Macrobrachium rosenbergii

This giant prawn lives in fresh water.

It can grow very large. Most of them are brown. Some small ones are green. They have stripes on their bodies.

Some males have big orange claws. Other males have blue claws. The blue claw males are the bosses. They protect the females.

Females carry many tiny eggs. They carry them for a few weeks. The eggs hatch into tiny babies. These babies live in salty water first. Then they move to fresh water. They are very special animals.

The giant river prawn is a very big prawn.

It lives in fresh water. Many people eat them for food. They are found in many warm places. This includes parts of Asia and Australia. Some have even moved to Africa and the Americas.

Males come in three different stages. The first stage is the small male. These males have short claws. If they can, they grow into orange claw males. Their claws are bright orange. The final stage is the blue claw male. These are the bosses. Their blue claws can be twice as long as their bodies. Blue claw males protect the females. They keep them safe until their shells are hard.

Females carry many eggs. They can lay up to 50,000 eggs at once. A female can do this five times a year.

The eggs hatch into tiny larvae. These babies live in brackish water. Brackish water is a mix of fresh and salty water. After about 35 days, they change. They become post-larvae. Then they move into fresh water to live.

Females are easy to spot. They have wider bodies than males. They also have smaller walking legs.

The giant freshwater prawn is a very important animal for people to eat. It is one of the largest prawns found in fresh water. You can find these prawns in many warm areas. They live in the Indo-Pacific region, from India to Australia. Some live in Southeast Asia too. Because they are so large, many countries grow them on farms. Sometimes, they end up in new places like Africa or the Americas. This happens because they were released by accident.

These prawns have a very interesting way of growing. Males actually change their look in three different stages. The first stage is the small male. These males have short, clear claws. If they have enough room, they grow into orange claw males. Their claws turn bright orange and grow quite large. The last stage is the blue claw male. These males are the bosses of their area. Their blue claws can grow to be twice as long as their whole body.

There is a strict order among the males. The blue claw males are at the top. They rule over the orange claw males. The orange claw males rule over the small males. The big blue claw males even stop the others from growing too fast. This keeps the small males from changing too soon. When it is time to make babies, the males help. The blue claw males will protect the females. They stay with them until the female's shell becomes hard again.

A female prawn can lay a huge number of eggs. She might lay 10,000 to 50,000 eggs at one time. She can do this up to five times every year. After the eggs hatch, the tiny babies are called zoeae. These larvae must live in brackish water. Brackish water is a mix of fresh and salty water. They stay in this water for about 32 to 35 days. After that, they change into post-larvae. Finally, they move back into fresh water to live their lives.

Scientists study these prawns to learn more about them. Professor Amir Sagi and his team study how they grow. They use special tools to learn about their genes. This helps people farm them better in places like China and Israel. Some people use prawns to help control pests. For example, they can eat invasive snails. This makes them helpful for nature and for farmers. They are very useful creatures in our world.

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.