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Comprehensive Description

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General Description

The shell is rather small for a bathymodioline mussel, only 40-56 mm long, rather light but strong. The outline of the shell is quite variable, rather elongae-modioliform and straight to slightly curved dorsoventrally. The shell is moderately inflated, most so just in front of the middle. The valves have a length/height ratio of 2.3-2.6 and the shell is equivalve. The valves gradually become higher posteriorly, with the highest point being between the second and third third of the shell or somewhat more anterior. The beaks are slightly subterminal to almost terminal, and in adult specimens there is only a very short anterior part of about 1-4 mm in front of the umbones. The anetrior margin is narrow but evenly rounded. The ventral margin is almost straight to more or less concave in the anterior half. The posterior margin is evenly rounded in its ventral part and convex dorsally. The postero-dorsal angulation is rounded to indistinct, and is situated above the posterior part of the posterior retractor scar.

The ligament plate is slightly convex, often more so in its anterior part. The shell exterior has well-marked irregular growth lines and very fine and faint radial striae or vermiculations that are only visible under a lens and are slightly visible on the interior. The umbo is very broad and flattened.

The periostracum is thick, hard, dark brown to almost black in colour, smooth and glossy to somewhat dull. There are sometimes irregular co-marginal folds that are not reflected on the chalky surface, often eroded in the umbonal region. The surface is frequently covered with the byssal endplates of other specimens.

The hinge is toothless, and the anterior hinge margin is almost not broadened towards ventrally. The ligament is opisthodetic, strong and extends over half the length of the posterior part of the shell and ends abruptly posteriorly. The ligamental plate in gerontic specimens is often 'secondarily' thickened. The subligamental shell ridge is very thin to indistinct. The anterior adductor scar is small, somewhat circular and is situated just under the umbo. The posterior adductor scar is rather large and is united with the posterior-most part of the posterior pedal and byssus retractor muscle complex. The anterior scar of the same complex is usually separated from it, and it is elongated and situated under the posterior-most part of the ligament. Occasionally, both retractor scars appear united, forming one long scar. The anterior byssus retractor muscle scar is situated in the posterior part of the umbonal cavity and is visible only from the ventral perspective. The pallial line is parallel to the ventral margin, and is straight or nearly concave.

The shell beneath the periostracum is dull-white in colour, internally nacerous, and has a pinkish hue on the dorsal part.

The animal has long fleshy ctenidia. The outer and inner demibranchs are of equal length. The ascending lamellae of the outer branch are anteriorly fused to the mantle for a very short distance. Those of the inner demibranch are fused to the visceral mass. The food-groove is very shallow and narrow.

The inner mantle folds separate along the entire length of the ventral margin from in front of the anterior adductor to the posterior margin. The edges are extremely frilled and undulate on the posterior part under the posterior adductor for a length of about 12 mm in a 50 mm specimen, but are only slightly frilled towards the anterior. At the anterior, the inner mantle folds terminate over the anterior adductor and fold back directly onto the muscle, but do not visibly continue transversally over it to meet the mantle fold of the opposite side. The valvular siphonal membrane is short and thin, without a median papilla, although some specimens a vestige of a papilla is visible.

The foot is slender and more or less pointed. The foot-byssus retractor muscle complex is somewhat variable, and is moderately elongated when compared to other bathymodioline mussels. The anterior retractor is long, and is attached to the shell in the posterior part of the long umbonal cavity, with the anterior-most part in or just behind the middle of the cavity. The posterior pedal retractor is thin to rather thick, with a few slight transversal grooves anteriorly. The posterior byssus retractor comprises of several diverging muscle bundles with a common base at the base of the byssus. The anterior part is attached to a long scar at about the middle of the valves, sometimes at almost a right angle (anterior-most bundle) and at about 80° and 70° to the longitudinal shell axis (second and third bundle). The posterior part is about the same length as the anterior retractor or only slightly longer, and passes towards one long attachment point directly in front of the attachment point of the posterior adductor. The posterior pedal retractor muscle is rather thin, and arises from the anterior side of the foot base behind the base of the anterior retractor muscles, passing towards the anterior bundle of the posterior byssus retractor, reaching the shell inside closely appressed to the bundle for a short part of its length only.

The labial palps are strong. The palps of the anterior pair are narrow-triangular, 2 mm long in a 50 mm specimen, whilst the palps of the posterior pair are long and narrow-triangular, measuring 5mm long in the same specimen.

The stomach is situated just behind the middle of the anterior half of the shell length and is rather large, with thick walls and a distinguishable anterior chamber. The posterior chamber is longer and broader. No entrances of digestive diverticula were detected on the inner surface, presumably an artefact of preservation). On the posterior left side there is a shallow depression that corresponds to the left pouch. The stomachs observed were filled with some mucus and sediment grains. The midgut runs posteriorly straight and medially from the stomach to under the ventricle, bending upwards and entering the ventricle in a vertical S-curve. Behind the heart, the intestine passes over the posterior adductor and towards ventrally on its posterior side, anus at mid-diameter of the posterior adductor.

The heart is small and narrow, situated in the posterior half of the shell, with the anterior extremity of the ventricle just behind the mid-shell/mantle. The auricles are long and fused posteriorly in front of the posterior adductor and extend forward in narrow lobes to the middle of the anterior part of the posterior foot byssus retractor.

(Cosel, 2008)

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