The neuropeptide FILa modulates larval settlement and ciliary function in the echiuran worm <i>Urechis unicinctus</i>

Larval settlement is a crucial stage in the development of most marine benthic invertebrates, but the molecular mechanisms remain unclear. Here, we have functionally characterised FILa1, a mature peptide derived from the previously reported FILa precursor, and show that it promotes larval settlement. Sequence clustering and alignment analyses revealed that the FILa precursor shows similarity to annelid FLGa/FVLa and crustacean AST-A. Among its mature forms, FILa1 was the most effective inducer of settlement. Transcriptomic and qRT-PCR analyses showed that FILa1 rapidly activates cAMP-PKA signalling and is associated with altered expression of ciliary genes, including DYT2B, localised in the circumoral cilia. FILa1 inhibited DYT2B mRNA expression through the cAMP-PKA-CREB and Ca²+ pathways at early and later time points. Knockdown of DYT2B disrupted ciliary structure, reduced cilia number and length, and decreased beating frequency, demonstrating its essential role in ciliary function. Notably, larvae with reduced DYT2B expression exhibited significantly increased settlement rates at later stages. Our results demonstrate that FILa1 rapidly triggers larval settlement through cAMP-PKA signalling, while longer term regulation of ciliary genes contributes to proper ciliary function associated with successful settlement.