Economic Importance of Phylum Molluscs

1. Edible Molluscs:

(i) Edible Oysters:

From ancient times the Romans and Greeks used the oysters as a main dish in their meals. The Red Indians were great oyster eaters. The maximum amount of oys­ter meats are collected from two genera— Ostrea and Crassostrea, which belong to the family Ostreidae and class Bivalvia.

In Japan, United States and Europe over billion pounds of oyster meat are sold each year. In Europe, Ostrea edulis and Crassostrea angulata are very popular among oyster eaters. The Virginia osyter, Crassostrea virginica is harvested from Southern Canada to Mexico.

In the brackish water, Crassostrea lives and the gametes are shed into the open water but Ostrea lives in clear water with higher salinity. The oyster fishery is related to the conditions of the substratum suitable for attachment, depth of the water and the salinity of the medium.

The fishermen use long-handled pair of tongs for detachment from the substratum. Modern trawlers with motor driven dredgers are used in many parts of the world.

In India though molluscan species are varied and plentiful, but a small number of poor population in the coastal areas utilise molluscs as food. The edible oysters are found along the sea shores, estuaries and back wa­ters where the substratum is suitable for the attachment.

The Indian edible oysters are Crassostrea cuculatta found on the intertidal rocky coasts, Backwater oyster, Crassostrea madrasensis, found in the estuaries and back­waters, and disc oysters, Crassostrea discoidea of the coastal areas. Oysters are sold in the markets alive or after freezing.

(ii) Edible Clams:

The edible clams are harvested through­out the world and its meat is a source of food since prehistoric times. Major portion of the edible clams are harvested in Japan and United States.

The Quahogs (family Veneridae), Razor clams (family Solenidae) and soft shelled clam (family Myacidae) of the class Bivalvia are in great demand in the markets of United States. The Quahog clam. Mercenaria mercenaria is harvested in the Gulf of Mexico and in the Gulf of St. Lawrence.

The largest clam fishery in the world is based upon Surf clam, Spisula solidissima which is found in the United States. The most favour­ite American clam is soft shelled clam, Mya arenaria which is used extensively for the nature of sweet meat.

Next is the United States, Japan, Malayasia, and Europe which consume a large proportion of clams; mainly cockles are harvested in North-West Europe and Malayasia. In Malayasia the Ark shells (Anadara sp.) are used extensively.

In India, poor coastal people consume the edible clams mostly. The edible clam species in India are Bay clam, Meretrix meretrix, the Inflated clam, Katelysia opima, the Backwater clam, Meretrix casta, the cockcle clam, Gafrarium tumidum, etc., belonging to the family veneridae of the class Bivalvia. They are found in the coastal shallow waters and are collected by hand-picked method or often used mechanical devices.

(iii) Edible Scallops:

The sweet, delicate meat of the scallops helds a high rank to the people of shell fish consumers, especially to the gourmets. The people of United States, Japan and Southern Australia consume a large amount of scallop meats. The Deep sea Scallop, Placopecten magellanicus is very popular in the east coast of the United States. The scallops are also extensively harvested in Japan and Australia.

(iv) Edible Sea Mussel and other Edible Molluscs:

The edible Sea Mussels mainly include Mytilus, Perna under the family Mytilidae which remain attached to the rocky substra­tum by byssus threads. The European edible Mussels, Mytilus edulis, though not popular in the United States but in France it has gained vast popularity.

In Chile, the mussel fishing has acquired the front rank among the South American countries. Along the Pacific coast of the United States, the people do not prefer mussels extensively for the dangers of paralytic mussel poisoning in some cases. The people of the West Indies use the meat of Pink Conch, Strombus gigas extensively.

In India, the Sea Mussel, Mytilus is mainly restricted from Quilon to Kanyakumari in the south-west coast and up to Tirunelveli district in Tamil Nadu in south-east coast.

The Green Mussel, Perna viridis is found along Mumbai, Ratnagiri and Karwar coasts and also in Cochin (Kerala), Tamil Nadu and in some parts of Odisha. The Green Mussels not only found in coastal waters but also in Bays and backwaters. The mussels are fished by iron chisels and are kept in coir bag or in nylon bag.

The gastropoda includes a number of species for edible purposes; such as the Top shell, Trochus niloticus, the Turban shell, Turbo marmoratus, the winged shell, Strombus canarium, the Purple shell, Thais bufo, etc.

(v) Edible Cephalopods:

The edible cephalopods include cuttle fish, squid and octopuses and occupy the second place next to the oyster fishery. A vast amount of octopuses are consumed by the Japanese, Spaniards, Italians and Austral­ians. Over 8000 tons of squid are sold annu­ally in the fish markets of New York city, Washington and California.

The squids are eaten mainly by Chinese, Italians, Spaniards and Puerto Ricans. The Australian giant cuttle fish, Sepia apama found in the southern part of Australia is sold in the fish markets for human consumption. The golden cuttle fish, Sepia esculenta is fished in Western Ja­pan and Shantung and Kiangsu provinces of China.

The flesh is used as food in Japan and south-east Asian countries. It is caught by otter trawls and hook-and-line. The flesh of kisslip cuttle fish, Sepia lycidas is in great demand in Japan and Hong-kong.

The com­mon cuttle fish, Sepia officinalis, occurs along the eastern Atlantic from the Baltic Sea to South Africa and the meat is highly appreci­ated food item in Italy, Spain, Japan and Republic of Korea. The catch is usually marketed fresh and frozen.

The edible squids are predominantly caught by Mediterranean’s and south-east Asian countries which are traditionally the major consumers. The Sword-tip squid, Loligo edulisi occurs in the Western Pacific, Philip­pine Islands, South China to central Japan and along northern Australia.

The species supports local fisheries in Japan, Philippines and Indonesia. The meat is of good quality and demands a high price in the markets. The veined squid, Loligo forbesi, an inhabit­ant of Mediterranean Sea, Red Sea, north­west coast of Africa is appreciated for its flesh and is used as food. The Japanese squid, Loligo japonica, is often consumed raw flesh for its excellent quality.

The Opalescent inshore squid, Loligo opalescens, of western coast of United States and the Longfin in­shore squid, Loligo pealei, of the Gulf of Mexico and the Caribbean Sea, demand a high price for the excellent quality of the flesh. The squids are fished with purse seines trawlers from December to April and the catches are marketed fresh, frozen or canned for human consumption.

The Northern shortfin squid, Illex illecebrosus is fished in the northwestern Atlantic with otter trawls. The major share of the catch is taken by Japan and Canada. The flesh is in high de­mand for consumption and as bait. The flesh of the species is deteriorated easily because it contains high percentage of water.

The Japanese flying squid, Todarodes pacificus, is found in Western Pacific, Northern and East­ern Pacific, and in north Japan. Fisheries exist around Japan, and to some extent off the Korean Peninsula and off China. The catches of this species were 380000 metric tons in 1983 and accounted for about 18% of world cephalopod catches and major portion is consumed by the people of Japan.

The species, T. pacificus, are dried after eviscera­tion and this dried product (surume) is used for domestic consumption and export. The Neon flying squid, Ommastrephes bartrami, occurs in subtropical, and temperate oceanic waters is being exploited more with the decline of the catches of T. pacificus.

The flesh is of excellent quality for human consump­tion and the lion’s share of the catches are consumed by the Japanese.

Since prehistoric times octopuses are ex­ploited for human consumption. The most commercial fisheries and markets are located in Japan and in Mediterranean countries. Octopuses are fished by spears and hooks in shallow water and seines and bottom trawls in open water.

Octopus briareus, Octopus cyaneus, Octopus dofleini (Japan), Octopus maya (Bay of Campeche), Octopus vulgaris (Japan), Cistopus indicus (Asian market), Eledone cirrosa (Mediterranean countries) are used as food in different countries.

In India, not well known cephalopod fishery has developed along our coastal ar­eas. Sepia pharaonis, Sepia brevimana, Sepia prashadi, Sepia aculeata, Sepiella inermis, Sepioteuthis lessoniana, Loligo indica, Loligo hardwickii, Loligo duvaucelli, Octopus hummelincki and Octopus incertus are the known cephalopods used for food.

In the south-east coast mainly in the Palk Bay, and in the Gulf of Mannar the seasonal fishery of the squid is observed. Pharaoh cuttle fish, Sepia pharaonis is caught throughout the year with trawls and hand jigs.

The flesh is excel­lent for human consumption. The short club cuttle fish, Sepia brevimana is caught in the eastern Indian waters between July to Febru­ary. The species appears as by-catch in trawls off waltair and Chennai.

The Needle cuttle fish, Sepia aculeata, is found mainly in south west India with peak landing months be­tween October to December and caught mainly by trawl. The Spineless cuttle fish, Sepia inermis, occurs both in south-east and south-west India and is caught by trawls, fixed nets or beach seines, etc.

The Bigfin reef squid, Sepioteuthis lessoniana is com­monly found in south Indian waters. They are caught by trawls and purse seines and the peak landing season of this cuttle fish is between March to June. In the Palk Bay the Bigfin squid is fished by a sein called Ola Valai but in the Gulf of Mannar the squid is caught by shore seines.

In Sri Lanka, it is the common squid and caught by beach seines. In western Japan and in Philippines the fishermen use spears, set nets and jigs. It has a high commercial value throughout south­east Asia and is in high demand for food. In Rameswaram the reef squid specimens are sundried on sand extracting shell and ink gland. Most of the catches are consumed by the local men and the rest is exported.

The huge number of Nautilus are caught each year in Philippines, Caledonia, and Fiji, etc. for the human consumption of the meat.

In Bihar and West Bengal (India) fresh­water gastropods are also much in demand for food, and heaps of these molluscs are kept for sale in the market.

2. Medicinal and Harmful Molluscs:

(i) Medicinal Value:

Cockcle clam is supposed to be the good for heart trouble. Molluscan food is an im­portant ingredient for good health and is helpful for sexual activities. Pliny recom­mended that raw molluscs can be used for sore throat and cough. It is reported that the extracts of hard-shelled clams are the growth inhibitors of cancers in mice.

Placuna placenta (window pane oyster) helps in the prepara­tion of “Mouktik Bhasma” which helps in gaining of youth and vitality. The pearl pow­der mixing with milk and various herbs are used for the cure of stomach troubles in China. A virus preventive, Paolin, derived from some oysters, is useful against polio­myelitis and influenza. The venom from Conus can be used as a muscle relaxant during heart operations.

(ii) Harmful Molluscs:

Freshwater snails of Philippines, China, Venezuela are responsible for the death of thousands of people because the snails are the intermediate host of blood fluke disease of Schistosomiasis. In India the disease Fasciolopsiasis is caused because the snails are the intermediate host of the intestinal fluke, Fasciolopsis fuelleborni.

Many peoples in the United States become paralyzed when they consume infected clams and mussels which are carriers of paralytic shell fish poisoning.

The garden snail, Achatina sp., is the most terrestrial destructive gastropod which eats not only leaves but also destroys tender stems of the gardens. The cone, conus omaria, possesses venomous stings which can inflict injuries and may even death. Teredo, Bankia, Pholas damage the wooden portion of the ship immersed in water by boring through it.

3. Chank in Religion:

The most popular sacred chank is Xancus pyrum belonging to the family xancidae and class Gastropoda. The sacred chank is a white, massive, pear-shaped shell with three spiral ridges on the inner lip. They are ma­rine and gregarious in nature. They are com­mon in Indian Ocean and are found in the south-east coast and west coast of India.

In the south-east coast the chanks are found along Tirunelveli to Nellor districts, and in the west coast they are found in the Gulf of Kachchh of Kathiawar coast near port Okha and in the South Kerala coasts and Andaman Islands. The chank beds are found up to the depth of 13 m and are collected by skilled divers.

The collection and management of the chank beds in Tamil Nadu are controlled by Tamil Nadu Fisheries Department. The fish­ing season of the chanks is between October and May. The sacred chanks has played a main role in Indian religion. It held in high esteem by Hindus.

The sacred chank is one of the symbols associated with god Vishnu and his many incarnations are also associ­ated with the chank. It is widely used as trumpet in the temples and as woman’s wrist bangles. Chank blowing is a common custom in Hindu marriages of some castes, specially in Bengal.

The chanks after collec­tion from the bed sent to Kolkata market where it is processed for selling. The small processing shops are located in different areas in Kolkata, specially in Amherst Street (North Kolkata). The highly priced sacred chank varieties of the different places are tutikkudi of Tirunelveli, duani from Travancore and surati of Kathiawar coast.

These varieties are large with fine texture and colour. The ancient bangles made from chanks have unearthed from the Neolithic sites of Tungabhadra River, situated in South India. The sacred chanks are generally dex­tral form (rt. handed) but sinistral (left handed) coiled chanks are very rare and demand very high price in the market.

These rare sinistral coiled shells had reached to Europe in the early part of 1700 by European collectors. These are now on display in some zoological museums in Europe. Some of these rare sinistral form of sacred chanks are now in the temples of South India and also in the hands of some rich private collectors.

4. Cowries and Money:

In earlier days cowries were used as money. The mostly two species of cowries—money cowries, Cypraea moneta and gold ringer cowries, Cypraea annulus are also referred to as money cowries. The, earliest graves in Egypt contained numerous cowries.

Since 2000 B.C. the cowries were used as coins in China and lasted about 600 B.C. Till recently Cypraea moneta was used as money in many parts of Africa. Even in 19^th century they were used as coins in Uganda and Central New Guinea (Africa).

During Livingstone’s travelling period (1871-1876) in Congo and Lake Tanganyika regions, Cypraea moneta was used extensively as coin. In the years of the English East India Company, specially in Bengal, the cowries were exchanged for rice from the Maldive Islands.

During these years the cowries were shipped to Europe, Eng­land, the Netherlands and Portugal in Bar­rels or sacks. It is known from the record that about 1,000 million cowries were exported from Maldives only in the year of 1800.

5. Decorative Ornamental and other Purposes:

Shells are collected from Sri Lanka, Western Australia, Philippines, Thailand, Fiji, Japan, South Africa and Hawaiian Islands for commercial purposes. In India the shell is collected from Andaman and Nicobar Islands, and from South Indian coasts. The shells are used for making ornaments and jewelleries.

The shell curio trade of Nautilus and other shells has developed in India, Phil­ippines and Indonesia. In India, the shell curio trade is seen in some tourist places or pilgrim centres near the shore, such as Digha (W. Bengal), Puri (Orissa), Chennai, Rameswaram, Kanyakumari (Tamil Nadu), Kovalam beach near Tiruvantapuram (Kerala), etc.

Several hundred tons of internal calcareous cuttle fish bone are collected from Sepia and squids along the shore of Mediterraneen Sea and are sold to the manufacturers of tooth paste. For centuries, the ink of Sepia has been used as a brown colouring matter and also as a writing ink.

Nowadays it has been replaced by aniline dyes because Se­pia’s ink fades in light. ‘Pearl’ buttoms pre­pared from shells of bivalves are extensively used.

The top shells (Trochus sp.), turbans (Turbo sp.) are used for beautiful shell lamps and are collected from the rocks and coral reefs of Andaman and Nicobar Islands. The royal tyrian purple dye was the source of two snails—Murex brandaris and Nucella lapillus and was used by the ancients.

The manufacture of the dye was perfected by Phoenicians and the use was known as early as 1600 B.C. in the Mediterranean Islands. This dye was very strong and did not fade after 100 years. The dye was collected as a fluid first from the hypobranchial gland of the snail situated on the roof of the mantle. Then it would turn to purple red gradually in presence of direct sunlight.

Shell lime is prepared from shells and is used for white washing on the walls of buildings and betels. The shells are collected along the East coast near Berhampur, Visakhapatnam, Kakinada and Chennai, and Cochin from West coast. The subfossil de­posits of Pulicat Lake near Chennai and Vembanad Lake in Kerala support a prosper­ous lime industry.