Shedding and moulting are very common forms of behavior in the animal world. All mammals, including humans, are known to shed, that is, to get rid of hair and skin as a means to cope with or prepare for seasonal changes, while crustaceans undergo a process called moulting during which they get rid of their external protective shells, their exoskeleton, to be able to grow.
The group of crustaceans is huge and incredibly diverse, the most common families being shrimps, barnacles, krill, crabs, prawns, crayfish and lobsters. There are more than 67,000 species that are considered to be members of the group, some of them tiny, such as the Stygotantulus stocki, an arthropod that is only 0.1 mm long, and some of them huge, such as the Japanese spider crab, which can grow to weigh 20 kg.
The exoskeleton is the hard external shell that protects the body of crustaceans against external impacts and fulfills a number of other functions as well, such as enabling smelling, excretion and feeding. Unlike the skin of mammals, the exoskeleton is rigid and it does not grow together with its owner; therefore, the animal must moult it from time to time to be able to develop.
The exoskeleton of crustaceans contain a substance called chitin as well as calcium carbonate. In terms of structure, chitin is similar to the keratin that makes up human hair – it is a glucose derivative, a polymer that is very hard, resilient, but lightweight. When combined with calcium carbonate, chitin becomes even stronger, creating a shell almost impossible to break for the predators that feed on crustaceans.
The process of moulting in crustaceans is called ecdysis. It takes place when the old skeleton becomes too tight, so it is probably the discomfort that prompts the animal to prepare for the moulting. Before the process actually commences, the animal prepares for it by becoming inactive. During this inactive period, the crustacean grows new epidermal cells underneath the old shell and it starts secreting a fluid from its moulting glands to loosen the underpart of the shell. When the old shell becomes completely separated from the epidermis underneath, the animal starts secreting a different kind of substance, a digestive liquid which becomes active when the upper layer of the new shell is fully formed. When the new shell is ready, the animal starts the process of actually getting rid of its old shell. It moves until the old shell breaks, freeing itself from the old housing and also digesting it at the same time.
It is not only the outer layer of the crustacean’s body that is actually shed – along with the old skeleton, the animal sheds some of its inner parts as well, including their alimentary tracts and their trachea if they have any. The length of the periods between two moultings depends entirely on the growth rate of the animal, which is, in turn, dependent on the temperature of the habitat and the food available. Crustaceans, like all living organisms, grow faster when conditions are more favorable, which makes the periods between the shedding sessions shorter when the weather is warm and food is plentiful. The molting itself is usually very quick and followed by a period of hiding because the animal’s new exoskeleton is not as hard as the old skeleton used to be, therefore it needs more shelter against predators.