Comprehensive Description

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General Description

Shell morphology: The shell is large, up to 180 mm long and is modioliform, elongate, elliptical, thin and fragile, essentially equivalve. The anterior margin is moderately 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 that becomes more pronounced in larger specimens. The dorsal margin is very broadly convex, more or less straight over the span of the ligament. The umbones of the largest specimens are eroded, prosogyrate, subterminal and positioned within the anterior tenth. An indistinct, raised, broadly rounded ridge extends from the umbonal region to the posterior-ventral margin. The external surface lacks sculpture and is generally smooth, except for concentric growth lines, fine radial lines in the periostracum that extend from the umbo to the ventral margin, and fine periostracal corrugations in the median ventral area. The shell has a dull-white colour beneath a dark-brown to straw-yellow periostracum. The periostracum is often marked by irregularly shaped dark brown pigment patches, and is overlain by numerous byssal thread attachment plates. The interior is off-white in colour, and predominately nacreous. The ligament is opisthodetic, parivincular, and extends posteriorly from the umbones to occupy between 41 - 59% of the dorsal margin. The adult hinge is edentulous, except for a posteriorly directed projection of the anterior hinge margin beneath the anterior end of the ligament. The hinge is thickened below and anterior to the umbo. Hinge denticles were absent in the smallest specimen (36 mm length).

Muscle scars: The anterior adductor scar is rounded but truncated posteriorly, and is located below and partially anterior to the umbo, distant form the antero-ventral margin. The posterior adductor scar is round and contiguous with a small siphonal retractor scar ventrally and the posterior portion of the posterior byssal retractor scar dorsally. The anterior retractor scar is located in the posterior portion of the umbonal cavity. The posterior byssal retractors form two scars with a large intervening gap. The anterior scar is elliptical, directly beneath or slightly anterior of the posterior end of the ligament. The second scar is also 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, and extends from the postero-ventral edge of the anterior adductor scar to the posterior adductor. It curves slightly upwards and then more strongly downwards to form an indentation in the byssal gape region at ca. one-quarter to one-third of the distance from the anterior. The small siphonal retractor scar is located at the posterior end of the ventral pallial line, usually but not always contiguous with the posterior adductor.

Internal morphology: The posterior byssal retractors are divided into two widely divergent main bundles that attach separately to the shell. A posterior portion inserts along the postero-dorsal edge of the posterior adductor and an anterior portion attaches to the shell just below the ligament's posterior end. The posterior pedal retractors are very thin, arising from the antero-dorsal part of the foot, passing the lateral to anterior retractors and inserting on the shell anterior and lateral to the posterior portion of the posterior byssal retractors. The siphonal retractors are integrated with the pallial musculature. The anterior retractors arise from the dorso-lateral section of the byssal-pedal mass and pass anteriorly to insert in the antero-dorsal extremity of the umbonal cavity. The pair of slender labial palp suspensors 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 oblong; whilst the anterior adductor is round in cross-section and about one-half the size of the posterior adductor.

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

Mantle & mantle cavity: The connections between the edge of ascending lamellae and the surface of the mantle lobes and the visceral mass are weak or lacking, resulting in the 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 thickened and muscular. The excurrent tubuliform siphon is capable of a slight extension beyond the perimeter of the shell, but 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 axes attach to the ventral surface of the horizontal branchial septum. There is a short extension as the valvular siphonal membrane joins the right and left mantle lobes, extending anteriorly a distance into the pedal gape. Small central papilla on the valvular siphonal membrane extends anteriorly into the pedal gape. The pedo-byssal gape is extensive, with the incurrent aperture extending from the anterior end of the valvular siphonal membrane to the posterior edge of the anterior adductor.

Ctenidia: The demibranchs are approximately equal sized, thick and short with poorly developed food grooves on the ventral edges. Dorsal food grooves are present in the 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 is connected apically to one-quarter of the height of the demibranch. Every second to fifth filament is a 'principal filament' with a septum rising to greater than one-third the height of the demibranch. 'Tubular connections' between the free edges of the ascending lamellae and gill axes are lacking.

Labial palps: The paired labial palps are broadly triangular, thick and muscular. The inner pair are more posterior than the outer pair, but not markedly so. Plicate ventral to the oral groove on the inner surface of the inner palp and dorsal to the oral groove on the inner surface of the outer palp. The outer palp surfaces are smooth. The mouth is situated in the normal anterior position at the basal junction of the inner and outer palps. The antero-ventral portion of demibranchs is situated between the inner and outer palps coincident with the plicate palp surfaces.

Digestive system: The alimentary tract is essentially straight, without a recurrent loop and is situated directly on the body mid-line. The intestine leaves the posterior end of the stomach and traverses posteriorly ventral to the pericardium to a level just posterior to the ventricle's mid-point. The rectum enters the pericardium and ventricle from below at the mid-point of the ventricle, but anterior to the level of the auricular openings.

(Gustafson et al., 1998)


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