Required for the morphogenesis and for the elongation of the flagellar filament by facilitating polymerization of the flagellin monomers at the tip of growing filament. Forms a capping structure, which prevents flagellin subunits (transported through the central channel of the flagellum) from leaking out without polymerization at the distal end. Essential for swarming motility. Homopentamer. V.parahaemolyticus possesses two flagellar systems: a single polar flagellum propels the bacterium in liquid (swimming), while multiple lateral (peritrichous) flagella move the bacterium over surfaces (swarming). The polar flagellum is synthesized constitutively but lateral flagella are produced only under conditions in which the polar flagellum is not functional. Belongs to the FliD family.