Internal Anatomy and Function

Pinctada margaritifera conforms to the general structure of the monomyarian lamellibranchs, with a single, posterior adductor muscle, which is crescent- shaped, with a constant thickness and an enlarged ventral area. The two symmetrical adductor muscles have considerable power and a rapid ratchet-like action. The retractor muscles are white oval masses and project laterally from the visceral mass. They are V-shaped and surround the byssal organ.

Gills (ctenidia)

There are two symmetrical, flat, crescent-shaped and filamentous gills, characteristic of bivalves with lamellibranch gills. The gills function in filter feeding and respiration. The feeding function is via their connection to the labial palps to form the pallial organ. Each gill has a dorsal main axis that is vascular and muscular. It has two long branchial sheets that arise from the main axis to form a narrow inverted V-shape in transverse section. Each branchial sheet is folded back on itself to create a descending, and an ascending lamellae. Thus, each gill is composed of four elongate lamellae and is W-shaped in transverse section. The anterior insertion point of each gill, left and right, is located laterally between the labial palps of the same side. Each gill is fused anteriorly with the visceral mass by its branchial axis and then it is fused with the adductor muscle in the anterior-ventral region of the pearl oyster. The gills then fuse via their axes and pass round the ventral zone of the adductor muscle. The fused gills end at the pallial fold facing the anal papilla, in a posterior-ventral position. Each gill is attached to the mantle lobe and gill is attached to gill, where this occurs, by long interlocking cilia. The indentation of the epithelium in these joining areas also strengthens the cohesion of the whole.

Mantle

The mantle is the most external organ, lining the insides of the shell valves and enclosing all other soft tissues and organs. Its fundamental function is to secrete the shell valves and ensure their growth. It consists of two large lobes, with each lining one valve of the shell. These two lobes are separated anteriorly, ventrally and posteriorly. Fused to the visceral mass and the adductor muscle, they join together dorsally along the hinge to form the isthmus. The black pigmentation of the external edge of the mantle and the outer nacre of the shell gives Pinctada margaritifera its common name of black-lip pearl oyster.

Each mantle lobe may be divided into four zones:

Foot and Byssal Gland

The foot is a highly mobile, tongue-shaped organ capable of great elongation and contraction. The main component of its bulk is composed of a network of fibers running in various directions, thus ensuring a broad range of movement. Control is provided by the foot retractor and elevator muscles with extensive blood spaces providing hydrostatic strength and flexibility.

The byssal gland is situated close to the end of the foot. Byssal fibres are secreted by the byssal gland and pass down the pedal grove, which is formed into a tube. These greenish grooves are initially clear. They darken gradually until they are deep bronze-green where the byssus is formed. Muscular contractions of the foot forms the discoid attachment and stem of the thread that is attached to the byssal root. Attachment takes place as the tip of the foot touches the substrate, and the byssal secretions set rapidly in seawater. The byssus is maintained throughout its life and if severed, a new byssus may be secreted within a week but both adults and juveniles will survive unattached.



The circulatory system

The circulatory system consists of a heart, and a system of cavities and blood vessels, which is responsible for the circulation of the hemolymph throughout the pearl oyster. After one of the shells is removed, the heart can be seen through the transparency of the mantle lobe. It is located within a pericardial cavity in the posterior region of the visceral mass, and covered by a thin pericardial membrane. It is limited dorsally by the small portion of free rectum, located between the visceral mass and the adductor muscle, and limited ventrally by the retractor muscles. It consists of a ventricle and two auricles, which are all triangular. The yellowish ventricle is located dorsally and in the middle, in comparison with the dark lateral symmetrical auricles. Each of the auricles communicates with the ventricle at its dorsal end. They join ventrally at their base and are in close connection with the excretory system.

Two large blood vessels, the anterior aorta and the posterior aorta, are visible to the naked eye. They emerge from the dorsal end of the ventricle and the former plunges into the anterior left side of the visceral mass, while the latter connects with the rectum. Each aorta subdivides into smaller vessels which irrigate the various organs of the animal.

The excretory system

The excretory system consists of a pair of light brown nephridia, on each side of the visceral mass, and numerous small pericardial glands. They join each other ventrally, beneath the auricles. The excretory system is covered by a fine epidermis of mesodermal origin, forming the renopericardial cavity. A distal branch of the nephridium follows the branchial axis, while a proximal branch goes to the heart. The excretory products are released through the buttonhole-like urogenital orifices that are located under the anterior area of the gills.

The nervous system

Like all bivalve molluscs, the nervous system of pearl oysters is not very apparent to macroscopic observation but consists of three pairs of ganglia:

1. The cerebral ganglia, located at the sides of the esophagus.

2. The pedal ganglia joined together as a single mass at the base of the foot.

3. The visceral ganglia or parieto-splanchnic ganglia, lying upon the anteroventral surface of the adductor muscle.

Commissures connect homologous ganglia. Consequently, the supra-esophageal commissure joins the cerebral ganglia and there is a large commissure between the visceral ganglia. There are further links to non-homologous ganglia and other nervous tissues connect ganglia to organs.