Unravelling the neuroendocrine signalling pathways guiding the developmental transition of marine invertebrate larval settlement

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Many marine invertebrates have a life cycle with a free-swimming larva that spends time in the plankton before settling to the sea floor. Here, the larva undergoes metamorphosis to its adult form. Larval settlement and metamorphosis is an excellent example of the important role that environment can play during animal development, since for many larvae, this process is initiated by specific environmental cues. While the regulation of insect, frog and fish metamorphosis is well known, metamorphosis occurs in at least 15 other animal groups. To fully understand how the environment regulates development, we need to investigate the external cues and internal neuroendocrine signalling that guide metamorphosis beyond insects and vertebrates, and in different environments. Currently we lack a complete understanding of the settlement process, from environmental cue to internal signalling, in any marine invertebrate.In my research at the University of Exeter, I will explore how larvae detect specific environmental cues which activate internal neuroendocrine signalling. I will use the marine worm, Platynereis dumerilii, as my main research organism due to its ease of culture in the lab, the availability of molecular tools and resources in this species, and the wealth of prior knowledge regarding its larval settlement that I can build on. Previous data shows that Platynereis are induced to settle by microalgae known as diatoms, and that a neuronal signalling molecule known as myoinhibitory peptide (MIP) regulates larval settlement behaviour. My research project has four main aims: 1. Characterize the response of Platynereis larvae to diatom cues.In collaboration with Dr Glen Wheeler and Dr Katherine Helliwell at the Marine Biological Association, Plymouth, I will test the response of Platynereis larvae to a variety of diatom species and extracts. I will also measure the neuronal activity of individual larvae exposed to diatoms to identify the larval cells responsible for detecting these cues.Aim 2. Determine whether MIP is linked to the detection of diatom cues.Using recent advances in genome editing technology, I will generate mutant Platynereis larvae that lack either MIP, or one of its two receptors. I will measure the response of mutant larvae to diatoms to test the hypothesis that if MIP signalling is directly activated by diatoms, then mutant larvae will not respond to the diatoms.Aim 3. Determine if MIP induces larval settlement in other marine invertebrates.In collaboration with Dr Rob Ellis at the University of Exeter, I will test whether synthetic MIP treatment induces larval settlement and metamorphosis in a prawn, mussel and oyster species used in local aquaculture. This will establish whether the function of MIP was conserved during the evolution of different marine invertebrates. Aim 4. Identify the downstream neuroendocrine signalling regulated by MIP to control Platynereis larval settlement. Using largescale sequencing of expressed genes, I will identify genes that are activated or repressed by MIP signalling and diatom exposure, providing insight into the physiological changes occurring at settlement. Thyroid hormone has long been suspected to play a role in marine invertebrate larval settlement. In collaboration with Professor Vincent Laudet at the Oceanological Institute of Banyuls-sur-mer, France, I will measure changes in thyroid hormone levels during larval settlement to investigate whether it is a regulator of settlement. Understanding of the importance of different microhabitats as inducers of larval settlement, and identifying downstream hormonal signalling pathways, can be exploited to induce or deter larval settlement. This knowledge has the potential to provide insights into improving invertebrate aquaculture productivity and developing novel antifouling strategies. Project findings may also assist in the repopulation of declining natural marine habitats with key invertebrates.