Macrobrachium rosenbergii

The giant freshwater prawn, known scientifically as *Macrobrachium rosenbergii*, is a highly significant species of palaemonid prawn. It is one of the largest prawns found in freshwater environments globally. This species is native to the tropical and subtropical regions of the Indo-Pacific. Its natural range extends from India through Southeast Asia to Northern Australia. Because of its size and value, it is widely cultivated for food in many countries. Through accidental releases, it has also been introduced to new regions like Africa, the Americas, and New Zealand.

Physically, these prawns show clear differences between males and females. Females generally have wider abdomens and smaller second pereiopods, which are walking legs that end in pincers called chelae. The genital openings are located on different body segments depending on the sex. Males have these openings on the segments containing the fifth pereiopods. In contrast, females have them on the segments with the third pereiopods. Most individuals are brownish, though smaller ones may appear greenish with faint vertical stripes. Their rostrum, a prominent part of the head, contains many teeth on both the top and bottom edges.

One of the most fascinating aspects of *M. rosenbergii* is the social hierarchy among males. Males exist in three distinct morphotypes, or physical forms, based on their social dominance. The first stage is the small male (SM), which has short, nearly translucent claws. If conditions allow, they metamorphose into orange claw (OC) males. These males possess large orange claws that can reach 0.8 to 1.4 times their body size. The final stage is the blue claw (BC) male. These dominant males have blue claws that can grow to be twice as long as their entire bodies.

This hierarchy is strictly enforced through biological interactions. The territorial blue claw males dominate the orange claw males. The orange claw males, in turn, dominate the small males. Interestingly, the presence of blue claw males actually inhibits the growth of small males. This presence also delays the metamorphosis of orange claw males into the blue claw stage. An orange claw male will only transform once it grows larger than the largest blue claw male in its local area. All three male stages remain sexually active throughout these transformations.

The life cycle of the prawn involves a complex journey between different water types. While adults live in fresh water, the larval stages require brackish water, which is a mix of fresh and salt water. During mating, the male deposits spermatophores on the underside of the female's thorax. The female then extrudes her eggs, which pass through these spermatophores to become fertilized. She carries these eggs until they hatch, which usually takes less than three weeks. A single female can lay between 10,000 and 50,000 eggs, and she may do this up to five times per year.

After hatching, the first larval stage is known as zoeae. These zoeae undergo several larval stages while living in brackish water. After approximately 32 to 35 days, they undergo metamorphosis into post-larvae. At this stage, they look much more like adults and are long in shape. Once they become post-larvae, they migrate back into fresh water to complete their development.

Scientists have conducted extensive research to improve how we farm these prawns. Professor Amir Sagi and his research group at Ben-Gurion University of the Negev have studied the IAG physiological sexual switch. This research involves investigating gene expression and the molecular mechanisms of sex determination. By using technologies like RNA interference (RNAi) and CRISPR, researchers can develop monosex biotechnologies. This allows for the creation of all-male or all-female cultures, which can improve production yields and profitability in countries like Vietnam, Thailand, and China.

Beyond food production, *M. rosenbergii* can serve important ecological roles. Because these prawns are efficient predators, they can be used as biocontrol agents. This means they can be used to manage invasive or pest species in a sustainable way. For example, they have been studied for their ability to control invasive snails like *Pomacea canaliculata*. This application helps protect ecosystems and can support ecological farming practices around the world.