Unio is a bivalve mollusc that inhabits freshwater rivers and ponds and lakes. It burrows in a furrow in sand with the help of a hatchet-shaped foot, keeping the inhalant and exhalent siphons above the sandy surface for maintaining a current of water. It is a filter feeder of planktons which are trapped by the specially modified gills.
The bivalve shell is held together by anterior and posterior adductor muscles and it can be opened by retractor muscles. Protractor muscle moves the foot while burrowing. Just below the two valves of the shell there are flap-like mantle lobes on either side which provide insulation between the hard shell and soft body. Mantle is also respiratory in function. Gills or ctenidia are large and made of two gill plates on either side of body under the mantle flaps. They are respiratory as well as food gathering in function.
Unio is filter feeder and traps planktons by the enlarged sieve-like gills plates or demibranchs. Water current enters the mantle cavity through the inhalant siphon and moves upwards through the gills to the suprabranchial chamber. Gills are coated with mucous which traps the planktons on the gill surface. Movement of cilia on the gill surface pushes the food particles downward into the food groove present on the lower margin of gill lamina. Inside the food groove the food is constantly pushed forward towards the mouth where two pairs of feeding palps sort out the sand particles from the mucous cord and push the food and mucous into the mouth.
The stomach is bag-like with ciliated lining and a crystalline style that rotates at the bottom, churning the food and mixing it with the digestive juices. Digestive gland is dark brown to greenish in colour and surrounds the stomachs and opens into it through many ducts.
The intestine is coiled and passes through the gonad and pericardial cavity to end in rectum that opens by anus at the base of the exhalent siphon. Rectum carries a typhlosole to increase the absorptive area. Most of the digestion and absorption takes place in the intestine.
Intracellular digestion also takes place in the digestive gland by wandering amoebocytes.
The chief respiratory organs are a pair of ctenidia and mantle. Each side of the gill or ctenidium has inner and outer gill lamina or demibranch or gill plate, which is made of inner and outer layers called lamella. All lamellae have vertical gill filaments which are joined by the interfilamental junctions at short intervals. The lamellae and the interfilamental connections together make the gill lamella a sieve-like structure through which the water is filtered and food particles are trapped. At the same time exchange of gases also takes place through the epithelium of the lamellae. During breeding season the outer gill lamina enlarges and serves as a brood pouch to hold the eggs and glochidium larvae.
Blood vascular system is open type comprising heart, arteries, veins, lacunae and sinuses. Blood is colourless but in some cases is slightly bluish due to the respiratory pigment haemocyanin. Only Solen has haemoglobin dissolved in plasma in bivalves. Large number of granular and agranular leucocytes and amoebocytes are present in the blood.
Heart is 3-chambered, with one ventricle and 2 auricles. Pericardium encloses the heart. Ventricle gives off anterior and posterior aortae, the former carrying blood to foot, mantle and visceral organs. The posterior aorta supplies blood to mantle only.
The blood from the posterior part of mantle is collected by the pallialvein and is taken to the auricle. On the anterior side the visceral vein and pedal veins collect blood and take it to the kidneys or organ of Bojanus. From kidneys either the blood can go directly to auricles via the efferent renal vein or can be taken to gills for oxygenation through the afferent ctenidial vein. From the ctenidium the blood is collected by the efferent ctenidial vein and taken to auricles. Thus auricles receive the venous blood from all over the body and pump it to the ventricle.
There are no capillaries in Unio. The arteries end in lacunae and sinuses and veins collect blood from major sinuses and take it to the heart. This type of circulatory system is called opentype.
Excretory organs are a pair of kidneys collectively known as organ of Bojanus that lies below the pericardial cavity. Kidney is a u-shaped tube that on one side opens into the fluid filled pericardial cavity and drains its contents. A reddish-brown Keber’s organ or pericardial gland is attached on the anterior side of pericardium. It discharges waste products into the pericardial cavity. Another opening of the kidney is renal pore that opens into the suprabranchial chamber for releasing the nitrogenous wastes into the outgoing current of water. The kidney has glandular part that extracts nitrogenous wastes from the surrounding blood and its ciliated part forces them to the outside. The excretory products are mainly ammonia and ammonium compounds and traces of urea and uric acid.
Nervous system is symmetrical having a pair of cerebral ganglia on the dorsal side of oesophagus and connected to each other by a cerebral commissure. A pair of pedal ganglia is located in the foot and connected to the cerebral ganglia via a pair of cerebro-pedal connectives. Visceral ganglion forms an X-shaped bilobed mass in the posterior side of the body. It is connected directly to the two cerebral ganglia via a pair of cerebro-visceral connectives.
Statocysts are connected to the pedal ganglia in the foot.
Owing to the near sedentary habit sense organs are not very well developed in Unio. Osphradium is located at the base of gills and is made of yellow coloured group of sensory cells that test the chemical nature of the incoming current of water.
Statocyst is one pair of sense organs of balance and posture located in the foot near the pedal ganglia, to which it is connected by pedal nerves.
Tactile cells are distributed on the edges of mantle and on the inhalant siphon.
Photoreceptor cells are also present on the margins of siphons which detect the intensity of light.
Unio is a dioecious animal in which sexes are separate. Gonads are paired structures made of branching tubes that lie in the coils of intestine. Testes are whitish in colour and ovaries reddish. Ducts of gonads open into the suprabranchial chambers of gills.
The sperms are released into the suprabranchial chamber from where they pass out of the body through the outgoing current of water. However, ova that are released in the suprabranchial chamber are held by mucous cords into the brood pouches of the outer gill lamina. Sperms of other mussels that enter the brood pouches with the current of water fertilise ova to form zygotes within the water tubes. Development of the eggs takes place inside the brood sac of the outer gill lamina and a glochidium larva is formed.
The glochidium larva has a bivalve shell whose two valves are attached together with adductor muscle. The free ends of the valves carry sharp hooks that help the larva to attach on the gills of fish. On the inner side of the shell there is mantle bearing bunches of cilia and a glandular pouch at the base. The glands secrete sticky mucous onto the byssus thread that helps the larva to firmly anchor on the gills of fish.
The glochidium larvae escape from the brood pouches into the surrounding water and along with the water current reach the gill chambers of fish, where they attach to the gills by hooks and byssus and forms a cyst. The cyst remains on the fish for about 10 weeks and is carried to long distances by the host. Then the cyst detaches from the gills of fish and drops on the sea floor where it gradually metamorphoses to become adult. The fishes help in the dispersal of the species.