Christineconcha, Krylova & Cosel, 2011

Krylova, Elena M. & Cosel, Rudo Von, 2011, A new genus of large Vesicomyidae (Mollusca, Bivalvia, Vesicomyidae, Pliocardiinae) from the Congo margin, with the first record of the subfamily Pliocardiinae in the Bay of Biscay (northeastern Atlantic), Zoosystema 33 (1), pp. 83-99 : 88-98

Genus Christineconcha n. gen.

Genus N 2 – Krylova & Sahling 2010: 6.

TYPE SPECIES. — “ Calyptogena ” (s.l.) regab Cosel & Olu, 2009 .

SPECIES INCLUDED. — The type species only.

ETYMOLOGY. — This new genus is dedicated to the daughter of the second author, Marie Christine von Cosel; gender feminine.

DISTRIBUTION. — Eastern Atlantic along the Equatorial African margin, off northern Angola and north of the Congo Canyon, there only known from the two pockmarks REGAB and ASTRID, from 2820 to 4017 m (Cosel & Olu 2009) ; in the Bay of Biscay (Gulf of Gascony), 4125 m ( Table 1) and also in the Gulf of Cadiz, 3060 m (C. F. Rodrigues & M. R. Cunha pers. comm.) .

DIAGNOSIS. — Shell rather large, to 122 mm long, thin-shelled and fragile, elongate-oval, quite variable in outline, with usually convex ventral margin, occasionally straight or slightly concave in its middle part. Anterior margin broadly rounded, posterior margin tapering and more or less narrowly rounded in the middle or more posterior-ventrally. Surface chalky, with fine and dense, irregular commarginal striae and small lamellae and growth lines. Extremely shallow and hardly visible escutcheon limited by sharp ridge, lunular incision missing. Posterior angle (between beaks and posterior margin) very broad and rounded, weak to obsolete. On the posterior third or fourth of the valve often broad, wave-like commarginal undulations which cross the posterior angle. Periostracum rather thin, dark brownish, dull, eroded, persistent on the antero-ventral part of the valves, where it becomes somewhat leafy near the margins. Pallial line close to the ventral margin, starting from the middle of the ventral margin of the anterior adductor scar; pallial sinus absent but pallial line in its posterior part slightly irregular. Dorsally to the pallial line a few small secondary pallial attachment scars arranged along the anterior 2 to 3 cm of the pallial line or its whole length in adult specimens (see Fig. 7C). Anterior adductor scar elongate in dorso-ventral direction, drop-shaped, its posterior margin somewhat impressed. Anterior pedal retractor scar irregularly elongate, slightly impressed, just above and behind the anterior scar, fused with it but occasionally separate. Posterior adductor scar larger, rounded, almost circular, fused with the posterior pedal retractor scar. Ligament external, parivincular, opisthodetic, on a well developed nymphal plate. Anterior lamellar layer not creating subumbonal pit. Hinge plate narrow, teeth arrangement only slightly radiating. Right valve with rather strong and well-developed anterior ventral cardinal (1), almost parallel to the antero-dorsal margin and with its posterior end situated more or less beneath the umbo. Subumbonal cardinals 3a and 3b assumed to be fused, anterior cardinal 3a thin and short, its anterior end more or less directly under the tip of the umbo. Posterior cardinal 3b lower than (1), stronger than 3a, and in its posterior part parallel to the ventral margin of the hinge plate. Left valve with a rather thin and short, somewhat variable cardinal 2a ascending towards the umbo and reaching the subumbonal and stronger, triangular wedge-like posterior cardinal 2b. Postero-dorsal cardinal 4b more or less long, thin and radiating. Ctenidia with inner demibranchs only, with descending and ascending lamellae; interlamellar septa divided into cylindrical channels. Ventral margin of the demibranch with very shallow food groove. Alimentary canal straight. Ventral margins of the mantle not fused forming the inhalent siphon. Inner valve of inhalent siphon without processes. Margins of inhalent siphon with close-set short tentacles on both sides towards antero-ventrally, then passing into the papillate inner mantle fold. For detailed description of the type species, see Cosel & Olu (2009).

REMARKS

Christineconcha n. gen. has a very characteristic anatomical feature that distinguishes it from all known vesicomyid genera: the mantle fusion forms two pallial apertures instead of three as in other vesicomyids, the pedal gape and the opening of the exhalent siphon only. There is no ventral mantle fusion under and in front of the inhalent siphon, that means that structurally the inhalent siphon is not separated from the pedal gape ( Figs 3E, F ; 5A, B ; 6A ). This “incomplete” siphon seems to be capable to function like a “normal” closed tubular siphon since there is probably an obturative mechanism allowing to make a temporally functional tube ( Fig. 5B ). At the base of the inhalent aperture there is a strongly developed muscle band and additionally on the inner surfaces of the mantle lobes, vascularized thickenings are present, which obviously can be enlarged and reach each other, which means that this incomplete siphon is probably able to function by the formation of a temporary functional tube. In other respects there is no principal difference between the siphons of C. regab n. comb. and “typical” vesicomyid siphons. Both siphons are very short, the inhalent siphon a little bit shorter; they are united over their entire length. The margins of both apertures bear 2-1 rows of small tentacles, those of the exhalent siphon are much shorter and look like papillae. The tentaculate margin of the inhalent siphon grades into the papillate margin of the pedal gape. Internally, the inhalent siphon has on its dorsal side a ventrally directed, triangular flap of vascularized tissue, and the exhalent siphon has a thin, short sleeve ( Fig. 3E ). A well-developed

E

E, F

FIG. 7. — Christineconcha regab (Cosel & Olu, 2009) n. comb.: A, D, half-schematic drawings of valves; A, holotype,rv; B, holotype lv; C, paratype MNHN, BZ 2 , ROV PL 147 ,SL 105.1 mm, rv (note the secondary scars above the pallial line) ; D, specimen from stn CP 20, BIOZAIRE 3, 107.1 mm, rv.; E, F, hinges of the holotype: E, lv; F, rv. Abbreviations: see Material and methods. Scale bars: 10 mm.

muscular septum is present between the siphons; the intersiphonal septal retractors are fused with the ventral surface of the posterior adductor.

Similar to other vesicomyids, the foot of C. regab n. comb. is well developed; posterior to the foot stem, lateral wing-like widenings, not recorded in vesico- myids before, are present ( Figs 3A, B ; 6B ); there is no byssal groove but a small byssal aperture only.

The ctenidia of C. regab n. comb. ( Fig. 4 ) are thick and non-plicate; they comprise the inner demibranch only, with descending and ascending lamellae; the ascending lamellae with half to two thirds the size of descending lamellae. The entire interlamellar space is filled with the interlamellar septum which is divided into separate cylindrical tubes at most parts or at full width within the septum.

BIOLOGY

Christineconcha regab n. comb. and Laubiericoncha chuni (Thiele & Jaeckel, 1931) are found on REGAB frequently in the same beds in more or less close agglomerations. On submarine photographs ( Fig. 9B ) they can be recognized at once by the entirely different form of the siphon: in L. chuni , they are fused, tubuliform and well visible, they can be extended to 2-3 cm beyond the shell margin, whereas the siphons of C. regab n. comb. hardly

A new genus of Vesicomyidae (Mollusca, Bivalvia) from eastern Atlantic

surpass the posterior and postero-ventral shell margin ( Figs 5C ; 9A ). Both species being situated vertically side by side ( Fig. 10 ), C. regab n. comb. is buried in the sediment with the anterior half or two thirds of the shell, with the posterior part sticking out into the free water (in monospecific clam beds, C. regab n. comb. may also be buried more deeper (Olu-Le Roy, pers. comm.). On the contrary, L. chuni is completely buried, and only the extended siphonal tube and perhaps the posteriormost shell margin are visible. However, because of the extendibility of the siphons, L. chuni is able to take respiration water at the same height above the sediment surface or clam bank surface (about 3-4 cm) as C. regab n. comb.. As L. chuni is buried deeper in the substrate, with the extended foot it can reach the sulphides deeper in the sediment. In fact this species is found on sites where seepage is rather feeble whereas C. regab n. comb. lives and always dominates or is exclusively present on sites with higher concentration of sulphides and stronger seepage. Quite probably the short siphons of C. regab n. comb., which can be better protected by the shell are an adaptative feature for living on sites with such a high sulphide concentration.

Measured data on sulphide concentration in the agglomerations of the two vesicomyid species are not yet available.However, Olu-Le Roy et al. (2007) mention that “first environment characterisation suggested that “ C. ” (s.l.) regab can be associated to higher methane fluxes than Laubiericoncha chuni (4.4 vs 0.9 mm), which dominates the clusters in the periphery of the REGAB pockmark”. This was verified and confirmed during the GUINECO campaign, where L. chuni was found present on sites with less active methane emission and so probably also a lower sulphide level (Olu-Le Roy pers. comm.).

Another photograph ( Fig. 9C ) shows the shrimp Alvinocaris muricola (Williams, 1988) at the posterior end of a specimen of C. regab n. comb. trying to feed on pseudofaeces of the bivalve. These pseudofaeces consist of undesired particles which enter the mantle cavity with the water current via the inhalent siphon, where they are enclosed by mucus and transported by means of cilia back to the inhalent siphon and to the free water.

The toxoglossan gastropod Phymorhynchus coseli Warén & Bouchet, 2009 also occurs on these vesicomyid beds and is visible on photographs ( Fig. 9A ; see also Cosel & Olu 2009: figure 30e). The genus Phymorhynchus is known to feed on Bathymodiolus ( Warén & Bouchet 2009: 2344) but its presence near and at the vesicomyid banks suggests that it may also feed on the Vesicomyidae . Partial nutrition on vesicomyids is also suggested by a stable isotopic study indicating that signatures of both Phymorhynchus and vesicomyids are very close ( Olu et al. 2009).

Figures

FIG. 3. — Christineconcha regab (Cosel & Olu, 2009) n. comb. anatomical details: A, lateral view of foot from the left; B, ventral view of foot; C, ventral view of labial palps; D, heart; E, lateral view of siphons from the left, part of left mantle lobe removed; F, interior view of inhalent siphon. Abbreviations: see Material and methods. Scale bars: A-C, E, F, 5 mm; D, 1 mm.

FIG. 5. — A, B, Christineconcha regab (Cosel & Olu, 2009) n. comb: A., posterior part of specimen from the left side; B, posterior view of siphons; C, D, Laubiericoncha chuni (Thiele & Jaeckel, 1931): C, posterior part of specimen from the left side; D, posterior view of siphons. Abbreviations: see Material and methods. Scale bars: 1 mm.

FIG. 6. — A, B, Christineconcha regab (Cosel & Olu, 2009) n. comb.: A, specimen from PL 147, BZ 2, SL 89.9 mm, posterior end with siphons, right mantle edge removed;B, paratype MNHN, PL 147, BZ 2, close-up view of foot with broadened base (both taken from Cosel & Olu 2009); C, D, hinges; C, hinge of rv, PL 147, 105.3 mm; D, hinge of lv of holotype. Abbreviations: see Material and methods.

FIG. 4. — Christineconcha regab (Cosel & Olu, 2009) n. comb. ctenidia: A; lateral view of right gill from the inside; the dotted line represents the transverse sections of images B and C; B, C, transverse section through medial part of ctenidia; B, specimen from ROV Victor 6000 PL 74-14; C, specimen from RV Jean Charcot, DS 55, stn 3. Abbreviations: see Material and methods. Scale bars: A, 5 mm; B, C, 1 mm.

FIG. 9. — In situ images of Christineconcha regab (Cosel & Olu, 2009) n. comb. and Laubiericonche chuni (Thiele & Jaeckel, 1931) from the SW of REGAB, cruise GUINECO, (P. I.: A. Boetius), RV Meteor Dive 225, site 3, 16.VIII.2008: A, field with almost exclusively Christineconcha regab n. comb.; B, C. regab n. comb. with some Laubiericoncha chuni scattered in between and recognizable by the tubular siphons and the fully buried shell, on the right is a specimen of Phymorhynchus coseli; C, the shrimp Alvinocaris muricola Williams, 1988, trying to feed on the pseudofaeces of C. regab n. comb. (this photograph from another site at REGAB, 19.VIII.2008, same cruise). Images courtesy Karine Olu (IFREMER Brest), © MARUM, Bremen.

FIG. 10.— Schematic drawing of the Laubiericoncha-Christineconcha-Phymorhynchus-Alvinocaris community on the REGAB pockmark. The siphonal openings of both Vesicomyidae are on the same level over the sediment surface.