Mesoglea biogenesis reveals a cryptic aboral valve for pressure regulation in cnidarian morphogenesis.

Cnidarians are classically defined by a single oral opening, a hallmark of the "blind gut" model in early animal evolution. Here, we identify a pressure-sensitive aboral valve in Nematostella vectensis that operates independently of egestion. This valve dissipates elevated hydraulic pressure during morphogenesis, expelling fluid through transient epidermal ruptures triggered by muscular ring opening. This unexpected function was revealed by a comprehensive analysis of mesogleal basement membrane biogenesis. We show that the global dynamics of this extracellular matrix transduce muscular hydraulics to drive tissue rearrangement and stabilize shape, while localized FGFRb-dependent matrix remodeling establishes the aboral valve. By positioning the mesoglea as an integrator of biomechanics, tissue remodeling, and aboral valve function, these findings expand nonbilaterian openings beyond the digestive paradigm as a hydraulic regulator.