Insects, roundworms and tardigrades share one habit: they grow by shedding skin
A beetle, a nematode and a water bear look like they have nothing in common. Yet all belong to Ecdysozoa, a huge branch of the animal family tree defined by one habit: growing up means periodically splitting open and discarding an outer cuticle, then expanding before the new one hardens.
The name joins two Greek words, ekdusis for shedding and zoion for animal. Members include the arthropods, meaning insects, spiders and their relatives, crustaceans and myriapods, plus velvet worms, tardigrades, roundworms, horsehair worms and several obscure phyla such as priapulids, kinorhynchs and loriciferans. The cuticle usually has three layers, four in tardigrades, and moulting is steered by the hormone ecdysone rather than by cell division in the underlying skin.
Other shared traits are subtler. Ecdysozoans have no cilia for moving about, produce mostly amoeboid sperm, fertilise internally and skip the spiral pattern of early embryo division seen in most of their protostome cousins. Ancestrally they had hardened teeth in the front gut plus a circle of spines ringing the mouth opening, though some lineages lost them. Only velvet worms and arthropods have circulatory and breathing systems, and even tiny arthropods such as mites often lack them. To recharge ATP, every member uses arginine phosphate, whereas vertebrates rely on creatine phosphate.
Eernisse and colleagues first proposed the grouping in 1992 after comparing 141 anatomical and embryological features. Aguinaldo and co-workers gave it a formal name in 1997, working mainly from 18S ribosomal RNA gene trees. That overturned the older Articulata idea, which paired arthropods with segmented annelid worms, and helped retire a scheme that sorted animals by the type of body cavity they had.
Placing roundworms here drew early resistance, but support has been broad since 2003, and a large 2008 study by Dunn and colleagues strongly backed the group as a single lineage. In 2011 James Lake received the Darwin-Wallace Medal for discovering the New Animal Phylogeny, which divides bilateral animals into Ecdysozoa, Lophotrochozoa and Deuterostomia.
Source: Ecdysozoa
