a bathyal range of - 331–825 m (minimum depth range).

Remarks: The most closely resembling species is Propeamussium siratama ( Oyama, 1951) recorded from Japan (- 100–300 m) (Hayami 2000: 913), which differs from the present species in being smaller (typically up to 10 mm, present specimens up to 15mm in height), in having a weaker latticed sculpture on the left valve, in having more internal ribs (typically 8 with sometimes a few rudimentary interstitial riblets in late growth stage near the ventral margin, present specimens 7 without rudimentary riblets), and in colour (typically left valve whitish opaque or transparent, present specimens reddish or sporadical whitish transparent). Another resembling species is Propeamussium andamanense (Bavay, 1905) from the Andaman Sea. This species is somewhat smaller in size (up to 9 mm in height), also almost circular of shape and having also 7 internal ribs, but is different in sculpture of the left valve (throughout the disc a fine latticed sculpture of numerous closely spaced radial riblets and weak interstitial commarginal lirae), whereas P. arabicum has a more delicate latticed sculpture in early growth stage to the central part of the disc. Moreover, P. andamanense is more solid, opaque white ( P. arabicum transparent reddish or sporadical whitish).A rather resembling species is Propeamussium malpelonium (Dall, 1908) , recorded from the tropical eastern Pacific. Propeamussium malpelonium differs in having a more prominent sculpture on the entire disc of the left valve and in having 11 internal ribs. This species is only recorded from abyssal depths, 2690–4505 m ( Grau 1959: 14). Propeamussium steindachneri ( Sturany, 1901) recorded from the Red Sea, Gulf of Aden,Arabian Sea and Gulf of Oman (- 439–457 m) ( Knudsen 1967: 280; Oliver 1992: 73; Dijkstra & Janssen 2013: 191) differs from P. arabicum in having a more oblique shape, smooth left and right valves, fewer internal ribs (usually 6), smaller auricles, and creamy and white dots on the left valve.

Propeamussium caducum (E.A. Smith, 1885) Fig. 1E, F

Amussium caducum E.A. Smith, 1885: 309 , pl. 23, figs 1–1c; Knudsen 1967: 274, pl. 1, fig. 17, text-fig. 16. Type locality: Philippines, West of Luzon (12°21'N 122°15'E), - 1280 m, live, blue mud, 16.i.1875 (Challenger, stn 207).

Amussium weberi Dautzenberg & BavaY, 1912: 32, pl. 28, figs 9–13. TYpe localitY: Indonesia, Bali Sea (7°19.4'S 116°49.5'E), - 538 m, live, fine dark brown sandY mud, 19.ii.1900 ( Siboga , stn 316).

Propeamussium nakazawai Kuroda , [1929–35] 1932: 87, figs 101–102 (ICZN Art. 11, nomen nudum). Synonymy established by Oyama (1951), and subsequently followed by Habe (1958, 1977) and Knudsen (1967).

Propeamussium caducum: Dijkstra 1991: 6 , figs 1–2; 1995: 15, figs 9–10, 129–132; 2013: 10, pl. 1, figs 1a–d,

pl. 4, fig. 1; Dijkstra & Maestrati 2008: 82; 2012: 391; Dijkstra & Janssen 2013: 187, figs 10–12. Description: Shell up to 29 mm in height, fragile, slightly higher than wide to circular, slightly inequivalve, almost equilateral, semi-transparent to opaque, glossy, creamy, left valve slightlY more inflated than right valve, umbonal angle about 90°. Prodissoconch 200 µm high. Left valve smooth, with commarginal growth lines, no radial striations. Auricles small, without sculpture, somewhat raised near margins. Right valve with widely spaced commarginal lirae, commencing at 3 mm shell height and extending to submarginal area, with interstitial microscopic scratches.Auricles with fine commarginal striae, strong scales on the marginal areas of hinge. Hinge line straight near umbo, gradually raised to the anterior and posterior margins. Interior radial lirae generally 10, sometimes 9 or 11, slightly nodulose at distal ends, more prominent on right valve. No byssal notch, ctenolium lacking, lateral gape present.

Type material examined: Amussium caducum : Lectotype (pr) NHMUK (1887.2.9.3310), designated by Dijkstra (1995: 17), 4 paralectotypes NHMUK (1887.2.9.331/1-4). Amussium weberi : Lectotype ZMA (Moll. 3.12.013), designated by Dijkstra (1995: 17), paralectotypes in KBIN, MCZ and RMNH.

Other material examined: NW MADAGASCAR: Off Mahajamba Bay (14°48'S 46°60'E), - 620–637 m, live, campaign Miriky, stn CP3249, 07.vii.2009 ( MNHN) ; Off Majunga (15°25'S 45°57'E), - 609–800 m, live, campaign Miriky, stn CP3251, 08.vii.2009 ( MNHN) ; Off Majunga (15°22'S 45°58'E), - 850–900 m, live, campaign Miriky, stn CP3252, 08.vii.2009 ( MNHN) ; Off Majunga (15°25'S 45°55'E), - 243–950 m, live, campaign Miriky, stn CP3253, 08.vii.2009 ( MNHN) ; between Majunga and Cap Saint-André (15°31'S 45°45'E), - 640–800 m, live, campaign Miriky, stn CP3268, 11.vii.2009 ( MNHN) ; between Majunga and Cap Saint-André (15°31'S 45°46'E), - 760–1000 m, live, campaign Miriky, stn CP3269, 11.vii.2009 ( MNHN) ; between Majunga and Cap Saint-André (15°24'S 45°56'E), - 750–780 m, live, campaign Miriky, stn CP3278, 12.vii.2009 ( MNHN) ; between Majunga and Cap Saint-André (15°22'S 45°57'E), - 780–1020 m, live, campaign Miriky, stn CP3279, 12.vii.2009 ( MNHN) ; in front of Narendry Bay (14°27'S 47°25'E), - 347–408 m, live, campaign Miriky, stn CP3291, 14.vii.2009 ( MNHN) .

Distribution and habitat: Propeamussium caducum has a large distribution throughout the Indo-West Pacific from southern Japan (Okutani 2000), southwards to the East China Sea and South China Sea ( Xu & Zhang 2008), the Philippine Archipelago ( Knudsen 1967; Dijkstra 2013), the Indonesian Archipelago ( Dijkstra & Kastoro 1997), and eastwards to the Solomon Islands ( Dijkstra & Maestrati 2008), the Vanuatu Archipelago and New Caledonia (Dijkstra 1995, 2001; Dijkstra & Maestrati 2012), westwards to the Bay of Bengal, the Arabian Sea and Gulf of Aden, and southwards to Zanzibar area (Dijkstra 1995; Dijkstra & Janssen 2013). Now also extended more southwards to the northwestern area of Madagascar (new record). This species is living on soft substrata of mud or muddy sand at a bathyal range of - 90–1500 m ( Dijkstra 2013). Present material live at - 243–1020 m. Remarks: The present specimens from northwestern Madagascar are almost morphologically completely similar to the type material from the Philippines. Only the present shells are slightly more circular in outline and the commarginal sculpture of the right valve is somewhat more prominent than that of the type specimens. Other characters are identical, i.e. smooth surface of left valve, weak commarginal sculpture on the right valve and in late growth stage on the left valve, number of internal riblets, and coloration and transparency of disc surface.

Shell crystallography is given by Hayami (1988), and Knudsen (1967) reported on the soft parts, diet and reproduction.

Propeamussium caducum is the type species of Flavamussium Oyama, 1951 , proposed as a subgenus of Parvamussium Verrill, 1897 (Dijkstra 1995: 12) . However, P. caducum is a typical Propeamussium .