Identification of the principal neuropeptide FxFa and its function in larval settlement of the echiuran worm Urechis unicinctus

Larval settlement and metamorphosis are of great importance in the life history of marine benthic invertebrates, and these events are regulated by neuroendocrine signals such as neuropeptides. However, our knowledge of the molecular mechanisms which neuropeptides regulate downstream genes to induce larval settlement behavior is limited. Here, we compared the effect of eleven FxFa mature peptides, and identified FxFa5 as the principal neuropeptide in triggering the larval settlement of Urechis unicinctus (Xenopneusta, Urechidae), a benthic burrowing echiuran worm in the intertidal zones. A total of 579 differentially expressed genes (DEGs) were identified by analyzing the transcriptome data of the early-segmentation larvae from both FxFa5-treated and control groups, including a cilia-related gene such as FLNB ( Filamin B ) which may play vital roles in the larval settlement. Whole-mount in situ hybridization revealed FLNB was located in circumoral cilia of the early-segmentation larvae. FLNB knockdown caused the closure, shorter, even disappearance of circumoral cilia, and led to a significant increase in the larval settlement rate. The results of our work indicate FxFa5 triggers the U.unicinctus early-segmentation larval settlement by inhibiting the function of the ciliary gene FLNB , and this study contributes important data for elucidating a complete understanding of neuropeptides regulation in larval settlement of marine benthic invertebrates. • Neuropeptide FxFa5 inducing larval settlement of U. unicinctus by downregulating the expression of a ciliary gene FLNB .