Comparative approach to barnacle adhesive-surface interactions

Abstract

Barnacles are considered to be one of the major marine fouling organisms. Their settlement behaviour has been investigated using mainly Balanus amphitrite as a model organism. To better understand the mechanisms involved during the colonisation of surfaces by cypris larvae we have investigated another species, B. improvisus, which is reported to have different surface preferences compared to B. amphitrite. This study aims to unravel the effects of surface physicochemical cues, in particular surface free energy (SFE), surface charge and elastic modulus on the settlement of cyprids of both species. The use of well-defined surfaces under controlled conditions further facilitates comparison of the results with B. amphitrite. Furthermore, since this phase of presettlement behaviour is characterised by temporary adhesive (footprint) deposition, considered to be fundamental to surface exploration and surface discrimination by cyprids, some of the chemistries used for the settlement assays were used to investigate temporary adhesive-surface interactions. Cyprids were exposed to a series of model surfaces, namely self-assembled monolayers (SAMs) of alkanethiols with varying end-groups, homogenously applied to gold-coated polystyrene Petri dishes. The settlement response was significantly higher on negatively charged SAMs and lower on positively charged surfaces, while intermediate settlement occurred on neutral SAMs. Furthermore, no effects were observed when data were plotted against surface free energy after 48 hr of exposure. Temporary adhesive on SAMs was investigated using imaging ellipsometry and atomic force microscopy. Relatively thick footprints with low wetting were found on positively charged surfaces. Settlement of both species was also low on these surfaces. Footprints were thinner and spread more on hydrophobic surfaces. The adhesion force of temporary adhesive measured with functionalised AFM tips was higher on hydrophobic and negatively charged surfaces for both species. Furthermore, PDMS-based surfaces were prepared varying the elastic modulus, keeping constant other parameters, settlement behaviour and strength of adhesion of juveniles and adults were tested. We conclude that cyprid settlement behaviour of both species is influenced more by surface charge than SFE under controlled conditions. The temporary adhesives (footprints) of the two species had a stronger affinity for hydrophobic surfaces. Contrary ii to previous reports, therefore, the settlement preferences and adhesive secretion of these two species are similar. Elastic modulus influences settlement, juveniles and adults removal of both species, although B. improvisus is more sensitive if compared with B. amphitrite. This finding will be important for understanding the mechanism of surface selection by cyprids and for the development of future antifouling technologies.