Abstract
When exploring immersed surfaces the cypris larvae of barnacles employ a tenacious and rapidly reversible adhesion mechanism to facilitate their characteristic ‘walking’ behaviour. Although of direct relevance to the fields of marine biofouling and bio-inspired adhesive development, the mechanism of temporary adhesion in cyprids remains poorly understood. Cyprids secrete deposits of a proteinaceous substance during surface attachment and these are often visible as ‘footprints’ on previously explored surfaces. The attachment structures, the antennular discs, of cyprids also present a complex morphology reminiscent of both the hairy appendages used by some terrestrial invertebrates for temporary adhesion and a classic ‘suction cup’. Despite the numerous analytical approaches so-far employed, it has not been possible to resolve conclusively the respective contributions of viscoelastic adhesion via the proteinaceous ‘temporary adhesive’, ‘dry’ adhesion via the cuticular villi present on the disc and the behavioural contribution by the organism. In this study, high-speed photography was used for the first time to capture the behaviour of cyprids at the instant of temporary attachment and detachment. Attachment is facilitated by a constantly sticky disc surface – presumably due to the presence of the proteinaceous temporary adhesive. The tenacity of the resulting bond, however, is mediated behaviourally. For weak attachment the disc is constantly moved on the surface, whereas for a strong attachment the disc is spread out on the surface. Voluntary detachment is by force, requiring twisting or peeling of the bond – seemingly without any more subtle detachment behaviours. Micro-bubbles were observed at the adhesive interface as the cyprid detached, possibly an adaptation for energy dissipation. These observations will allow future work to focus more specifically on the cyprid temporary adhesive proteins, which appear to be fundamental to adhesion, inherently sticky and exquisitely adapted for reversible adhesion underwater.
Highlights
Cyprids are the final larval stage of barnacles and are superbly equipped for surface exploration, selection of a suitable location for settlement and final attachment prior to metamorphosis to the adult form [1,2]
Full contact between the disc and the surface is very brief during wide searching, similar in essence to the contact between a foot and a surface during a human stride
Previously undescribed behaviours have been identified during wide searching, inspection and detachment by exploring cyprids that tailor temporary adhesion to the requirements of each exploration stage
Summary
Cyprids are the final larval stage of barnacles and are superbly equipped for surface exploration, selection of a suitable location for settlement and final attachment prior to metamorphosis to the adult form [1,2]. Using adhesive discs on the 3rd segments of paired antennules, cyprids are able to walk in a rapidly reversible bi-pedal fashion over immersed surfaces [3] (Figure 1), evaluating the properties of the substratum using a range of sensory structures [4]. Most of the cyprid’s external sensory setae are concentrated on the 3rd and 4th segments of the antennules, including on and around the adhesive disc, with putative chemo-, contact-chemoand mechano-sensory setae having been described in detail [4,5,6]. Specific details of the antennule-associated structures differ between barnacle species, perhaps corresponding to preferred habitat type [5], the gross morphology of the antennules and the sensory and adhesive structures they support are universally conserved. The similarity of adhesive structures between species, regardless of adult form, is suggestive of a supremely adapted adhesion mechanism
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