Direct observation of the setular web that fuses thoracopodal setae of a calanoid copepod into a collapsible fan

Summary Copepods are numerically dominant planktonic grazers throughout the waters of Earth, preyed upon in turn by a wide diversity of pelagic animals (1,2). Their feeding and swimming performance thus has global importance to aquatic food webs and oceanic carbon flux. These crustaceans swim and feed using cuticle-covered, segmented, muscular appendages whose reach is extended greatly by setae, extracellular chitinous extensions with diverse structure and function (3). Plumose setae, with subsidiary setules arranged like barbs on a feather, have well-documented roles in generating feeding and swimming currents (4,5). Recent work showed that plumose setae of barnacle cyprid thoracopods are permanently linked by setules into a single fan that opens and closes as one sheet during high-speed swimming (6). Intersetular linkage across cyprid thoracopods may greatly decrease leakage between extended setae, ensure even spread of setae within the fan, and promote ordered collapse of the fan to avoid entanglement of adjacent appendages. Here we demonstrate similar setular webbing amongst thoracopod setae in the calanoid copepod Acartia sp. High-speed video directly documents the existence of such links, and reveals that individuals experience apparently-irreparable degradation of the setal array due to de-linkage, with likely consequences for swimming performance.