Land crabs are no different from other crustaceans in that growth can take place only by means of intermittent shedding of the more or less inextensible integument – the process of molting or ecdysis. For aquatic crabs, molting is a time of stress and mortality, resulting both from the dangers inherent in the molt process itself, and from the high risk of predation while the newly molted crabs are soft and relatively immobile. For land crabs there are added complications: The risk of desiccation is greater at this time, and there is the problem of obtaining the quantity of water needed for the postmolt increase in size, which must occur rapidly before the new integument hardens.
Although molting appears superficially as a short and intermittent interlude, it has a pervasive effect on the whole of the life cycle, and the period between molts is one of continuous morphological and physiological change (also see Chapter 5). These changes enable the crab to prepare for molting, and to recover from it. Some basic elements relating to molting and growth of crabs will now be outlined; a more detailed account for crabs in general is provided by Hartnoll (1982, 1983).
The molting cycle can be divided into the following stages.
Premolt (proecdysis). The new integument is being laid down beneath the old, and stored energy reserves are being mobilized to enable the new structures to be formed. Calcium is being resorbed from the old integument and stored in the tissues.
Molt (ecdysis). The old integument is shed, and the crab rapidly increases in size by the absorption of water. The process takes at most only a matter of hours.