Permanently Fused Setules Create Unusual Folding Fans Used for Swimming in Cyprid Larvae of Barnacles

Many crustacean swimming appendages carry arrays of plumose setae-exoskeletal, feather-like structures of long bristles (setae) with short branches (setules) distributed along two sides. Although closely spaced, setae are not physically interconnected. Setal arrays function during swimming as drag-based leaky paddles that push the organism through water. Barnacle cyprids, the final, non-feeding larval stage, swim with six pairs of legs (thoracopods) that open and close setal arrays in alternating high-drag power strokes and low-drag recovery strokes. While studying cyprid swimming, we found that their thoracopods contained setae permanently cross-linked by fused setules. These cuticular connections would seem highly unlikely because setae are individually produced exoskeletal secretions, and the connections imply unknown processes for the production or modification of crustacean setae. We describe the morphology and function of plumose setae on cyprids of Balanus glandula and other species across the clade Cirripedia. Setules from adjacent plumose setae are seamlessly joined at their tips and occur in three distinct linkage patterns. Thoracopods lack muscles to open and close the array; interconnected setae are instead pulled apart, producing a paddle-like fan with high drag when appendages spread laterally during power strokes. Setules are spring-like, passively closing setae into tight bundles with low drag during recovery strokes. The linked setules occur in the three main clades of the Cirripedia. This cuticular arrangement is effective in swimming, may eliminate the need for muscles to close the setal array, and may represent a unique swimming structure within the Crustacea.