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Commercial Prawn Production

Biology

Species of the freshwater prawn genus Macrobrachium are distributed throughout the tropical and subtropical regions of the world. Although several species have been cultured, the predominant species being produced is Macrobrachium rosenbergii. Macrobrachium rosenbergii is indigenous to South and Southeast Asia, northern Oceania, and the western Pacific islands. This species has been transferred from their natural location to many parts of the world, initially for research purposes and ultimately for commercial production. All further reference to freshwater prawns is specific to this species.

Life History

Although freshwater prawns require brackish water in the initial stages, most of their lifecycle is spent in turbid, riverine systems. They are not cold tolerant so production in northern climates can be limited.

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; egg, larvae, postlarvae, and adult.

Growth chart. Enlarged imaage. 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.

After hatching, larvae are released and swim upside down and tail first. Although larvae can survive for 48 hours in freshwater, they must be transferred to brackish water (9 to 19 parts per thousand) for optimum survival. Larvae undergo 11 molts over a period of 15 to 40 days before transforming into postlarvae. The rate of this transformation depends upon food quantity and quality, temperature, and other water quality variables. Larvae feed primarily on zooplankton and larval stages of aquatic invertebrates. See larger chart of the larvae stages.

At this point, the prawns resemble small adult prawns, about 0.3 to 0.4 inch long and 50,000 to 76,000 per pound. They change to principally bottom dwelling, crawling individuals. Postlarvae can tolerate a range of salinities. The postlarvae prawn’s diet expands considerably and they may become cannibalistic under conditions of food limitation. Although no standard definition exists, the term juvenile is used to describe the freshwater prawn between postlarvae and adult.

Morphology

Prawns being held in someone's hand.Adult males are larger than the females and the sexes are easily distinguishable. The second pair of walking legs or claws and the head of males are larger than those of the females. The base of the last pair of walking legs of males is expanded inward to form a flap that covers the opening through which sperm are released. The males’ walking legs are close together in nearly parallel lines, with little open space between them. A wide gap exists between the last pair of walking legs in females and they have a genital opening at the base of the third pair of walking legs.

Males are classified by external characteristics. Blue-claw males (BC) are characterized by long, spiny blue claws. The two other classes are orange claw (OC) and strong orange claw (SOC) males. The males progress from OC to SOC to BC. See larger photo of male and female.

Social Hierarchy

Freshwater prawns develop a social hierarchy based on size and sex. BC males maintain a territory with a group of females that are ready for mating. Until the BC male either dies or molts and returns to a growth phase, the growth potential of the subordinate males (OC and SOC) is suppressed. This results in three size classes in the pond; large BC males, females, and OC/SOC males. This suppression of growth when coupled with the physical constraint of how many territories can “fit” in the bottom of a pond are limiting factors in pounds of prawns produced.

MSU Publications & Information

Other Freshwater Prawns Information