Habit habitat Structure and Pearl Formation of Unio. Freshwater Mussel

Habit Habitat of Unio: Freshwater mussels.

Unio is found in freshwater lakes, rivers, streams, inhibiting the surface layers of the muddy beds of lakes and rivers.In the posterior tip of its shell, the buried position remains exposed in water to facilitate entry and exit of the water current, which is useful both for breathing and feeding.The food of Unio consists of microscopic organism, both animals and the plants. They usually feed upon by filter-feeding mechanism involving both ciliary as well as mucoid moment.These animals crawl slowly with the help of its plough-like, wedge-shaped muscular foot. It usually moves night to shallow places and retires to deeper places by day. These animal responses light, touch, or some other stimulus by withdrawing its food. The incoming and the outgoing water currents mean for closing the syphons.

The structure of Unio: Freshwater Mussel.

[I] Shell.

These are a soft-bodies animal .It is enclosed within a calcareous shell which represent the exoskeleton. The shell is measured about 10 cm in length and 5 cm in width.It consists of two similar, more or less oval, convex valve, that joined at the dorsal margin or hinge line by a strong ligament. Shell valve is similar in shape and size. This ligament is elastic and causes the shell to gape ventrally. Dorsally and somewhat anteriorly, each shell valve has a slightly raised part, called the umbo. It represents the oldest part of the shell and the concentric lines around it are lined of shell growth representing intervals between successive growth stage. The anterior end of the shell is somewhat rounded. The posterior end is tapering and projecting. There are two short tubes or syphons, one for the entry and other for an exit of water current.

Internally, the shell valve is attached to the mantle of the body with three anterior and two posterior sets of muscles, constituting the anterior protracter, reactor and adductor muscle. On the inner surface of each valve are seen characterised marking or scars of the attachment of these muscles. Primarily the two adductor muscle are equal in size, but in many families, the anterior adductor become reduced, and in oysters and scallops it disappears completely, then the posterior adductor moves to the centre of the shell valve. All the muscle present in Unio are un-striped, they gradually shift with the growth of the animal, their faint lines may be traced to the umbo.

Histology of the shell.

The shell of Unio is composed of three layers.

1. Outer layer periostracum,

2. Middle prismatic layer, and

3. Inner nacreous layer.

(i) Outer periostracum.

It is a thin horny layer of an organic substance, conchiolin, which imparts brownish colour to the shell. It is subjected to erosion by the action of carbonic and humic acids present in water and is thus often absent on the umbo which represents the oldest part of the shell.

(ii) Middle prismatic layer.

It lies perpendicular to the surface of the shell. It is composed of crystalline calcium carbonate separated by conchiolin layer.

(iii) The inner nacreous layer.

These layers lie parallel to the surface of the shell. The first two layers are secreted by the edge of the mantle whereas the innermost layer by its whole surface.These layers are also called the mother of pearl’ . it is formed by the many thin and alternating layers of calcium carbonate and conchiolin.

[II] Mantle and mantle cavity.

It consists of two fleshy lobes, which are attached dorsally to the body and ventrally to the shell valve.At the posterior end, the edge of two mantle lobes touches each other at four points. It extends slightly forming two tubes. The dorsal surface of the Unio is narrow tube is excurrent or exhalant syphon, through which leave the body.The ventral surface of a union is somewhat broader and a longer tube is for in current or inhalant syphon, through which water enter the body.The two mantle folds also form a dorsal pore on the dorsoposterior side.

Histologically, the mantle consists of:

An outer columnar epithelium based with nacre-secreting cells, a middle fibrous connectives tissue, and an inner ciliated epithelium containing mucus-secreting cells.The two mantle lobes enclose a bilaterally compressed cavity, the mantle or pallial cavity. It consists of following parts.

(i) Visceral hump.

(ii) Foot.

(iii) Labial palps.

(iv) Mouth.

(v) Gills.

(i) Visceral hump.

It contains various organs including the alimentary canal, gonad, digestive gland, pericardial cavity enclosing the heart, blood vessels, kidney, and bladders. It is compressed mid-dorsal part of the body.

(ii) Foot.

The thick basal part of the foot contains a portion of the alimentary canal, digestive gland, and gonad.Foot of Unio is a large, muscular,and wedge-shaped structure. It extends from the anteroventral of the visceral hump into the mantle and laterally compressed and terminates below into an elongated keel.

(iii) Labial palp.

It comprises the two parts of large somewhat oval, flattened fleshy lobes, located just below the anterior protractor muscle. The external and internal palps form the upper and the lower lips of the mouth, respectively.

(iv) Mouth.

The mouth of Unio is a wide slit-like transverse located in a depression formed by the labial palps between the anterior adductor muscle and the foot.

(v) Gills.

The gill or ctenidia ate two in number. They lie in the mantle cavity, one on either side of the foot.The gills are plate-like structures, forming by the fusion of successive branchial or gill filaments. They are perforated by minute pores and-and are lined with cilia. Each gill is characteristically folded to form two laminae each comprising two lamellae.Out of two gills, the outer gill lamina is somewhat smaller than the inner.

The mantle cavity is divided into two chambers, the large ventral, infra-branchial chamber and the smallest dorsal, suprabranchial chamber. The basis of the gills form a horizontal partition between these two chambers.

The process of pearl formation in Unio.

When a foreign object, such as sand grain or an egg or parasites, gets between the shell and mantle, the mantle is stimulated to secret concentric layers of nacre around it. This results in the formation of the pearl. The pearl of Unio is not commercially precious. The bivalves which commercial pearls are called oysters.e,g Pinctada vulgaris, etc.

Reference:

Agrawal Sarita. A Text Book of Biology.,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L., Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

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