Bathymodiolus mauritanicus, Cosel, 2002

Cosel, Rudo von, 2002, A new species of bathymodioline mussel (Mollusca, Bivalvia, Mytilidae) from Mauritania (West Africa), with comments on the genus Bathymodiolus Kenk & Wilson, 1985, Zoosystema 24 (2), pp. 259-271 : 261-265

Bathymodiolus mauritanicus n. sp. ( Figs 1 ; 2 ; 4 )

TYPE MATERIAL. — Holotype: off Banc d’Arguin, Mauritania, reportedly, 18°41’N, 16°45’W, 1200 m, I.1994, by the commercial trawler Peixe de Mar , 1 complete stell ( MNHN); paratypes: same locality (2 MNHN, 1 USNM, 1 NSMT, 1 NMNZ).

TYPE LOCALITY. — Off Banc d’Arguin, Mauritania, 1200 m.

MATERIAL EXAMINED. — Off Mauritania, reportedly 400 m, 2 shells, live-collected, in coll. J. Pélorce ex. coll. M. Pin.

DISTRIBUTION. — Mauritanian slope, said to be from 1000-1200 m (the “ 400 m ” of one of the lots is probably erroneous).

DESCRIPTION

Shell large, up to 110 mm long, rather thickwalled and solid, moderately short, wedge-shapedmodioliform, little variable, modestly inflated, equivalve, length/height ratio 1.8-1.9. Beaks almost terminal to terminal or even slightly protruding anteriorly. Anterior margin narrowly rounded, ventral margin visibly concave in the middle part, occasionally with a well-marked crease at the end of the first fourth or fifth of the valve, which makes the anterior part appear bent ventrally. Posterior margin ventrally broadly rounded, postero-dorsal margin still markedly convex; postero-dorsal corner well rounded. Ligament plate more or less arched. Exterior with well developed, dense, irregular growth lines. Umbone broad and somewhat flattened.

Periostracum thick, dark chestnut brown, smooth, dull, with no periostracal hairs and no byssal endplates of other specimens. Umbonal region generally eroded in half-grown and larger specimens.

A

Hinge toothless, anterior hinge margin strengthened to a buttress along the valve edge and protruding ventrally. Posteriorly, buttress not abruptly ending but gradually passing into the subligamental shell ridge. Ligament opisthodetic, strong, extending to more or less in front of the postero-dorsal corner (depending on the situation of the latter), ending abruptly without any taper. Subligamental shell ridge faint, visible along two thirds of ligament length and from both ventral and lateral perspectives. Anterior adductor scar small and oval, under or just behind the umbone. Posterior adductor scar large, rounded-oval, united with the long scar of the posterior pedal and byssus retractor muscle. Scar of the anterior bundle of the posterior retractor not at all or hardly separated from that of the posterior bundle, the anterior end situated under the posterior third or fourth of the ligament. Anterior byssus retractor scar in the posterior part of the umbonal cavity, well behind the beaks, visible only when viewed from posterior and ventrally. Pallial line only weakly concave in the middle.

Shell without periostracum dull whitish, interior white and slightly nacreous.

Animal not known, but judging from the muscle scars, it is evident that it has an almost continuous multibundle posterior retractor.

See Table 1 for measurements.

Biotope not known. All known specimens (about 200) of B. mauritanicus n. sp. were taken in one single trawl haul (G. Poppe pers. comm.), which means the trawl went over an isolated, densely populated mussel bank, obviously located on a seeping site.

TABLE 1. — Measurements of Bathymodiolus mauritanicus n. sp.

REMARKS

Among the large mussels from the Atlantic, B. mauritanicus n. sp. most closely resembles Bathymodiolus childressi Gustafson, Turner, Lutz & Vrijenhoek, 1998 , from the Gulf of Mexico (Alamiños Canyon and Louisiana continental slope) ( Figs 3A ; 5A, B ), and an undescribed species from the Barbados Accretionary Prism, currently under study by Cosel & Olu ( Fig. 5C, D ). These three species are distinguished from other Bathymodiolus species by the comparatively small anterior adductor scar, the almost continuous posterior byssus retractor scar, and the rather low and almost terminal umbones. The anterior retractor scar is placed considerably further posteriorly within the umbonal cavity than, for example, in B. puteoserpentis Cosel, Métivier & Hashimoto, 1994 , B. azoricus Cosel & Comtet, 1999 , and B. thermophilus Kenk & Wilson, 1985 . The new species is distinguished from B. childressi by the thick-walled valves, the narrower anterior end, and the still more anteriorly situated umbones. This and the position of the broadest part of the valves more posteriorly than in B. childressi makes the shell outline of B. mauritanicus n. sp. appear more wedge-shaped rather than modioliform.

The undescribed mussel from the Barbados Accretionary Prism also has the broadest part of the valve situated rather far back, although the anterior part is broader than in B. mauritanicus n. sp., and in most specimens the umbones are slightly subterminal, leaving 1/12 to 1/15 of the shell length in front of it. Bathymodiolus sp. I from Barbados shares with B. mauritanicus n. sp. the small anterior adductor scar, the continuous posterior retractor scar ( Fig. 5C, D ) and the posteriorly situated anterior retractor scar.

An Indo-Pacific species close to B. mauritanicus n. sp. is Bathymodiolus platifrons Hashimoto & Okutani, 1994 . This also has a narrow anterior end and terminal umbones. Hashimoto & Okutani (1994: fig. 2) stated that it has an interrupted posterior byssus retractor scar, though in one of the two paratypes seen (MNHN), the impression appeared continuous; the other (larger) specimen shows a short interruption only within the scar ( Figs 3B, C ; 5E, F ). These short interruptions were also observed in fully grown specimens of B. mauritanicus n. sp. and in specimens of Bathymodiolus sp. I ( Barbados) ( Fig. 5D ), the continuous scar seems to become more or less broken up during ontogeny. However, in most of the specimens of B. mauritanicus n. sp. and Bathymodiolus sp. I examined, the posterior byssus retractor scar appeared uninterrupted. In B. platifrons , the anterior retractor scar is situated in the posterior part of the umbonal cavity, another feature in common with the other mussels. Like B. childressi and unlike B. mauritanicus n. sp., B. platifrons has rather thin and light valves.

Another species, superficially resembling the four species mentioned above, with a more cuneiform shell shape with narrow anterior margin and broad posterior margin, as well as a strong multibundle posterior byssus retractor occurs east of the North Island of New Zealand. It is currently under description by Cosel & Marshall and will be discussed there (figured by Lewis & Marshall 1996: fig. 4A). The New Zealand mussel however is much more slender than B. mauritanicus n. sp., it has markedly subterminal umbones, and the highest point of the shell is still situated further backwards.

Figures

FIG. 1. — Bathymodiolus mauritanicus n. sp., holotype (MNHN), length 98.1 mm; A, exterior and interior of left valve; B, exterior, lateral and ventral views of interior of right valve.

FIG. 2. — Bathymodiolus mauritanicus n. sp., paratype (MNHN), length 99.8 mm; A, exterior and interior of left valve; B, exterior and interior of right valve; C, dorsal view.

FIG. 4. — Half-schematic drawings of the insides of the right valves of Bathymodiolus mauritanicus n. sp.; A, holotype (MNHN); B, paratype (USNM); C, paratype (NSMT). Scale bars: 10 mm.

FIG. 3. — A, Bathymodiolus childressi Gustafson, Turner, Lutz & Vrijenhoek, 1998, paratype 3129-52 (MNHN), length 83.7 mm, Bush Hill hydrocarbon seep, about 210 km SSW of Grand Isle, Louisiana, 27°46.9’N, 91°30.4’W, 546 m, Johnson Sea-Link-1 cruise, dive 3129, lateral and ventral views of interior of right valve; B, C, Bathymodiolus platifrons Hashimoto & Okutani, 1994, paratypes (MNHN), Hatsuashima Site, Sagami Bay, 35°00.2’N, 139°13.5’E, 1170 m, Shinkai 2000, dive 666; B, paratype I, length 96.1 mm, lateral and ventral views of interior of right valve; C, paratype II, length 65.4 mm, lateral and ventral views of interior of left valve.

FIG. 5. — Half-schematic drawings of the insides of the right valves of Bathymodiolus species; A, Bathymodiolus childressi Gustafson, Turner, Lutz & Vrijenhoek, 1998, paratype 3137-21 (MNHN), Green Canyon-272, Louisiana continental slope, 27°41.1’N, 91°32.2’W, 723 m, Johnson Sea-Link-1 cruise, dive 3137; B, Bathymodiolus childressi Gustafson, Turner, Lutz & Vrijenhoek, 1998, paratype 3129-52 (MNHN); C, D, Bathymodiolus sp. I, Barbados Accretionary Prism, Orénoque site, 1688 m, Diapisub cruise, stn DS 05; E, Bathymodiolus platifrons Hashimoto & Okutani, 1994, paratype II (MNHN); F, Bathymodiolus platifrons Hashimoto & Okutani, 1994, paratype I (MNHN); G, Bathymodiolus puteoserpentis Cosel, Métivier & Hashimoto, 1994, “Les Ruches” site, Snake Pit hydrothermal field, MAR, 23°22’N, 47°57’W, 3478 m, HYDROSNAKE cruise,stn HS 10 (MNHN) (for comparison). Scale bars: 10 mm.