Functional morphology of cirri in the barnacle Amphibalanus improvisus (crustacea: Balanidae)

Barnacles rely heavily on their mobile cirri for food capture because of the sessile lifestyle. These filamentous food capturing devices are extended into the water current and perform undulating movements. Cuticular structures with corresponding musculature work together, to allow these highly repetitive movements. This paper studies the interplay between structure and function of the cirri using scanning electron microscopy (SEM), microcomputed tomography (µCT) and high-speed video recordings (HSV) in Amphibalanus improvisus. Barnacles use the external cuticle structures (denticles and setae) for efficient grooming, food capturing, and providing support for the muscular system responsible for movement control. Entanglement of the cirri during extension is probably avoided through an interlocking of the serrate setae on cirri IV-VI, creating a "zipper-like" effect, which was recorded here using the HSV. We analyzed the muscular arrangement using µCT and found a new flexor muscle in both the endo- and exopod of cirrus II. Supported by the intrinsic cirral muscles, the new flexor muscles may provide variable movements of the anterior cirri (cirri I-III), which is important for further food handling. Our results provide a foundation for further comparative studies of the feeding apparatus of barnacles and for possible implications in the area of bio-inspired robotics.