Parvamussium australanum Dijkstra & Maestrati, 2010

Dijkstra, Henk H. & Maestrati, Philippe, 2013, Pectinoidea (Bivalvia: Propeamussiidae, Entoliidae and Pectinidae) from the Tarava Seamounts, Society Islands and the Tuamotu Archipelago (French Polynesia), Zoosystema 35 (3), pp. 361-375 : 363-367

Parvamussium australanum Dijkstra & Maestrati, 2010

Parvamussium australanum Dijkstra & Maestrati, 2010: 335 , fig. 2A-E.

MATERIAL EXAMINED. — Austral Islands. East of Rapa Island, 800-850 m, holotype spm ( MNHN 21377).

Tarava Seamounts. Tarasoc, stn DW 3297, 18˚19’S, 148˚32’W, 380 m, 1 rv ; stn DW 3301, 19˚18’S, 151˚01’W, 700-990 m, 1 lv; stn DW 3302, 19˚15’S, 150˚57’W, 600- 660 m, 1 lv; stn CP 3303, 19˚15’S, 150˚56’W, 587-704 m, 1 lv, 1 rv; stn DW 3309, 19˚12’S, 151˚35’W, 614-664 m, alive,1 spm;stn DW 3325,19˚17’S, 151˚33’W, 595-628 m, 1 lv; stn DW 3330, 18˚45’S, 152˚16’W, 717-794 m, alive, 1 spm, 1 lv, 1 rv; stn DW 3331, 18˚45’S, 152˚17’W, 766 m, alive, 1 spm, 6 lv, 15 rv; stn 3332, 18˚45’S, 152˚18’W, 790-880 m, alive, 1 spm, 9 lv, 8 rv; stn DW 3333, 18˚45’S, 152˚18’W, 795-975 m, alive, 1 spm, 16 lv, 18 rv; stn DW 3334, 18˚22’S, 154˚06’W, 635-760 m, 1 rv; stn DW 3336, 18˚23’S, 154˚06’W, 573-619 m, 1 rv; stn DW 3338, 18˚22’S, 154˚08’W, 713 m, 1 rv; stn CP 3350, 15˚05’S, 148˚03’W, 981-992 m, alive, 1 spm.

Society Islands. Tarasoc, stn DW 3406, 16˚31’S, 152˚32’W, 504-763 m, alive, 1 spm; stn DW 3415, 16˚34’S, 151˚47’W, 614-730 m, 1 lv; stn DW 3418, 16˚33’S, 151˚48’W, 580-618 m, alive, 2 spms; stn DW 3420, 16˚46’S, 151˚04’W, 550 m, 1 rv; stn DW 3429, 16˚43’S, 150˚38’W, 493-540 m, 1 rv; stn DW 3442, 16˚41’S, 151˚26’W, 515-550 m, 1 rv; stn DW 3451, 16˚53’S, 151˚21’W, 440-490 m, 2 lv, 3 rv; stn DW 3457, 16˚45’S, 151˚24’W, 520-572 m, alive, 3 spms, 1 rv.

Tuamotu Archipelago. Tarasoc, stn DW 3349, 15˚05’S, 148˚03’W, 976-997 m, alive, 1 spm; stn DW 3350, 15˚05’S, 148˚03’W, 981-992 m, alive, 1 spm; stn DW 3351, 15˚04’S, 148˚01’W, 976-983 m, 1 lv; stn DW 3373, 15˚41’S, 146˚54’W, 507-607 m, 1 rv; stn DW 3374, 15˚39’S, 146˚54’W, 703-790 m, 2 lv, 7 rv; stn CP 3376, 15˚41’S, 146˚54’W, 646-737 m, alive, 1 spm; stn DW 3378, 15˚38’S, 146˚51’W, 887-890 m, 1 lv; stn DW 3380, 15˚39’S, 146˚56’W, 970-1060 m, alive, 1 spm.

DISTRIBUTION. — Austral Islands, alive at 500-800 m (minimum depth range). Now also Society Islands and Tuamotu Archipelago (new record), alive at 572-981 m (minimum depth range).

REMARKS. — The present specimens are identical to the type material. The intervals of the commarginal lamellae on the left valve are variable in development

and the interstitial radial sculpture varies in prominence. However, these variations could also be observed in the type material from the Austral Islands.

Parvamussium lamellatum n. sp. (Fig. 1)

TYPE MATERIAL. — Society Islands . Tarasoc, stn DW 3463, 17˚34’S, 149˚54’W, 460-505 m, alive, paratypes 4 spms, 1 lv ( MNHN 25697 ) ; stn CP 3464, 17˚34’S, 149˚54’W, 460 m, alive, holotype spm ( MNHN 25695 ), paratypes 3 spms, 1 lv ( MNHN 25696 ) ; stn DW 3476, 17˚29’S, 149˚45’W, 435-490 m, alive, paratypes 3 spms (two MNHN 25698 , one ZMA Moll. 398322) .

TYPE LOCALITY. — Society Islands, Moorea, 17˚34’S, 149˚54’W, 460-505 m (Tarasoc, stn DW 3463).

ETYMOLOGY. — Left valve covered with lamellae (latin: lamellatus, adjective meaning thinly layered).

OTHER MATERIAL EXAMINED. — Society Islands . Tarasoc, stn DW 3406, 16˚31’S, 152˚32’W, 504-763 m, 1 lv ; stn DW 3408, 16˚25’S, 152˚17’W, 437-536 m, 34 lv, 91 rv; stn DW 3415, 16˚34’S, 151˚47’W, 614-730 m, 1 lv; stn DW 3455, 16˚52’S, 151˚20’W, 430-527 m, alive, 1 spm, 3 lv, 3 rv; stn DW 3482, 17˚29’S, 149˚45’W, 440 m, 2 lv; stn unknown, 300-900 m, 1 lv, 1 rv.

Tuamotu Archipelago. Tarasoc, stn DW 3378, 15˚38’S, 146˚51’W, 887-890 m, 1 lv.

DISTRIBUTION. — Society Islands and Tuamotu Archipelago, alive at 460 m depth.

DESCRIPTION

Shell up to c. 10 mm in height, fragile,opaque whitish or pigmented with white and creamy dots and streaks, almost circular, inequivalve, nearly equilateral,weakly inflated, left valve slightly more convex than right valve, auricles unequal in shape and size (anterior larger than posterior). Prodissoconch 230 µm long. Left valve sculptured with strongly developed,closely spaced, commarginal lamellae and closely arranged radial rows of imbricated vesicles (damaged on many specimens), which are part of the commarginal lamellate sculpture, commencing at 1.5 mm from dorsal margin (radial rows of vesicles c. 12-15 mm), with increasing intercalated radial row of vesicles to ventral margin (c. 30 mm). Anterior auricle sculptured with prominent, closely spaced, commarginal lamellae and one or two hollow bands, posterior auricle almost identically sculptured with slightly more closely spaced and less prominent commarginal lamellae.

Right valve with closely spaced, regularly arranged, weak commarginal lirae,lamellate near ventral margin. Marginal apron (c. 1.5 mm) pressed against left valve, broken off many specimens. Anterior auricle with coarse, closely spaced commarginal lamellae, posterior more delicate. Hinge line straight. Byssal notch shallow, byssal fasciole narrow, ctenolium lacking.

Internal riblets (ten and one weak auricular on each side, plus one or two rudimentary in some specimens) commence in early growth stage and extend to ventral submarginal area.

Dimensions of the holotype: H 9.5 mm, W 9.7 mm, D 2.2 mm.

REMARKS

The present species is morphologically close to Parvamussium australanum , known from the Austral Islands, but differs somewhat in size ( Parvamussium lamellatum n. sp. up to 10 mm high, P. australanum up to 12 mm), in having radial rows and intercalated secondary radial rows of vesicles on the left valve,which are lacking in P. australanum ( Dijkstra & Maestrati 2010: fig. 2A), in having closely spaced, prominent commarginal lamellae ( P. australanum with widely spaced commarginal sculpture), in lacking intercalated radial microsculpture on the left valve (present in P. australanum ), in having more prominently developed internal ribs (10 ribs) commencing early ingrowth ( P. australanum 12 ribs, commencing later on the central part of the inner surface).

Parvamussium undosum Dijkstra, 1991 , known from the southwestern Pacific, is similar in shape, size and internal ribs, but differs from P. lamellatum n. sp. by having more delicate and more closely spaced lamellae on the left valve and auricles and by lacking the radial rows of vesicles. Moreover, P. undosum has small solid radial lirae on the left valve that are lacking on P. lamellatum n. sp.

Parvamussium scutulatum n. sp. (Fig. 2)

TYPE MATERIAL. — Tarava Seamounts. Tarasoc, stn DW 3314, 19˚14’S, 151˚39’W, 803-815 m, paratypes 2 lv, 1 rv ( MNHN 25700 ) ; stn DW 3328, 18˚46’S,

FIG. 1. — Parvamussium lamellatum n. sp.: A, holotype, external view of the left valve; B, holotype, internal view of the left valve; C, holotype, external view of the right valve; D, holotype, internal view of the right valve; E, prodissoconch and early stage of the left valve; F, details of the right valve; G, H, details of the left valve; I, details of the right valve. Scale bars: A-D, 2 mm; E, 200 µm; F, 100 µm; G-I, 200 µm. All SEM pictures.

152˚15’W, 788-836 m, paratype 1 lv ( MNHN 25701 ) ; stn DW 3333, 18˚45’S, 152˚18’W, 795-975 m, paratypes 3 lv, 4 rv (five MNHN 25702 , two ZMA Moll. 398323) ; stn DW 3340, 18˚24’S, 154˚09’W, 787-792 m, paratypes 4 lv ( MNHN 25703 ) .

Society Islands. Tarasoc, stn DW 3421, 16˚46’S, 151˚04’W, 782-847 m, alive, holotype spm ( MNHN 25699).

Tuamotu Archipelago. Tarasoc, stn DW 3354, 15˚55’S, 147˚06’W, 860-1030 m, paratypes 1 lv, 1 rv (MNHN 25705).

TYPE LOCALITY. — Society Islands, Huahine, 16˚46’S, 151˚04’W, 782-847 m (Tarasoc, stn DW 3421).

ETYMOLOGY. — Left valve with reticular sculpture (latin: scutulatus, adjective meaning with lozenge-shaped pattern).

DISTRIBUTION. — Society Islands and Tuamotu Archipelago, 782-1030 m, living in 782- 847 m.

DESCRIPTION

Shell up to c. 5 mm in height, fragile, opaque to semi-transparent white, circular, inequivalve, left valve slightly more convex than right valve, almost equilateral, auricles subequal in shape and size, anterior demarcated from disc, posterior continuous

FIG. 2. — Parvamussium scutulatum n. sp.: A, holotype, external view of the left valve; B, holotype, internal view of the left valve; C, holotype, external view of the right valve; D, holotype,internal view of the right valve; E, prodissoconch of the left valve; F, G, details of the right valve; H, I, details of the left valve. Scale bars: A-D, 1 mm; E, 50 µm, G, H, 100 µm; F, I, 200 µm. All SEM pictures.

with disc, umbonal angle c. 110˚. Prodissoconch 185 µm long.

Left valve with reticulate sculpture of widely spaced commarginal and more closely spaced radial threads, commencing at 2 mm shell height, extending to ventral margin. Umbonal region glossy. Auricles with some closely spaced commarginal lamellate riblets near lateral margins.

Right valve with regularly spaced commarginal lirae. Auricles with commarginal lamellae laterally, slightly stronger on anterior, more closely spaced on posterior. Hinge line straight. Byssal fasciole

narrow. Each valve with 12 short internal riblets, commencing in late growth stage.External sculpture clearly visible from interior side. Resilifer triangular. Byssal notch moderately deep, ctenolium lacking.

Dimensions of the holotype: H 4.7 mm, W 4.7 mm, D 0.9 mm.

REMARKS

The present species is morphologically closest to Parvamussium multiliratum Dijkstra, 1995 , known from the southwestern Pacific. The two species are similar in shape, but differ in size ( Parvamussium

FIG. 3. — Parvamussium vesiculosum n. sp.: A, holotype, external view of the left valve; B, holotype, internal view of the left valve; C, early stage of the left valve; D, prodissoconch of the left valve; E, details of the left valve. Scale bars: A, B, 1 mm; C, E, 500 µm; D, 50 µm. All SEM pictures.

scutulatum n. sp. up to c. 5 mm high, P.multiliratum up to c. 9 mm), in the sculpture on the left valve ( P. scutulatum n. sp. reticular, P. multiliratum commarginal), and in the number of internal riblets ( P.scutulatum n. sp. 12 short riblets, P.multiliratum 14 longer riblets plus one or two rudimentary).