Neuromodulation of Swarming Behavior in C. elegans: Insights into the Conserved role of Calsyntenins.

Collective animal behaviors arise from a complex interplay between internal physiological states and external environmental cues. In Caenorhabditis elegans, favorable conditions promote dispersal, while stressors like food scarcity or overcrowding trigger aggregation. Here, we describe a distinct behavior termed as swarming, where C. elegans move and feed in aggregates despite abundant food availability. While environmental factors have been implicated in this behavior, the underlying genetic and molecular mechanisms remain unclear. We identify a novel role for the conserved calsyntenin protein CASY-1 in regulating swarming. Through genetic, behavioral, and optogenetic approaches, we show that CASY-1 functions in sensory neurons to modulate the neuropeptide pigment-dispersing factor (PDF) signaling. Mutants in casy-1 show impaired PDF-1 signaling and reduced inhibition of the serotonin pathway, a known regulator of social behaviors. This dysregulation, along with its associated mechanosensory and foraging defects, likely contributes to the swarming phenotype. Our findings reveal a putative neuromodulatory pathway critical for swarming behavior in C. elegans.