Mud Crab (Scylla spp.) Growth and Molting | Ecdysis

Mud crabs are invertebrates, where their shells are skeletons. Unlike humans, they do not grow continuously. Mud crabs must undergo a process called molting (or ecdysis) to grow. Understanding the molting process is the key to ensure a profitable mud crab farm. In this article, we will dive in deeper on the mud crab molting process.



The size of the mud crab is determined by the size of its exoskeleton. The hard shell protects the mud crab from predation, but also restricts the growth of mud crabs. Hence the mud crab molting process is characterized by the removal of the shell to grow. Before molting, the crabs will reabsorb some of the calcium in the exoskeleton to create an underlying skin. The underlying skin serves as a barrier that protects the crabs after shredding. Just before molting, the crabs would attempt to balloon itself up by drawing in seawater. This action facilities to expand the current exoskeleton, cracking the shells at certain points to facilitate molting. Upon expansion, the crabs would attempt to pull out its limbs, starting from the swimming legs, walking legs, and lastly the claws. The actual molting process visible to us will only last about 30 minutes. However, the mud crab would also run into some problem while trying to molt. The typical problem involves getting stuck while the crabs try to remove its limbs or claws, due to failure of the shell to break at the designated points. One unique adaptation that the mud crab has developed to overcome this problem is to voluntarily drop-off their limbs to escape. The mud crab also can regenerate its claws and limbs lost during the molting process. At this point, the crabs are vulnerable and would most likely be cannibalized by other crabs.




After molting, the crabs remain soft and continue to absorb water to inflates its body. It will start to mineralize its new exoskeleton, forming a paper-thin shell (commonly known as the paper shell). The duration before the paper shell is formed will depend on the availability of the minerals in the water, with a typical window of 4 hours with brackish water. It is also important to note that mud crabs will not feed right after molting as their mouthparts remain soft. Predation by other crabs during this period is also common due to the absence of the hard shell. While crabs are mineralizing their shell right after molting, some farm operators would harvest them and sell as soft-shelled crabs. These soft crabs are prized in many countries like Japan, Taiwan and Korea. Unlike the traditional mud crab trade, harvested soft shell crabs can be frozen and stored in containers waiting to be shipped and exported. Soft-shell crab producers often incur higher labor cost than other farms, and crabs must be monitored and harvested upon molting. Failure in harvested in the crab before the formation of the paper shell would result in financial losses.



As the paper shell continues to harden, the mud crab would enter the intermolt period. These crabs are commonly traded and sold to end consumers where they are cooked. However, it is important to note that a newly hardened crab would have little to no meat, as they are mainly water. Mud crabs at this stage would have hardened mouth and claws, where they can forage for food to build up meat mass. It is only after a period where they can reach the full-bodied crab to repeat the molting cycle. The intermolt period will depend on the age of a crab, where older crabs have longer intermolt period. A successful molt will require a huge amount of energy to execute, and mud crabs generally accumulate these resources during the intermolt period. Should the mud crab be deprived of nutritious food, they are most likely going to face mortality during the molting process. It is also crucial for mud crab traders to be able to differentiate whether the crabs have full-bodied meat or water. The empty crabs are categorized as rejects and called many different names throughout the industry “Peyat”, “Thin Crabs”, “Water Crabs”, “BS”, etc.




Apart from the changing shells, the mud crab also swaps into a new pair of gills and guts. It is important to note that molting can provide portals for entry of pathogen. One notable problem prevalent in the South East Asia is the emergence of rhizocephalan barnacle parasite that infects the crabs during the molting process. The pathogen that exists right before molting could also weaken the ability to store energy in the hepatopancreas, resulting in death before, during and after molting.


For many physiological functions that take place in crab, none as significant and traumatic as molting (Passano, 1960).

A mud crab essentially spends its lifetime, foraging for food and getting ready to for the next molt cycle. Many scientists have spent decades researching on methods to improve the molting success, and the fundamental understanding would pave the way for many profitable crab farms.



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