The 3D architecture of the ctenophore aboral organ and the evolution of complex integrative centers in animals

The ability to sense and respond to environmental cues is fundamental to animal behavior and survival. In ctenophores-early-branching marine animals-a subepitheliall nerve net (SNN) underlies complex behaviors such as geotaxis, feeding, and escape. Central to this system is the aboral organ (AO), a sensory hub that detects motion, light, and pressure and coordinates ciliary movement. However, the AO's cellular architecture and its integration with the SNN remain poorly understood. Here, using volume electron microscopy in <i>Mnemiopsis leidyi</i>, we reveal that the SNN condenses around the AO, forming synaptic connections with effector cells. We annotated 17 cell types, including nonsynaptic vesicle-rich cells likely involved in volume transmission. Our data suggest that signal processing within the AO relies on synaptic and nonsynaptic communication. Profiling of shared cnidarian-bilaterian transcription factors revealed nonconserved expression, suggesting unique molecular pathways underlying ctenophore AO formation. Our findings redefine the ctenophore AO as a distinct, integrated, and potentially multimodal sensory system critical for behavioral regulation.