Sensory perception in marine larvae

The life cycle of sessile marine invertebrates relies on a successful transition from a free-swimming larval stage to a benthic adult form. Gaining a deeper understanding of this transition is essential, as this transition often serves as a bottleneck in the life cycle of marine invertebrates and shapes the marine communities, particularly in diverse ecosystems like coral reefs. Recent studies focusing on various marine invertebrate model organisms have shed light on how ciliated larvae navigate their environments, detect essential cues, and activate their specialised sensory nervous systems to initiate larval settlement and metamorphosis. This chapter synthesises current knowledge on the sensory abilities of larvae, the role of peptidergic and classical neurotransmitter signalling in neural control, the influence of environmental cues, and the evolution of intracellular pathways that underpin settlement processes. It focuses on the early-branching metazoans, such as sponges (phylum Porifera) and cnidarians. Despite lacking a true nervous system, sponges show coordinated sensory responses, illustrating that fundamental molecular elements of sensory functions predate the emergence of neural complexity. Cnidarians, as one of the oldest extant phyla with a nervous system, provide crucial insights into the early evolution of neural organisation.