ecdyses

Ecdysis is the biological process by which ecdysozoan animals shed their outer protective cuticle or exoskeleton. The plural form ecdyses refers to multiple instances of this molt across an organism’s life or across members of a group. This process is a defining feature of the Ecdysozoa, a large group that includes arthropods and nematodes, and it enables growth without compromising structural integrity.

In arthropods, ecdysis is driven by a hormonal cascade beginning with ecdysteroids such as ecdysone and its active form 20-hydroxyecdysone. These steroids interact with the nuclear receptor complex EcR/USP to regulate gene expression that initiates molt. Juvenile hormone levels influence developmental outcomes, determining whether a molt yields another larval stage or triggers metamorphosis. Neurohormonal signals, including ecdysis-triggering hormone (ETH) and eclosion hormone (EH), coordinate the motor behaviors required to shed the old cuticle.

The molt sequence typically includes three phases: apolysis, in which the old cuticle separates from the epidermis; ecdysis proper, when the animal actively sheds the exoskeleton; and postecdysis, during which the new cuticle expands and hardens (sclerotizes).

Scope and significance vary by lineage. In insects, crustaceans, and other arthropods, ecdysis is essential for growth and developmental transitions, including metamorphosis in many species. In nematodes and other ecdysozoans, molting and associated signaling are also present, though the hormonal control can differ. The term ecdyses emphasizes multiple molts rather than a single event.