Like all crustaceans, shrimp have external skeletons or exoskeletons that limit the extent of their development. In order to grow they need to moult, that is to say, to totally renew their carapace, or shell. How can the pivotal period of moulting be managed in order to optimise the growth of farmed shrimp?
Shrimp growth takes place in steps
The shrimp's exoskeleton is also called its shell or cuticle. Mainly composed of chitin, calcium salts, protein, and fat, this cuticle is a rigid element that protects the animal but limits its growth. As it grows, the shrimp reaches the limits of its exoskeleton; it therefore moults in order to build a larger exoskeleton. Hence the animal’s growth is not continuous as with fish but rather stepwise, i.e. it takes place in steps or successive stages.
Moulting is a hormonal phenomenon. As is often the case in such contexts, it appears that moults are synchronised among breeding populations of shrimp. Yet moult cyclicality may be disrupted by significant stress — for example, when ponds are too rapidly emptied — and this sometimes results in postponing the harvest.
In order to grow, a shrimp has to get out of its old shell (step E). When its new cuticle is in contact with the external environment, the animal undergoes a relatively critical period.
There are three stages that occur after the moulting itself:
- Post-moult: During this phase the shrimp recovers from its previous moult. In order to be able to extend and consolidate its cuticle as well as to adapt it to its new size, the shrimp absorbs a large volume of water (point A). The new shell hardens for several hours (stage B).
- Inter-moult: During this stationary phase, the shrimp’s cuticle is functional. Its growth in mass is continuous and its feeding activity is stable and at its maximum level.
- Pre-moult: The animal is getting ready for the next moult. Its feeding activities decrease; the new cuticle becomes visible to the naked eye (Figure 2).
The post-molt stage in shrimp: a period conducive to diseases and dysfunction
Moulting significantly disrupts the shrimp’s body. It is especially vulnerable as its shell is just newly formed during the post-moulting phase. Indeed, the physical barrier formed by the cuticle is not yet fully functional; the shrimp needs to mobilize its body reserves in order to harden and mineralize its weak cuticle. Some diseases are likely to emerge at this point; for example, shrimp's extreme vulnerability to the White Spot Syndrome Virus (WSS) has been demonstrated during stages A and B of the post-moult phase. Furthermore, the osmotic shock caused by the substantial intake of water strongly affects the shrimp’s internal environment; its cell functions are disrupted by this major variation. In such a context osmoregulators are a good first-line resource for maintaining cell integrity.
How to promote moulting in shrimp
For the moult to take place properly, the following guidelines are suggested:
- Check which moult phase the shrimp are at through regular pond sampling;
- Keep a register of moults, which may help in better anticipating the ensuing moults;
- Adjust the amount of feed distributed depending on the moult stage;
- Ensure that a proper intake of calcium and phosphorus is provided in order to help the new shell form itself;
- The use of osmoregulators can help reduce the extent of osmotic shock experienced by the shrimp, especially in farms in which the salinity level is low or high, or where the water exchange is limited.
With its expertise in the impact of the moult phenomenon on shrimp, the Techna Group has set up solutions to assist you in formulating and manufacturing shrimp feed so as to optimise the animal’s growth. For more information, please contact our experts.
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