Propeamussium watsoni (E.A. Smith, 1885 )

Dijkstra, H. H. & Maestrati, P., 2015, Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the southwestern Indian Ocean, African Invertebrates 56 (3), pp. 585-585 : 595-596

Propeamussium watsoni (E.A. Smith, 1885)

Fig. 1K, L

Amussium watsoni E.A. Smith, 1885: 309 , pl. 22, figs 8-8c. TYpe localitY: NE of New Guinea (2°23'S

144°04'E), - 1957 m, live, blue mud, 10.iii.1875 (Challenger, stn 218).

Propeamussium watsoni bayonnaisense Okutani, 1962: 15 , pl. 2, figs 1–2. TYpe localitY: Japan, 24 miles off

Bayonnaise Rocks (32°00'N 140°21.4'E), - 2140–2160 m, live, 05.vii.1960 (Soyo­Naru, stn B4). Amussium sewelli Knudsen, 1967: 279 , pl. 2, figs 1–2, text-figs 18A, B. TYpe localitY: Zanzibar area

(4°05'54"S 41°10'12"E), - 1789 m, live, globigerina ooze, Agassiz trawl, 17.i.1934 (John Murray,

stn 118) syn. n.

Propeamussium watsoni: Dijkstra 1995: 24 , figs 27–30, 123–124; Dijkstra & Maestrati 2010: 334; 2013 a:

362; 2013 b: 470.

Description: Shell up to 52 mm high (most specimens smaller up to 40–45 mm), inequivalve, almost circular, slightly higher than wide, somewhat inequilateral, umbonal angle 120°–125°. Opaque, left valve milky-white, right valve cream. Prodissoconch 240 µm high. Left valve more convex than right valve, with delicate radial lirae that commence at c. 3 mm shell length and extend to central part of disc, crossed by commarginal lamellae in central part and extend to the ventral margin, lamellae variable in prominence or even lacking. Auricles relatively small, equal, commarginal lamellae prominent on anterior, finer and more closelY spaced on posterior. Most specimens with 10 internal ribs and a small auricular riblet on each side, commencing just below resilifer and extending to pallial line. Right valve with fine regular commarginal lirae and granulate interstitial microsculpture (prismatic calcite layer). Auricles with commarginal lirae, anterior auricle with a few radial lines near suture, absent from posterior auricle. Prominent scales on anterior and posterior dorsal margins of auricles. Resilifer triangular, erect. No byssal notch.

Type material examined: Amussium watsoni : Lectotype (pr) NHMUK 1887.2.9.3307, designated by Dijkstra (1995: 24), 2 paralectotypes (pr) NHMUK 1887.2.9.3308/1-2. Propeamussium watsoni bayonnaisense : Holotype (pr) NSMT Mo.62759, 5 paratypes (pr) NSMT Mo.62760. Amussium sewelli : Holotype (pr) NHMUK 196641, paratype (pr) NHMUK 196642.

Other material examined: NW MADAGASCAR: In front of 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°42'E), - 777–800 m, dead, campaign Miriky, stn CP3270, 11.vii.2009 ( MNHN) ; between Majunga and Cap Saint-André (15°31'S 45°45'E), - 780–800 m, dead, campaign Miriky, stn CP3271, 12.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) . S MADAGASCAR: S Pointe Barrow (25°35'S 44°15'E), - 821–910 m, live, expedition Atimo Vatae, stn CP3595, 12.v.2010 ( MNHN) .

Distribution and habitat: Northern Indian Ocean, southern Japan, Papua New Guinea, New Caledonia, Vanuatu, Society Islands, Tuamotu Archipelago, and Austral Islands (Dijkstra & Maestrati 2010: 335; 2013 a: 362). Now also northwestern and southern Madagascar (new record). Living bathyally on soft substrata of mud or muddy sand at a depth range of - 650–2160 m. Present material live at - 243–1020 m (maximum bathyal range), minimum range - 780– 821 m.

Remarks: The present material from northwestern and southern Madagascar is almost similar to the type material, although the commarginal lamellae of the left valve are almost lacking (only a few near the ventral margin in some of the largest specimens) and the commarginal lamellae of the right valve are somewhat more widely spaced than the typical specimens. However, both features are variable in examined material from other localities in the southwestern Pacific (MNHN).

There are no basic morphological differences between the Japanese morph P. watsoni bayonnaisense and P. watsoni (Dijkstra 1995: 25) .

According to Knudsen (1967: 280) P. sewelli from the Zanzibar region differs from P. watsoni in having a different shape of the auricles and a different reflection of the posterior margin of the posterior auricle. In addition he also observed differences in the “arrangement of the tentacles of the mantle edge” ( Knudsen 1967: fig. 18b, c). However, these three characters of P. sewelli are also observed in specimens of P. watsoni from other localities of the southwestern Pacific (MNHN) and are interspecific variations of P. watsoni . Therefore, P. sewelli is treated herein as a junior synonym of P. watsoni .

Knudsen (1967: 281) also treated Propeamussium alcocki (E.A. Smith, 1894) as a junior synonym of P. watsoni , whereas Dijkstra (1995: 13) considered both as valid species. Propeamussium alcocki could be distinguished from P. watsoni by having a semi-transparent thin shell ( P. watsoni is opaque and more solid), by having a more oval shape ( P. watsoni is almost circular), by having a smooth left valve and sometimes some commarginal lamellae in late ontogeny ( P. watsoni has a radial sculpture in early growth stage and more prominent commarginal lamellae in later growth stage), and by having thin internal radial ribs ( P. watsoni has more solid and broader ribs, especially on the right valve).