Thyasira vulcolutre, Rodrigues, Clara F., Oliver, Graham & Cunha, Marina R., 2008

Rodrigues, Clara F., Oliver, Graham & Cunha, Marina R., 2008, Thyasiroidea (Mollusca: Bivalvia) from the mud volcanoes of the Gulf of Cadiz (NE Atlantic), Zootaxa 1752, pp. 41-56 : 43-49

Thyasira vulcolutre sp. nov. Rodrigues CF & Oliver PG

(Figs 2–23)

Material examined. Holotype: one live-collected specimen, MSM01-03 stn 217 GKG10, Captain Arutyunov mud volcano, Gulf of Cadiz, 35º39.643'N, 07º20.046'W, 1321 m, 30 April 2006, M.R. Cunha ( NMW.Z.2007.3.1).

Paratypes: 24 live-collected specimens as holotype. ( NMW.Z.2007.3.2-3; USNM 1110226, DBUA 00826.01).

Other material: All from Gulf of Cadiz; nine live-collected specimens, MSM01-03, stn 246/274 BIGO 3, Captain Arutyunov mud volcano, 35º39.738'N, 07º20.010'W, 1321 m, 10 May 2006 ( NMW.Z.2007.3.4-6, DBUA 00826.02); one live-collected specimen, TTR10, stn AT243G, Carlos Ribeiro mud volcano, 35º47.217'N, 08º25.313'W, 2200 m, 26 July 2000 ( DBUA 00827.01); one shell, TTR16, stn AT615Gr, Carlos Ribeiro mud volcano, 35º47.238,N, 08º25.272’W, 2200 m ( NMW.Z.2007.3.7).

Description. Measurements: Holotype, length 17.0 mm, height 16.5 mm, tumidity 10.4 mm. Summary statistics of morphometric ratios are presented in Table 1 .

Shell (Figs 2–11): To 17.2 mm in length; thin shelled; equivalve; subequilateral, beaks slightly in front of the midline, pointed prosogyrate. Relatively tumid but variable, (Length:Tumidity ratio 1.3:1 to 1.9:1, mean 1.6:1). Outline typically subcircular, with length similar to height, some specimens with length greater than height (see variation below). Lunule margin incurved, long. Anterior dorsal margin narrowly curved, ventral margin broadly curved. Posterior sinus distinct but shallow, submarginal sinus distinct. Posterior dorsal margin more or less straight. Posterior sulcus shallow but distinct, submarginal sulcus shallow but with a sharp, well-defined margin. Auricle elevated but low and very long almost filling the entire escutcheon. Ligament visible for most of the length of the auricle (Escutcheon:auricle ratio 1.1:1 to 1.5:1, mean 1.3:1). Lunule shallowly sunken, cordate, broad. Hinge plate narrow, lacking clearly defined teeth. Ligament long, sunken. Sculpture concentric, of lines and relatively regular narrow ridges. Radial lines visible in some, but are not surface features but are reflected inside the shell by numerous radial striations. Pallial line entire. Anterior adductor scar long; posterior adductor scar oval. Periostracum very thin, pale, greyish brown. Va r ia ti on: The outline is variable (Figs 8–10), this variability generally increases with the size of shell, and is reflected mostly in the disproportionate increase in tumidity in the largest size classes. The expression of the lunule is highly variable and is probably related to the variation in shell tumidity.

Anatomy: Mantle edge fused posteriorly between termination of gill axis and posterior adductor; exhalant siphonal opening simple, margins smooth (Fig. 13). Labial palps small, with indistinct sorting ridges. Posterior ventral mantle edge with a glandular area on its inner side (Fig. 16). Ventral mantle edge with a distinct median fold (Fig. 17). Anterior inhalant aperture with an elongate glandular area on its inner side (Fig. 14); ventral side of anterior adductor muscle with a digitate sensory papilla (Figs 14, 15). Ctenidium large, of outer and inner demibranchs, both as thick fleshy lamellae (Fig. 12). Outer demibranchs about 2/3 size of inner demibranchs. Foot elongate, vermiform (Fig. 12), with a muscular ring at the junction with the visceral mass; heel indistinct. Visceral mass, lateral body pouches large, extensively lobed (Figs 12, 18). Lobes flat-ended, roughly cuboidal. Lobes arise from large, single orifice at base of stomach (Figs 19, 20). Stomach cylindrical in a transverse longitudinal orientation; midgut a simple tight loop and lies over stomach before looping posteriorly into the straight hindgut and rectum (Fig. 20). Kidneys large (Fig. 19), with dense aggregations of crystalline granules and a yellow/white fibrous mass in the lumen.

Note. Dissection of the holotype revealed an identical gross structure to that described above but the ctenidia were thin and semi-transparent and the lateral body pouches were greatly contracted (Fig. 21). The kidney was extensively stained dark brown and was massively infested with a fibrous mass and crystalline granules (Figs 22, 23).

PLATE 1. Thyasira vulcolutre sp. nov.; Figs 2–5 Holotype, NMW.Z.2007.3.1; Figs 6 & 7 Juvenile shells, NMW.Z.2007.3.2-3; Figs 8–10 variations in shells form, NMW.Z.2007.3.4-6; Fig. 11 large shell, NMW.Z.2007.3.7.

PLATE 2. Thyasira vulcolutre sp. nov., NMW.Z.2007.3.4-6. Gross anatomy after removal of the right shell and mantle. Fig. 12 whole view; Fig. 13 enlarged view of area A, the posterior exhalant aperture; Fig. 14 enlarged view of area B, the anterior inhalant aperture; Fig. 15 detail of the sensory papilla; Fig. 16 enlarged view of area C, the posterior ventral area; Fig. 17 enlarged view of area D, the median ventral mantle edge.

PLATE 3. Figures 18–20: Thyasira vulcolutre sp. nov., NMW.Z.2007.3.4-6. Fig. 18 Gross anatomy after removal of right shell, mantle and ctenidium; Fig. 19 Gross anatomy after removal of right mantle, ctenidium and lateral body pouch; Fig 20 Dissection of the alimentary system; Figures 21–23. Thyasira vulcolutre sp. nov. Holotype, NMW.Z.2007.3.1; Fig. 21 Gross anatomy after removal of right shell and mantle; Fig. 22 kidney as viewed through the epithelial tissues; Fig. 23 Kidney dissected showing crystalline granules and fibrous mass in lumen.

Distribution. Most of the specimens were collected on the Captain Arutyunov mud volcano, some specimens were also collected on Carlos Ribeiro mud volcano; depth range between 1300 and 2200 m.

Etymology. Vulcolutre , Latin combination of vulcano from “volcano”, lutum from “mud”; suffix tre denoting “belonging to”.

Remarks. Species comparisons: This new species is morphologically similar to Thyasira sarsi (Philippi, 1845) , T. southwardae Oliver & Holmes, 2006 , T. oleophila Clarke, 1989 and T. methanophila Oliver & Sellanes, 2005 all known to be associated with chemosynthetic sites. Thyasira striata Sturany, 1896 has also been recorded from mud volcanoes in the eastern Mediterranean ( Olu-Le Roy et al. 2004) but that species has a very distinct posterior sulcus and is more similar to the T. flexuosa group than to the above taxa.

Thyasira sarsi has been reported from methane seeps in the North Sea ( Dando et al. 1994) but is also known from vegetation-derived organic-rich environments in Norwegian fjords ( Southward 1986). Thyasira sarsi differs by having a weakly developed posterior sulcus, a submarginal sulcus without a sharp, clearly defined margin and the lunule not so depressed and heart-shaped. Thyasira sarsi also possesses shell sculpture without concentric ridges and with a distinctly granular surface (see Oliver & Holmes 2006).

Thyasira southwardae has been reported only from the Logatchev hydrothermal vent in association with Bathymodiolus and Calyptogena . Only two shells and one valve of this species have been collected, making comparisons difficult but it is clear that the shells of T. vulcolutre and T. southwardae are similar. The shells of T. southwardae (see Oliver & Holmes 2006) are thinner and have irregular sculpture of concentric lines and corrugations and never display the regular concentric ridges typical of T. vulcolutre . The definition of the submarginal sulcus is not so sharp in T. southwardae and the submarginal sinus and posterior sinus are not so pronounced. From the anatomy, the gills of T. southwardae were pink in contrast to the light brown in T. vulcolutre .

Thyasira oleophila was described from the Louisiana oil and gas seeps at depths around 500 m. It is a large species reaching 22.0 mm in length and is almost devoid of a posterior sulcus, is not expanded anteriorly and has a strongly granular surface (see Oliver & Holmes 2006).

Thyasira methanophila Oliver & Sellanes, 2005 was described from a methane seepage area off Concepcion, Chile at bathyal depths. Thyasira methanophila differs from T. vulcolutre in that the lunule is not sunken, and the escutcheon is less defined.

From morphology alone, a new genus could be proposed for the group of species similar to T. vulcolutre , including T. methanophila , T. southwardae , T. sarsi and T. oleophila but this is not wholly supported by the molecular data.

An 18S rRNA gene Neighbour Joining tree (not shown) combining an unpublished sequence from Thyasira vulcolutre with previously published sequences from thyasirids analyzed by Taylor, Williams and Glover (2007) shows that T. vulcolutre is clustered with T. perplicata Salas, 1996 and is distant from T. methanophila and T. sarsi . (S Williams and JD Taylor pers. comm.). Payne and Allen (1991) described T. perplicata under the name T. excavata plicata ( Verrill, 1885) as having a shell with a strong posterior carina and thus most similar to shells of the genus Conchocele . Anatomically T. perplicata is also similar to Conchocele with a bulbous tipped foot, large fleshy gills and prominent anterior mantle edge folds ( Nakazima, 1958; Kamenev et al. 2001).

The lack of congruence between morphology and molecular data may be clarified once a larger set of taxa are included but until this study is completed the genus Thyasira is retained for the majority of species with sulcate shells, with two demibranchs and that are chemoautotrophic.

Functional morphology: The functional morphology is essentially similar to other thyasirids that are primarily chemosymbiotic and strongly associated with chemosynthetic settings ( Oliver & Sellanes 2005). The gills are thick and fleshy and of Dufour Type 3 ( Dufour 2005). The foot is vermiform and the lateral body pouches are large and multilobed. Stable isotopic ratios are in the range of other symbiotic chemosyntheticbased bivalves hosting sulfide-oxydizing symbionts (CF Rodrigues unpublished results).

The presence of a sensory papilla situated on the anterior ventral face of the anterior adductor muscle was first reported by Payne and Allen (1991) for the members of genus Axinus . A similar papilla is present in A. cascadiensis , and it was also reported on the dorsal surface of the adductor muscle in T. sarsi , T. flexuosa , T. obsoleta and Axinulus croulinensis (Oliver & Holmes in press). In Thyasira vulcolutre the papilla (Figs 14, 15) is situated in the inhalant flow and it probably functions in a sensory manner to monitor flow or water chemistry of the inhalant current.

An obvious feature of Thyasira vulcolutre is the extensive staining of the kidneys by both crystalline granules and fibrous material in the lumen. Similar deposits were seen in T. methanophila and could be related to metabolism in a mineral-rich environment or to the bacterial source of nutrition.

The unusual condition of the holotype suggests that the bacteria were lost from the gills and that digestion had slowed or ceased resulting in morbidity. The mortality of bivalves has been already studied and sulphide deprivation, sulphide toxicity, and disease were indicated as possible agents of mortality on the system ( Mills et al. 2005).

Tables

TABLE 1. Shell dimension ratios for 21 shells from type series of Thyasira vulcolutre sp. nov.

	Mean	Std. Dev.	Count	Minimum	Maximum
Length/Height	0.99	0.04	21	0.93	1.11
Length/Tumidity	1.60	0.17	21	1.29	1.86
Length/Length lunule	3.49	0.39	21	2.81	4.11
Escutcheon/auricle	1.25	0.08	21	1.14	1.47