Comprehensive Description

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General Description

The shell is large, up to 163 mm in length, and is rather thin-shelled but strong. The shell is variable in outline and in the valve length/height ratio. It is modioliform, equivalve, more or less elongate, and is rather inflated, mostly in front of the vertical midline. The valve length/ height ratio is from 1.73 to 2.17. The umbones are broad, and are flattened to more prominent. The beaks are slightly subterminal, at about one-eighth to one-eleventh of the total shell length in adult specimens. The anterior part is very short and rather rounded. The anterior margin is narrowly but evenly rounded. The ventral margin is slightly concave, but in juvenile and half-grown specimens it is nearly straight or even somewhat slightly convex, especially in the anterior half. The strongest concavity is situated at or just behind the middle. The posterior margin is broadly and evenly rounded ventrally, whilst being slightly convex dorsally. The postero-dorsal corner is very variable, from narrowly rounded and well-marked to very broadly rounded and almost indistinct, and is situated well in front of the posterior adductor scar, close to the posterior margin where the highest part of the valve is situated.

The ligament plate is straight to weakly arched. The exterior of the shell has well-developed, irregular growth lines and marked growth waves, especially on the ventral part, which are reflected on the interior. There are very faint radial striae, mostly developed on the antero-ventral part and that are hardly visible or becoming obsolete on the posterior part. In some specimens the striae are overlain by curved striae that are visible only under a lens, which end vertically to the postero-dorsal margin.

The periostracum is thick, hard, with a dark chestnut brown to almost black colour on parts of the valves. In juveniles the colour is light brown. The periostracum is smooth, dull and has no byssal hairs, but there are scattered byssal endplates from other specimens on the valves.

The hinge is toothless, with the anterior hinge margin slightly protruding ventrally but hidden by the ligament. The ligament is opisthodetic, narrow but strong, and extends over about six-sevenths or more of the postero-dorsal margin in front of the postero-dorsal corner, and ends posteriorly in a taper. There is a weakly developed to almost obsolete subligamental shell ridge. The anterior adductor scar is oval, and is situated partly in front of and partly under the umbo. The posterior adductor scar is rather large, and is united with the posterior most scar of the posterior pedal and byssus retractor muscle complex. The anterior scar of the same complex is more or less separated from it, and is very long. It is broken up into isolated impressions, extending from about the last third of the ligament length towards the posterior. The anterior byssus retractor muscle scar is situated on the anterior part and middle of the umbonal cavity, and is visible only from a ventral perspective. The pallial line is parallel to the ventral margin, and curves according to the degree of the convexity of the margin.

The shell beneath the periostracum is a dull-whitish colour, and the shell interior is nacreous.

The ctenidia are long and narrow, measuring 79 mm in length and 12.5 mm broad in a 105.4 mm specimen. The outer demibranch is slightly shorter anteriorly than the inner demibranch. The ascending lamellae of the outer demibranch are anteriorly fused to the mantle for a very short distance (a few mm), whilst those of the inner demibranch are fused to the visceral mass for an equally short distance.

The inner mantle folds separate along the entire length of the ventral margin from the anterior adductor to the posterior margin. The edges are variable, being rather thin and continuous to just behind the vertical midline or still more posterior, then become more undulate towards the posterior, with the posterior third to fourth being heavily frilled and thick. At the anterior end, the inner mantle folds end in a few very small leaf-like fills and do not continue transversally over the anterior adductor to meet the mantle fold of the opposite side. The valvular siphonal membrane is thin and short. It is more or less thickened along its margin, and is without median papilla.

The foot is medium-sized, measuring 37 mm in length (including byssus orifice) in a 105.4 mm specimen. The foot-byssus retractor muscle complex is not prolonged. The anterior retractor is rather short, becoming very broad towards its attachment point on the shell in the median and posterior part of the long umbonal cavity, the anterior most point being behind the beaks. The posterior byssus retractor is composed by of three groups of several diverging muscle bundles with a common base at the base of the byssus. The anterior part of the posterior byssus retractor is composed three muscle bundles and is attached to a very long scar at about the middle of the valves below the posterior part and end of the ligament, at an angle of about 80° (anterior most bundle) to the longitudinal shell axis. The posterior part of the posterior byssus retractor is composed of four muscle bundles that pass towards two attachment points, with three narrow anterior bundles passing to a point behind the anterior scar, and a thicker and long posterior bundle passing to a scar just in front of the posterior adductor scar. The posterior foot retractor is 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. It reaches the shell inside closely appressed to the bundle to a very short part of its length only.

The labial palps are short and very thick. The anterior (outer) pair is small and short, with a triangular shape, measuring about 3 mm long in a 105.4 mm specimen. The palps of the posterior (inner) pair are larger, with the free posterior part measuring about 6 mm. The inner surface of the palps in the posterior pair has a deep oral groove.

The stomach is slender and rather long, reaching from just behind the middle of the anterior half of the body length to just in front of the anterior bundle of the posterior byssus retractor. The stomach is simple, with very few entrances of digestive diverticula, and no other structure detected on the inner surface. The midgut runs posteriorly straight and medially from the stomach to deep under the ventricle, before bending abruptly more than 90° upwards, forming a weak vertical ‘S’-shaped curve. It enters the pericardium and ventricle in front of the auricular ostia, at about their anterior margin. Behind the heart, the intestine passes dorsally over the posterior adductor and towards ventrally on its posterior side, anus at mid-diameter of the posterior adductor, adjacent to the posterior end of the ctenidia.

The heart is rather short and broad, and is situated in the posterior half of the body. The anterior extremity of the ventricle is slightly behind the mid-shell/ mantle length. The ostia are long, and are about in the middle of the ventricle. The auricles are fused posteriorly above the anterior end of the posterior adductor and stretch out forward in rather short lobes to slightly before the middle of the body length, just at the anterior limit of the anterior muscle bundle of the posterior byssus retractor.

(Cosel & Janssen, 2008)


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