Aboral cell types of <i>Clytia</i> and coral larvae have shared features and link taurine to the regulation of settlement

Larval settlement is of interest both for ecologists and for evolutionary biologists, who have proposed that anterior sensory systems for substrate selection provided the basis for animal brains. Nevertheless, the cellular and molecular regulation of settlement, including in Cnidaria (corals, jellyfish, sea anemones, and hydroids), is not well understood. We generated and compared anterior (aboral) transcriptomes and single-cell RNA sequencing datasets from the planula larvae of three cnidarian species: the jellyfish <i>Clytia hemisphaerica</i> and the corals <i>Astroides calycularis</i> and <i>Pocillopora acuta</i>. Integrating these datasets and characterizing aboral cell types, we defined common cellular features of the planula aboral end and identified clade-specific specializations in cell types. Among shared features were genes implicated in taurine uptake and catabolism expressed in distinct specialized aboral cell types. In functional assays using both <i>Clytia</i> and <i>Astroides</i> planulae, exogenous taurine inhibited settlement. These findings define the molecular and cellular architecture of the planula aboral pole and implicate localized taurine destruction in regulating settlement.