Comparative analysis of Asterias forbesi development reveals distinct mechanisms of hydro-vascular organ formation across sea stars

A central question in developmental biology is how organs acquire their final shape. Since a conserved organ might form through distinct steps, even within animals belonging to the same class, comparative studies are needed to reveal conserved or unique principles. Sea stars are a useful system to investigate how organs develop their specific structure across various species. This species-rich and diverse class of animals provide embryos that can be easily cultured in large numbers and are optically clear, which allows morphogenesis to be observed live. All sea star larvae develop a coelom: a hollow tube that fills the body cavity. We recently named it the “hydro-vascular organ” (HVO), as most of this organ will form the adult water-vascular system after metamorphosis. Understanding the distinct steps that drive HVO development across species requires comparative studies, but detailed characterizations of embryonic and larval stages have been reported for only a few representatives of asteroidea. Here we use brightfield microscopy and live imaging to document for the first time the development of Asterias forbesi , an ecologically important species that lacks a detailed description of embryonic and larval stages. Leveraging the optical clarity of these larvae, we defined distinct stages of HVO morphogenesis. To understand whether the same organ forms through shared steps across different species, we compare A. forbesi HVO development with sea stars from two other orders, Patiria minata and Astropecten aranciacus.