Cooperative hydrodynamics accompany multicellular-like colonial organization in the unicellular ciliate Stentor

Evolution of multicellularity from early unicellular ancestors is one of the most important transitions since the origin of life. Multicellularity is associated with enhanced nutrient uptake, better defense against predation, cell specialization and division of labor. While many single-celled organisms exhibit both solitary and colonial forms, the organizing principles governing the transition and the benefits endowed by the colonial states are less clear. Here, we use the suspension-feeding unicellular protist Stentor coeruleus, in which unlike Volvox carteri, colony-formation is ephemeral. We show that hydrodynamic coupling between proximal neighbors results in faster feeding flows that depend on the separation between individuals, such that individuals in a dynamic colony have stronger feeding flows on average. Moreover, accrued feeding benefits are typically asymmetric; individuals with weaker solitary currents gain more from partnering than those with faster currents. Our finding that colonial organization in simple unicellular organisms is beneficial provides fundamental insights into the selective forces favoring the early evolution of multicellular organization.