Comprehensive Description

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General Description

Shell: The shell is whitish in colour, chalky, and is covered by dehiscent yellowish brown periostracum. The shell is large (up to 240 mm long, 95 mm high, and 63 mm wide), elongate, heavy and solid. The valves are strongly inequilateral, with a slightly inflated and incurved umbo positioned in the far anterior of the shell (18-20% of the length). The anterior margin is short, rounded, and slightly gaping due to the outward flexure of the left valve. The anterodorsal margin is short and slightly convex. The umbonal cavity is moderate, and the beaks are mildly inflated. The posterior margin is subangular, and is pointed near the ventral end. The lunule is short, sublanceolate, and poorly defined anteriorly. The posterodorsal margin is elongate, convex, and angular near the distal end. The escutcheon is steeply incised immediately posterior to the umbo in some specimens. The margin of the incision near the umbo forms a posteriorly directed ridge that extends toward the postero ventral margin. The ligament is deeply embedded and highly inflated. It is dark brown in colour, lanceolate, and is calcified along the hinge plate in large individuals, encompassing 16-25% (calcified portion) or 38-41 % (calcified and uncalcified portion) of the posterodorsal margin. The ventral shell margin is nearly straight along the midpoint in small individuals, and is mildly concave in large specimens. The sculpture of the shell exterior consists of a strong radial ridge from the umbo to the posteroventral tip, with similar adjacent ribs on some specimens. There are poorly defined commarginallirations on the shell and the periostracum, which are most crowded near the anterior end. The commarginal ridging is suggestive of growth rings that are weakly evident on some specimens. A slight flexure is evident along ventral margin when viewed ventrally, most notably near the posterior end. Large individuals have a flaky, mostly dehiscent periostracum, except along the shell margin, where the periostracum overlaps the shell margin to provide a complete seal when the shell valves are closed. The periostracum is inflated and ruffled along the anterior to the anteroventral margin. Dissolution of the external shell is moderate to extreme in some specimens, principally ventral and posterior to the umbones. The fenestrations resulting from dissolution are occasionally repaired by localized calcification of the inner shell in some specimens.

The right valve has two cardinal teeth beneath the umbo. The anterior cardinal tooth is strongly protuberant and has parallel to subtrigonal borders. It points ventrally from the umbo, and has a convex to slightly concave medial surface. The posterior tooth is dorsal to the anterior cardinal, and is protuberant, narrow, and slightly bifid in some specimens. The anterior and posterior cardinals are joined under the beak. Three sockets are formed by the cardinal teeth and the umbonal shell margin to accept the cardinal teeth from the left valve. The central socket is deepest, and is triangular in shape. The posterior hinge plate is massive, and forms a nymph subtending and partially enveloping the ligament.

The left valve has three cardinal teeth and two sockets to accept the central and dorsal cardinal teeth of the right valve. The anterior cardinal is strongly protuberant, narrow to massive, and is rounded medially. It has a convex anterior margin that merges ventrally with the hinge plate, and a flat posterior face that contacts the anterior cardinal of the right valve. The central cardinal tooth is massive, strongly protuberant, trigonal in shape, and is pointed to nearly blunt. It has a convex anterior surface, while the posterior contact surface is flat. The posterior cardinal is positioned dorsally, and is small compared to the other teeth. The posterior cardinal is long, narrow, produced only slightly above the hinge plate, and is nearly horizontal. It has a medial surface that is nearly smooth to mildly serrate.

The internal shell surface is porcellaneous and has faintly developed radial internal riblets and minor commarginal undulations. The anterior adductor muscle scar is recessed dorsally and posteriorly. It has an ovately conic to subelliptical shape, with minor concentric lirations, which extend to the anterior shell margin in small individuals. The posterior adductor muscle scar is larger, and has an irregularly ovate, teardrop shape, or is pear-shaped, pointed dorsally. The posterior adductor scar lacks the lacking supportive shell sculpture found in the anterior scar. The pallial line is weakly evident, broad, and has sinuous and irregular margins. It is mildly convex anteriorly and ventrally, but angular posteriorly, forming a small pallial sinus.

Soft anatomy: The most conspicuous features are the greatly enlarged and often sulphur-coloured ctenidia, a large and heavily vascularised foot, a reduced digestive system, and a red, haemoglobin-rich blood, which all relate to the chemosynthetic life style of the species.

Mantle and siphons: The mantle lobes are bilaterally symmetrical, and are thickened around the shell margin, particularly near the anteroventral margin. The lobes are attached to the shell by thick, broad pallial muscles. The mantle cavity opens to create a pedal gape from the ventral margin of the anterior adductor muscle to the ventral anterior margin of the incurrent siphon. The thick folds of the inner mantle are fused posteriorly to form separate incurrent and excurrent siphons. The fusion extends dorsally between the adductor muscles. The mantle margin is thickened and inflated along the anterior margin. There is a band of sensory papillae along the thickened anterior mantle margin. The incurrent and excurrent siphons are formed by fusion of the mantle, and are conical to cylindrical in a side view, and ovate in cross section. They are positioned in a pallial sinus that is formed by folds of thickened mantle musculature. There is highly developed pallial musculature near the posteroventral shell margin in the siphonal region. The incurrent siphon is larger and more ovate than the excurrent siphon. The distal margin of both siphons is uneven, and slightly serrate, but lacking in papillae. There is a densely branched structure near the base of the incurrent siphon that functions as a filter to reject large particles. The excurrent siphon is smaller in cross section than the incurrent siphon, with a mildly serrate distal margin. A thin collar of tissue lines the internal siphonal walls to form a one-way valve similar to other vesicomyids.

Ctenidia: Greatly enlarged ctenidia envelop the body along its length, from the umbonal cavity ventrally through much of the shell cavity. The inner and outer demibranchs on each side of the body have ascending and descending lamellae. The inner demibranchs are fused along the distal margins to the middle of the visceral mass and are joined posteriorly, isolating the incurrent and excurrent pallial chambers. The ctenidia are variously coloured among specimens, from a bright sulphur yellow to a purplish red, presumably depending upon the content of elemental sulphur in the endosymbiont bacteriocytes.

Foot and visceral mass: The foot is large, and is generally conical, highly muscular and distensible, particularly in its ventral half. The foot is highly vascularized, and deep red in colour owing to its haemoglobin content. Dorsally, the foot grades into the visceral mass, and houses a large gonad that is surrounded laterally and ventrally by the foot musculature, and dorsally by the stomach, digestive gland, intestinal tract, and heart. The labial palps, stomach, and intestine are greatly reduced, similar to other vesicomyids.

(Barry & Kochevar, 1999).


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