Some marine zooplankton are only temporary residents of the plankton, representing the larval stages of marine invertebrates such as corals, worms, snails, clams and crabs. After a period in the open water, these larvae must settle to the ocean floor where they will live out their adult lives. It is important that larvae are able to find the best place in which to settle, as this will affect their future success and survival.
Many marine larvae rely on a chemosensory organ in the head, known as the apical organ, to detect chemical cues from their environment that are indicative of favourable settlement sites. How environmental cues are interpreted by the apical organ to generate behavioural responses during settlement is not yet understood.
Studying Platynereis larvae, we have identified a neuropeptide, myoinhibitory peptide (MIP), as an apical organ signaling molecule that triggers larval settlement. In lab culture, 2 day Platynereis larvae actively swim in a water column. Exposure of the larvae to synthetic MIP however, rapidly induces sinking of the larvae followed by exploratory crawling on the bottom of the container. We have shown that MIP is expressed in chemosensory-neurosecretory cells of the apical organ and signals to its receptor, an ortholog of the Drosophila sex peptide receptor, which is expressed in neighbouring neurosecretory cells. The MIP-receptor cells likely then initiate further downstream endocrine signaling events that regulate the complex behaviors associated with settlement. The signaling pathways downstream of MIP and its receptor are a subject that we are currently investigating further.
Interestingly, MIPs are present in many protostomes (also called allatostatin-B) and are part of the Wamide family of neuropeptides, which are also involved in regulating settlement and metamorphosis in larvae of Cnidarians such as corals and sea anemones. This indicates that the initiation of life cycle transitions may have been an ancestral function of MIPs.