REPRODUCTIVE MECHANISMS IN THE GIANT FRESHWATERREPRODUCTIVE MECHANISMS IN THE GIANT FRESHWATER

The giant freshwater prawn, Macrobrachium rosenbergii, is a commercially important species of crustacean cultured extensively throughout Southeast Asia. In Vietnam, where Japan International Research Center for Agricultural Sciences (JIRCAS) is currently implementing a comprehensive project entitled “Evaluation and improvement of farming systems combining agriculture, animal husbandry and fisheries in the Mekong Delta,” M. rosenbergii is considered to be an important target species by the Vietnamese Government, and its aquaculture is being actively promoted. Farmers have traditionally depended on wild sources to obtain seed for aquaculture but are now faced with dwindling resources and a shortage of natural spawners. Development of improved means of artificial seed production for M. rosenbergii in the Mekong Delta is thus essential. JIRCAS is currently implementing basic studies on the reproductive endocrinology of M. rosenbergii as part of the Mekong Delta project. Biochemical and molecular biological research is being conducted both at the project site, Cantho University’s College of Agriculture, in Cantho Province, Vietnam, in collaboration with Vietnam counterparts, and on JIRCAS Tsukuba premises. As part of our on-site studies in Vietnam, we are conductingan assessment of typical feeds utilized in freshwater prawn culture in Vietnam, and evaluating the effects of this feed on reproduction. This research is expected to be relevant in controlling female reproduction in captivity and improving seed production technology.
source:Wei-Jun YangDo Thi Thanh Huong, Masachika Maeda,
Japan International Research Center for Agricultural Sciences

Scope for Fresh Water Prawn Culture

Considering the high export potential, the giant fresh water prawn, Macrobrachium rosenbergii, the scampi, enjoys immense potential for culture in India. About 4 million ha. of impounded freshwater bodies in the various states of India, offer great potential for fresh water prawn culture. Scampi can be cultivated for export through monoculture in existing as well as new ponds or with compatible freshwater fishes in existing ponds. It is exported to EEC countries and USA. Since the world market for scampi is expanding with attractive prices, there is great scope for scampi production and export.

Location

Giant freshwater prawns are native to the fresh and brackish waters across South and Southeast Asia, parts of Oceania, and the Western Pacific Islands.

The main sources of giant freshwater prawns are China, Bangladesh, Taiwan, Indonesia and Thailand. Prawns sold in the U.S. market are primarily from Bangladesh and China.

Diseases

Diseases so far do not appear to be a significant problem in the production of freshwater prawns, but as densities are increased to improve production, disease problems are bound to become more prevalent. One disease you may encounter is "blackspot" or "shell disease," which is caused by bacteria that break down the outer skeleton. Usually it follows physical damage and can be avoided by careful handling. At other times, algae or insect eggs may be present on the shell. This condition is not a disease, but rather an indication of slow growth, and is eliminated when the prawn molts.

Identification of Male and Female

The sexes are separate and the males are larger than females. The body can be divided into cephalothorox and abdomen and there are about 20 appendages of which six pairs are located on the head portion of the cephalothorox eight pairs on the chest portion of the cephalothorax, and the rest on the abdomen. These appendages are used for various purposes such as sensory, touching, cutting and grinding feed, capture of food and handling, waking, swimming and propulsion etc. The Male have a large cephalothorax and narrow abdomen. The second pair of chelate legs of male are long, robust and large. They have a gonopore at the base of the fifth pair of legs and an appendex masculine on the endopod of the second pair of swimming legs.

Culture of Fresh water prawn is Profitable Business

Culture of freshwater prawn is profitable and can be adopted throughout the asia pecific region, even in land-locked states, which are unsuitable for brakishwater shrimp farming. The giant freshwater prawn, known scientifically as Machrobrachium rosenbergi popularly known as scampi is now popularly cultivated in many parts of the region, in almost all the states. From a meagre production and export of about 2200mt during the early ninetie's (1994-95) it has seen a sevenfold increase in 2004-2005 with 15000 tonnes. The cause of successfully culture of freshwater prawn are many, such as better FCR and growth, diease resistance, demand in export market and amenability to cultivate, both in mono culture and in combination with carps. In India the development, hatchery production of larvae and post larval rearing, and grow-out culture have spurred an unprecedented increase in its adoption by farmers, and entrepreneurs.

Fresh water Prawn as an Aquaculture Product

The so-called giant freshwater prawn, Macrobrachium rosenbergii, is widely distributed in the Indo-Pacific region, ranging from Australia to New Guinea to Indus River delta. This species has emerged in the last few years as one of the aquatic animals having a very high potential for aquaculture.

This is based on a number of advantages of this species over many other crustaceans. It adapts to a relatively wide range of temperature from a minimum of 15 to a maximum of 35°C. This species has a relatively short larval life. It is also a fast growing species. Fast growing individuals reach market size in about 7–8 months, and the meat is of high quality in terms of tests and texture.

Food is normally the largest single item in the running expenditure of a prawn hatchery or prawn farm. If prawns are held in artificial confinement where natural food are absent or limited, an external food source should be added. For example, the external food of freshwater prawn larvae that are held in tanks may consist of live food (nauplii of brine shrimp) or artificial food (fish egg, fish flesh, formulated feeds, etc.)

The production of freshwater prawn stocked in pond depends on the ability of the environment to produce natural food. Various factors such as soil and water fertility, water temperature and intensity of solar radiation affect the production level of natural food in the pond. However, for maximum rate of performance, the supplemental feeding programme is supposed to be important. Hence, the knowledge of nutrient requirements, the preparation of suitable feeds from the local available ingredients, feeding techniques, and the cost effectiveness of prepared feeds is of paramount importance to commercial success.

Classification

Prawns are omnivores and essentially scavengers. Some are planktivores and vegetarians. Others feed on aquatic small insects and other animals. The type of food varies according to the developmental stage and species of the prawn. Some females carry fertilized eggs under their abdomen with the pleopods until they hatch, others shed the fertilized eggs in water. Eggs of most species usually hatch as a nauplius larva and pass few larval stages, similar in appearance with the primitive crustacean groups. Bangladesh has very rich source of prawns in the Bay of Bengal, estuaries and freshwater. A total of 56 species is reported, of which 37 are salt water, 12 are brackish water, and 7 are freshwater in habitats. However, some species migrate in between lower and higher saline zones. The prawns are classified under six families, Palaemonidae, Penaeidae, Pandalidae, Alpheidae, Hippolylidae, and Sergestidae. The freshwater species are included in the family Palaemonidae.

Prawn culture

Raising, processing and marketing of prawns/shrimps as a source of protein food. The most widely cultured fast growing species in the globe as well as in Bangladesh are the giant black tiger shrimp Penaeus monodon and the giant prawn Macrobrachium rosenbergii. Their culture techniques are traditional, extensive, improved extensive, semi-intensive, intensive and super intensive, named mainly based on the stocking density of post larvae (pl) and management techniques of the culture ponds. Both monoculture, polyculture and integrated culture with paddy (rotational and concurrent) and with fin fish (mullets, carps and tilapia) are the common systems practiced. But shrimp culture starts in saline water and prawn culture mainly in freshwater though they all need saline water for hatching of eggs in the hatchery.

Prawn Name and History

All the freshwater prawns that have been cultured so far belong to the genus Macrobrachium, Bate 1868, the largest genus of the family Palaemonidae. About 200 species have been described, almost all of which live in freshwater at least for part of their life.

The giant river prawn, Macrobrachium rosenbergii, was one of the first species to become scientifically known, the first recognisable illustration appearing in 1705. The nomenclature of freshwater prawns, both on a generic and a species level has had quite a muddled history. In the past, generic names have included Cancer (Astacus) and Palaemon. Previous names of M. rosenbergii have included Palaemon carcinus, P. dacqueti, and P. rosenbergii and it was not until 1959 that its present scientific name, Macrobrachium rosenbergii (De Man 1879) became universally accepted.

Some taxonomists recognize a western sub-species (found in the waters of the east coast of India, Bay of Bengal, Gulf of Thailand, Malaysia, and the Indonesian regions of Sumatra, Java and Kalimantan) and an eastern sub-species (inhabiting the Philippines, the Indonesian regions of Sulawesi and Irian Jaya, Papua New Guinea and northern Australia). These are referred to as Macrobrachium rosenbergii dacqueti (Sunier 1925) for the western form and Macrobrachium rosenbergii rosenbergii (De Man 1879) for the eastern form. However, from the perspective of freshwater prawn farmers, exact nomenclature has little relevance, especially because the species M. rosenbergii has been transferred within its natural geographical range and been introduced into many other zones where it may become established.

Life cycle of a prawn

Although freshwater prawns require brackish water in the initial stages, most of their lifecycle is spent in turbid, riverine systems.
Freshwater prawns have a hard outer shell that must be shed regularly in order to grow. This process is called “molting”. Because of these periodic molts, growth occurs in increments, rather than continuously. This results in four distinct phases in the life cycle;
1.Egg
2.Larvae
3.Post-larvae
4.Adult.

Male Prawns
Male prawns are larger than females of the same age. The male has a head (cephalothorax) proportionally larger than the abdomen, which is narrow, and the chelipeds are long, massive and large.

Females Prawns
Females become sexually mature before six months of age. Mating occurs only between hard-shelled males and ripe females that have just completed their pre-mating molt and are soft-shelled. Within a few hours after mating, eggs are laid and transferred to the underside of the tail where they are kept aerated and cleaned. Although first spawns are often not more than 5,000 to 20,000 eggs per female, mature females have been reported to lay between 80,000 to 100,000 eggs during one spawning. The eggs remain attached to the abdomen until they hatch. The bright-yellow to orange color of newly spawned eggs gradually changes to orange, then brown, and finally to grey-black. At 82º F, eggs hatch 20 - 21 days after spawning.

Biology of Macrobrachium rosenbergii

Giant river prawns live in turbid freshwater, but their larval stages require brackish water to survive. Males can reach a body size of 32 cm;females grow to 25 cm. In mating, the male deposits spermatophores on the underside of the female's thorax, between the walking legs. The female then extrudes eggs, which pass through the spermatophores. The female carries the fertilized eggs with her until they hatch; the time may vary, but is generally less than three weeks. A large female may lay up to 100,000 eggs.
Prawn move upstream, entering lakes and even paddy fields, up to about 200km from the sea. This type of migration is observed only in M.rosenbergii but also in other species of Macrobrachium

From these eggs hatch zoeae, the first larval stage of crustaceans. They go through several larval stages before metamorphosing into postlarvae, at which stage they are about 8 mm long and have all the characteristics of adults. This metamorphosis usually takes place about 32 to 35 days after hatching. These postlarvae then migrate back into freshwater.