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Fisheries > Culture Fisheries > Edible Oysters

Species

Thirteen species of edible oysters have been reported from India among which, four species viz., Crassostrea gryphoides, C. rivularis, C. madrasensis and Saccostrea cucullata are of commercial value. C. gryphoides and C. rivularis are mostly found in the North-Western coast of India. Crassostrea madrasensis enjoys a wider distribution along the South-West and East coasts of India and it is found in almost all the estuaries and backwaters.

C. madrasensis is euryhaline i.e., it tolerates wide variations in salinity. It inhabits backwaters, creeks, bays and lagoons and occurs, from the intertidal region to 17 m depth. It is predominantly found in Orissa, Karnataka and Andhra Pradesh. Tamil Nadu also has rich oyster resources. In the Pulicat backwater oyster beds cover 10 ha with an estimated standing stock of 1,320 t. In the Ennore backwater 45 ha oyster beds support a standing stock of 18,600 t and the oysters are regularly exploited by fisherfolk throughout the year. The Kerala coast supports small oyster populations. In the Ashtamudi and Vembanad lakes, Cochin backwaters, Mahe estuary and the creeks of Dharmadam, Valapatinam, Nileshwar and Chandragiri, oyster populations have been recorded and there is regular exploitation.

C. gryphoides is also a euryhaline species occurring along the north Karnataka, Goa and Maharashtra coasts. In Maharashtra it is exploited regularly from several creeks. It occurs in waters upto 7 m depth. C. rivularis is found along the Gujarat coast and also occurs in Maharashtra together with C. gryphoides in the Mahim, Ratnagiri and Jaytapur creeks. Saccostrea cucullata occurs all along the main land coast and Andaman and Lakshadweep Islands. It is found in shallow coastal waters, attached to the rocks and boulders and can withstand surf and wave action in the marine environment.

madrasensis

Crassostrea madrasensis

 

gryphoides

Crassostrea gryphoides

 

rivularis

Crassostrea rivularis

cucculata

Saccostrea cucculata

Distinctive Characters

The shape of the oyster shell is highly variable. If grown individually on a soft bottom, the shell tends to be smooth and elongate. If grown individually attached to a surface, the lower valve grows attached to the surface rather flat, thus loosing the normal cupped appearance.

The various genera and species are identified based on the shape, size, colour and other characteristics of the shell, and the breeding habits.

C. madrasensis : Shell valves irregular in shape, usually elongate. Left valve deep and the right one slightly concave; hinge narrow and elongated. Adductor muscle is kidney-shaped and dark purple. Grows to 22 cm.

C. gryphoides : Shell valves elongate and thick. Left valve cup like. Hinge area well developed and has a deep median groove with lateral elevations. Adductor muscle scar is broad, more or less oblong and striations on the scar are absent or obscure. Grows to 17 cm.

C. rivularis : Shell valves large, roughly round, flat, thick and with a shallow shell cavity. Left valve is thick and slightly concave and the right one is about the same size or slightly larger. Adductor muscle scar is oblong and white or smoky white in colour. Grows to 15 cm.

S. cucullata  : Shell valves hard, stony, trigonal and pear-shaped. Hinge straight, devoid of teeth and umbonal cavity well developed. The margins of both the valves have well developed angular folds sculptured with laminae. Small tubercles present along the inner margin of the right valve and there are corresponding pits in the left valve. Adductor scar is kidney-shaped, striated and white or grey in colour.

Biology

The edible oyster is a sedentary animal. It attaches to the substratum by the cupped lower valve (left) and the upper (right) valve acts as a lid. The antero-posterior axis (long axis) is the height but in common usage it is called length. The actual length of the oyster (dorso-ventral axis) is referred to as width in common usage. The food consists of organic detritus and phytoplankters such as diatoms and nanoplankters. The growth is related to food availability and also to the environmental conditions, particularly to salinity and temperature. In the Cochin backwaters, spat of 10 mm nodal length grow to 55 mm nodal length in about 6 months.

In the genus Crassostrea, sexes are separate but occasionally hermaphrodites occur. The ovary and testis consist of a series of branching tubules, also called follicles, on either side of the body. During spawning, ripe eggs and sperms are discharged into the exterior where fertilization takes place.

Exploitation

The oysters occur in shallow waters and form subsis­tence fisheries at several centres along the Indian Coast. The production is mostly utilised for local consumption. Men, women and children collect the oysters at low tide and various implements are used to dislodge the oysters from the bed. The production of the oysters from the natural grounds is not monitored regularly in the country.

Seed collection

Oyster seed are procured from the natural resources by placing suitable collectors called cultch in the water column at appropriate period. This practice is still in vogue in several countries even though hatcheries are increasingly supplying the seed for farming. During spawning seasons the spat collectors are laid on the bottom, on the racks or suspended from racks. If the collectors are laid in advance of spat fall, they may be covered with silt or settlement of foulers, making them unsuitable for the oyster larvae to settle. The larval period in C. madrasensis is 15-20 days. The ideal time for laying the spat collectors in the water is about 7-10 days after peak spawning.

Of the two spawning periods, April-May is more productive than August-September for laying the spat collectors. Tiles laid in the Bay gave a high production of 316 spat/m2 compared to 92.3/m2 in the oyster bed and 76.9/m2 in the creek.

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