Ecdysis, commonly called molting, is the biological process of shedding an exoskeleton. This process is fundamental for the growth of many invertebrate animals. Because their rigid external skeletons cannot expand, they must periodically cast them off and form new, larger ones to accommodate an increase in body size. The discarded exoskeleton is referred to as an exuviae.
The Biological Process of Ecdysis
The molting cycle is controlled by the hormone ecdysone. An increase in this hormone triggers the process, which begins with apolysis. In this first stage, the living cell layer, the epidermis, separates from the old cuticle, creating a space between them.
Following separation, the epidermis secretes enzymes that digest the inner layers of the old cuticle, and the digested material is absorbed for reuse. Simultaneously, the epidermis begins to secrete a new, soft cuticle. Once this new layer is formed, the animal initiates the physical act of shedding the old exoskeleton.
To break free, an animal might swallow air or water to swell its body, causing the old cuticle to split. After emerging, the new exoskeleton is soft and pliable, allowing the animal to expand its body. This is followed by sclerotization, a hardening process where the new cuticle becomes rigid over hours or days. This cycle repeats multiple times throughout an animal’s life.
Which Animals Molt and Why
The primary group of animals that undergoes ecdysis is the Ecdysozoa. This group is dominated by the phylum Arthropoda, which includes insects, arachnids (spiders), and crustaceans (crabs and lobsters). Examples range from a cicada leaving its nymphal shell on a tree to a spider shedding its skin to grow larger.
The fundamental reason for molting is to permit growth, as their rigid exoskeletons cannot grow with the animal. Beyond growth, ecdysis is also a part of metamorphosis, such as when a caterpillar molts to become a butterfly. The process also allows for the regeneration of damaged tissues and lost limbs, which can be reformed over a series of molts.
Dangers and Complications of Molting
The process of ecdysis, while necessary, is fraught with danger. An animal is extremely vulnerable during and immediately after molting. The new exoskeleton is soft, leaving the creature temporarily defenseless against predators. A newly molted crab, often called a “soft-shell crab,” is exhausted and an easy target.
A frequent and fatal complication is known as dysecdysis, where the animal fails to completely escape its old exoskeleton. It can become stuck, which often leads to death. The act of shedding is physically demanding and complex, involving the removal of linings from the respiratory system and parts of the digestive tract. Any minor issue during this stressful period can result in injury or death, making each molt a significant life-or-death event for the animal.