Stenoplax marcusi (Righi, 1971)

(Fig. 1 A-E)

Ischnochiton marcusi Righi, 1971: 129-131, figs 19-31. — Rios 1994: 18, fig. 21. — Dornellas & Simone 2011: 26, fig. 7.

Ischnochiton (Ischnochiton) marcusi – Kaas & Van Belle 1990: 102- 104, figs 1-8.

Stenoplax (Stenoplax) marcusi – Kaas & Van Belle 1994: 33.

MARION DUFRESNE MATERIAL. — Brazil • 2 specimens; Espírito Santo, off Vitória; 20°55’S, 34°01’W; 60 m; 1987; MD55, sta. DC15; Bouchet, Leal & Métivier leg., continental shelf; MNHN .

TYPE MATERIAL (EXAMINED OR NOT). — Holotype. Brazil • 1 disarticulated valve; Alagoas, off Maceió; 10°33’45”S, 36°12’00”W, 27 m; 2.IX.1965, “ Akaroa ” 104 leg; continental shelf; MZSP 36093; Paratype. Brazil • 1 specimen, 1.1 × 0.4 mm; Pernambuco, off Cabo de Santo Agostinho; 8°18’03”S, 34°50’07”W; 26.5 m; “Recife” 28; MZSP 25944 • 1 specimen, 1.2 × 0.4 mm; Pernambuco, off Cabo de Santo Agostinho; 8°16’09”S, 34°55’02”W; 11 m; “Recife” 34; MZSP 25943 • 1 specimen, 0.9 × 0.4 mm; Pernambuco, off Piedade; 8°13’06”S, 34°51’05”W; 23 m; 21.III.1967; “Recife” 07; MZSP 25945 • 1 specimen, 1.2 × 0.4 mm; Pernambuco, off Piedade; 8°15’03”S, 34°51’03”W; 26 m; “Recife” 18; MZSP 25946 • 1 specimen, 1.0 × 0.4 mm; Pernambuco, off Recife; 08°09’09”S, 34°45’08”W; 27 m; 17. V.1966; “Recife” 04, continental shelf; MZSP 36092 • 1 specimen, disarticulated valve viii; Pernambuco, off Recife, continental shelf; 08°07’01”S, 34°48’08”W; 19.5 m; “Recife” 59 leg.; MZSP 36094 • 1 specimen, 0.8 × 0.3 mm; Pernambuco, off Recife; 08°07’03”S, 34°48’01”W; 21.5 m; “Recife” 92, continental shelf; MZSP 36096 .

OTHER MATERIAL EXAMINED. — Brazil • 1 specimen, 14.1 × 4.9 mm; Paraíba, off João Pessoa; 7°26’00”S, 29°40’48”W; 21.4 m; MZSP 134341 • 1 specimen, 10.0 × 3.9 mm; Paraíba, off João Pessoa; 7°26’S, 34°30’W; 51 m; MZSP 25935 • 1 disarticulated valve viii; Pernambuco, off Recife; 08°13’00”S, 34°48’01”W; 28 m; continental shelf; MZSP 36096 • 1 specimen, 11.1 × 4.0 mm; MZSP 87403 • 1 specimen, 12.2 × 4.6 mm; Maranhão, off Barreirinhas; 01°49’S, 042°55’W; 62-64 m; MZSP 94569 • 9 specimens, varying from 9.0 × 3.1 mm to 15.3 × 5.4 mm; Espirito Santo, Trindade Island; 20°30’10.3”S, 29°20’36.1”W; 12.1 m; MZSP 108544 .

TYPE LOCALITY. — Brazil; Alagoas, Off Piaçabuçu; 10°33.45’S, 36°12.00’W.

DISTRIBUTION. — From Alagoas to São Paulo state (Righi 1971), Brazil.

DIAGNOSIS. — Animal small, elongate. Valves very elevated, with rounded posterior region, convex lateral slopes, non-beaked. Tegmentum sculptured by grooves following outline of valve in lateral and postmucronal areas, with grooves continuing in longitudinal direction in central and antemucronal areas; colour pink with randomly-distributed white spots. Valve viii with sloped antemucronal area; mucro slightly pointed; postmucronal area straight with slightly convex distal margin (Kaas & Van Belle 1990: 102).

REMARKS

The examined specimens were consistent with the original description (Righi 1971: 129-131, figs 21-31) and range in size from 4.0 × 2.5 mm to 15.3 × 5.4 mm. The shell was also coloured homogeneous pink, sometimes with randomly distributed white spots.

Stenoplax marcusi is much smaller than S. kempfi (Righi, 1971) (15.3 × 5.4 mm while S. kempfi is about 87 × 28 mm); In S. marcusi, the intermediate valves are moderately carinated, while in S. kempfi they are non-carinated; S. marcusi can present beaked intermediate valves, while the same valves in S. kempfi are never beaked. Stenoplax marcusi presents a slightly raised lateral area, and sculpture composed of regular and shallow longitudinal grooves in the central area, while in S. kempfi the lateral areas are raised, with sculpture composed of irregular and nodulose riblets and deep grooves. In S. marcusi the slit formula varies from 9-13/1/10-13, while in S. kempfi it is 16/2/10. Moreover, S. marcusi presents pink-coloured valves with sporadic white spots, while S. kempfi varies from brown to orange. Compared to Stenoplax boogii (Haddon, 1886), S. marcusi is quite smaller, with a maximum length of 15.3 mm, while S. boogii reaches 12-17 mm. The valves are also more elevated than S. boogii . In S. marcusi, the intermediate valves can be beaked, while the same valves in S. boogii are non-beaked. Finally, S. marcusi has a pink colouration with white spots, while in S. boogii colouration varies from milky white to reddish.

The range of S. marcusi is herein extended about 1130 km northwards, from Pernambuco to Maranhão, and about 1500 km southwards from Sergipe to Espirito Santo. The bathymetrical range is extended from 11 to 64 m in depth.