Crustaceans are an impressive group of animals that surprise us with a unique phenomenon, their molting cycle. On our site we want to offer you an explanation of this phenomenon that will help you understand a little more about the growth requirements of this fascinating group.
The phylum arthropoda, as its name suggests, is made up of animals with jointed legs. These animals have the joint characteristic that they have a more or less hard chitin exoskeleton and they have to change it in order to grow, and within this phylum we have the crustacean class. In this article we are going to focus on this class to study molting, so that next we will explain the molting cycle of crustaceans As a curiosity, the The word crustacean comes from the Latin word crusta, which means bark.
The exoskeleton of crustaceans
These animals have a calcareous protection called an exoskeleton. The exoskeleton gives them a rigid structure and a strong protection against predators This protection is so rigid that it becomes a problem as the individual grows, since the calcareous exoskeleton does not it stretches Restricting the size of the animal. It should be noted that it is not a rigid block, but a set of articulated plates that allow them to move.
How do crustaceans grow?
In order to grow, crustaceans must go through a delicate process of shedding their old exoskeleton and forming a new one. This trance involves a large energy expenditure, so they only carry it out when the animal is well nourished and ready to give the growth spurt. The molting moment of crustaceans, also called ecdysis, is the shedding of the exoskeleton It is a cyclical phenomenon affected by a large number of exogenous and endogenous factors (Hopkins et al. 1999). In a publication of the Journal of Marine Biology and Oceanography, it has been confirmed that the lunar phases highly affect the moment of molting, concluding that in the last quarter there are 50% more molts than in other lunar phases.
Moulting of crustaceans is more common in young individuals and less so in adults that have already reached standard size. In the rest of the animal groups, growth is somewhat continuous, but in the case of crustaceans, molting creates a discontinuity in development.
Moulting phases of crustaceans
Drach's investigations (1939, 1944) were the first to document the complete moult of crustaceans, dividing it into four phases:
- Intermolt: the suture of the previous moult can be seen but the shell is totally hard. This is the period from when the hardening process of the last moult has finished until the animal needs to grow again.
- End of intermolt and beginning of premolt: the suture becomes deeper and more marked. Internally, an internal skin begins to separate, becoming defined and darkening.
- Premolt: the scidial suture becomes decalcified and more sensitive. A very strong darkening of the inner layer is observed, which acquires turgidity.
- Ecdicis: it is the moment of breaking and abandoning the old exoskeleton.
It should be noted that before these four phases occurs phase 0, called post moult, in which a smooth suture can be seen at the cleavage lines where the crustacean has broken through the ancient exoskeleton. The still soft shell gradually hardens, it may take a couple of days.
At the time of moulting, the individual manages to break the old shell for two fundamental reasons. The first is that the scidial suture of the previous molt has decalcified, leaving it much weaker. In order to fragment it, it swells and stretches with the help of spasmodic movements. They also tend to swallow large amounts of water that helps them increase the pressure inside the exoskeleton and thus fragment it.
Once they have abandoned the old exoskeleton they are back in phase 0. At this moment they are still soft and stretching to the maximum, growing what their body needs. This is a very vulnerable time, so they tend to spend it hiding in crevices for the necessary days until they harden completely.
Experiment
To fully understand what the crustacean molting cycle consists of, it is possible to consult the following experiment of monitoring the growth of a rock crab for 300 days:
