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Comprehensive Description

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General Description

Shell morphology: The shell is large, up to 190 mm long, modioliform, thin, fragile, essentially equivalve and elongately elliptical. The anterior margin is sharply rounded, whilst the posterior margin is broadly rounded. The ventral margin is straight in young specimens, with a slight ventral concavity in medium sized specimens. The concavity becomes more pronounced in larger specimens. The dorsal margin is broadly convex, more or less straight over the span of the ligament. The umbones are often eroded. They are prosogyrate and subterminal, positioned between 6 - 16% of the length of the shell from the anterior end. An indistinct, raised, broadly rounded ridge extends from the umbonal region to the posterior-ventral margin. There is a lack of external sculpture to the shell. The surface is smooth expect for concentric growth lines and fine radial lines in the periostracum that extend from the umbo to the ventral margin which are most prominent posteriorly. There are also fine radial periostracal corrugations along the ventral margin in the region of the byssal gape. The shell has a dull-white colour, with the periostracum straw-yellow to light-brown in young specimens, whilst older specimens have a dark-brown periostracum that becomes straw-yellow peripherally. The periostracum of older specimens is sometimes marked by irregularly shaped dark brown pigment patches, overlain by numerous byssal thread attachment paltes. The interior of the shell is off-white in colour, predominately nacreous. The ligament is opisthodetic, parivincular and extends posteriorly from the umbones to occupy 32 - 49% of the dorsal margin. The adult hinge is edentulous, except for a small posteriorly directed projection of the anterior hinge margin beneath the ligament's anterior end. The hinge is somewhat thickened below and anterior to the umbo. The juvenile hinge has about 15 denticles immediately posterior to the ligament and ca. 10 denticles located immediately below the umbones. The hinge denticles become obsolete in specimens that are greater than 18 mm in length.

Muscle scars: The muscle scars and pallial line are indistinct. The anterior abductor scar is rounded but truncated posteriorly. It is located ventral and partially anterior to the umbo in small specimens, and entirely in front of the umbo in medium and large specimens. The posterior abductor scar is round to oblong, usually contiguous with a small siphonal retractor scar ventrally and the posterior portion of the posterior byssal-pedal retractor scar dorsally. The anterior retractor scar is located within the upper extremity of the umbonal cavity directly below the umbo. The posterior byssal retractors form two scars with a very large intervening gap. The anterior scar is obliquely elliptical, and is found directly beneath or slightly anterior to the posterior end of the ligament in small specimens. In medium to large specimens the scar is located well to the anterior of the posterior end of the ligament. The second scar is elliptical, parallel to the antero-posterior axis of the shell and located antero-dorsally to and bordering the posterior adductor scar. The pallial line is distinct from the shell margin, extending from the postero-ventral edge of the anterior adductor scar to the postero-ventral edge of the posterior adductor scar. The pallial line curves slightly upwards and then downwards to form a slight indentation in the byssal gape region at about one-quarter to one-third of the distance from the anterior end. A small siphonal retractor scar located at the posterior end of the ventral pallial line, usually but not always contiguous with the posterior adductor scar.

Musculature: The posterior byssal retractors are divided into two widely divergent main bundles that attach separately to the shell, with a posterior portion inserting along the antero-lateral edge of the posterior adductor and an anterior portion inserting below and anterior to the ligament's posterior end. The posterior portion of the posterior byssal retractor is long and slender, resulting in an elongate and quite narrow region of the shell attachment. The pedal retractors are large and prominent, arising from the dorso-lateral surface of the foot mass and passing posteriorly along the lateral aspect of the anterior byssal retractors to become integrated with the anterior and lateral region of the anterior portion of the posterior byssal retractors at the point of shell attachment. The siphonal retractors are integrated with the pallial musculature, although there does appear to be a siphonal retractor scar on the shell. The anterior retractors are long and slender, arising from the dorso-lateral aspect of the byssal-pedal mass and passing anteriorly to insert in the antero-dorsal extremity of the umbonal cavity. There is a pair of slender labial palp suspensors that extend forward as branches of the anterior retractors to attach to the shell just behind and adjacent to the anterior adductor. The posterior adductor is rounded, as is the anterior adductor, which is one-half the size of the posterior adductor.

Foot & byssus: The foot is long and thick. It has a variable shape in preserved specimens that is dependant on the degree of contraction. The byssal strands are gray to brown in colour, wide, flat and unornamented. The byssal gland extends down the foot behind the byssal groove, without extension dorsal to the origin of the anterior retractors.

Mantle & mantle cavity: The connections between the edge of the ascending lamellae and the surface of the mantle lobes and visceral mass is weak or lacking, resulting in an incomplete separation of the incurrent and excurrent chambers. The muscular longitudinal ridges for the attachment of the ascending lamellae to the mantle lobes and visceral mass are lacking. The ventral edges of the inner mantle lobes are not unusually thickened or muscular. The excurrent tubuliform siphon is short, and is not capable of extension beyond the perimeter of the shell, and lacking an internal diaphragm. The horizontal branchial septum is incomplete; fusion of the inner mantle immediately below the excurrent siphon forms a short horizontal shelf, not directly attached to the ventral edge of the posterior adductor. The incurrent and excurrent chambers are not completely separated posterior of the posterior adductor; the posterior end of the gill axis is attached to the ventral surface of the horizontal brachial septum. There is a short extension as the valvular siphonal membrane joins the right and left mantle lobes, extending anteriorly only a short distance into the pedal gape. There are small central papilla on the most ventral extension of the valvular siphonal membrane that extends anteriorly into the pedal gape. The pedo-byssal gape is extensive, with the incurrent aperture extending from the anterior end of valvula siphonal membrane to the posterior edge of the anterior adductor.

Ctenidia: The demibranchs are thick, short and ca. equal sized, with both demibranchs extending anteriorly to the same degree. The ascending lamellae are slightly shorter than the descending. The ventral edges of the demibranchs have poorly developed food grooves. The dorsal food grooves are present in deep folds just below the junction of the ascending lamellae and the areas of attachment to the mantle lobes and the visceral mass. The filaments are wide and fleshy. The ctenidia and filaments are light-brown in colour. The distal interlamellar junctions are lacking. The descending and ascending portion of each filament are connected apically to one-quarter the height of the demibranch. Every second to sixth filament is a 'principal filament' with septum rising to one-third of the height of the demibranch. A single posterior 'tubular connection' between the free edges of the ascending lamellae and gill axes are sometimes present, but can be indiscernible in some individuals.

Labial palps: The paired labial palps are greatly modified from the typical filter-feeding type, appearing to function as a sorting area for material gathered by foot rather than ctenidia. The base of the inner and outer palp pair are widely separated; the ctenidia lie lateral of the labial palps in preserved specimens. The mouth is situated at the basal mid-point of the anterior end of the inner pair of labial palps, farther to the posterior than is typical for mytilids. The inner palp pair is placed posteriorly, and is large and muscular, and elongately triangular in shape. The outer pair of palps are more anterior, triangular, muscular but smaller than the inner pair. The oral groove is on the inner surface of both pairs of palps, bordered by plications, running from near the tip of the proboscid-like extensions to the mouth. The outer surfaces of the palps are smooth and non-plicate.

Digestive system: The alimentary tract is straight with no recurrent loop, situated directly on the body mid-line. The intestine leaves the posterior end of the stomach and traverses a short distance posteriorly, merging with the rectum. The rectum enters the extreme antero-ventral aspect of the pericardium and ventricle, anterior to the level of the auricular openings into the ventricle.

(Gustafson et al., 1998)


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