Adaptive Adhesions of Barnacle-Inspired Adhesive Peptides

The strategy of robust adhesion employed by barnacles renders them fascinating biomimetic candidates for developing novel wet adhesives. Particularly, barnacle cement protein 19k (cp19k) has been speculated to be the key adhesive protein establishing the priming layer in the initial barnacle cement construction. In this work, we systematically studied the sequence design rationale of cp19k by designing adhesive peptides inspired by the low-complexity STGA-rich and the charged segments of cp19k. Combining structure analysis and the adhesion performance test, we found that cp19k-inspired adhesive peptides possess excellent disparate adhesion strategies for both hydrophilic mica and hydrophobic self-assembled monolayer surfaces. Specifically, the low-complexity STGA-rich segment offers great structure flexibility for surface adhesion, while the hydrophobic and charged residues can contribute to the adhesion of the peptides on hydrophobic and charged surfaces. The adaptive adhesion strategy identified in this work broadens our understanding of barnacle adhesion mechanisms and offers valuable insights for designing advanced wet adhesives with exceptional performance on various types of surfaces.

Sacculina-Induced Morphological Feminization in the Grapsid Crab Pachygrapsus crassipes

Rhizocephalan barnacles (Thecostraca: Cirripedia) are parasitic crustaceans that lack appendages, segmentation, and a digestive system in adults, while instead infiltrating their hosts with a nutrient-absorbing system of rootlets. Sacculinids, belonging to the Rhizocephala order, are known for their various parasitization-induced effects on their decapod hosts, such as parasitic castration, reduction in the growth of secondary sexual characteristics, feminization of male crabs, and alteration of host behavior. In this study, we conducted field surveys in Japan at Manazuru Town (Kanagawa) on the Pacific coast, and on Sado Island and Noto Peninsula on the Sea of Japan side, and found that sacculinid-parasite-ratios on the grapsid crab Pachygrapsus crassipes were particularly high on the Sea of Japan coast. Molecular phylogenetic analysis revealed that the Manazuru population forms a single clade with Sacculina yatsui, and both Sado and Noto populations form a single clade with S. confragosa. We further demonstrated that external morphologies of male P. crassipes parasitized by sacculinids were changed to female phenotypes. This host-parasite interaction will be a useful model for understanding molecular mechanisms underlying rhizocephalan-driven morphological and behavioral feminization and castration.

Efficacy of amorphous TiOx-coated surfaces against micro- and macrofouling through laboratory microcosms and field studies

In this study, Soda Lime Glass (SLG) and Stainless Steel (SS316L) substrata coated with Titanium oxide (TiOx) were tested for their efficacy in the laboratory microcosms and in field against micro- and macrofouling. Laboratory microcosm studies were conducted for five days using natural biofilms, single-species diatom (Navicula sp.), and bacterial biofilms, whereas field observations were conducted for 30 days. The TiOx-coating induced change in the mean contact angle of the substratum and rendered SS316L more hydrophilic and SLG hydrophobic, which influenced the Navicula sp. biofilm, and bacterial community structure of the biofilm. Overall, the TiOx-coated SS316L showed minimal microfouling, whereas non-coated SLG exhibited greater efficacy in deterring/preventing macrofouling organisms. Moreover, the reduction in macrofouling could be attributed to high abundance of Actinobacteria. Unraveling the mechanism of action needs future studies emphasizing biochemical processes and pathways.

Nitroxide-Containing Amphiphilic Random Terpolymers for Marine Antifouling and Fouling-Release Coatings

Two types of amphiphilic random terpolymers, poly(ethylene glycol methyl ether methacrylate)-ran-poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate)-ran-poly(polydimethyl siloxane methacrylate) (PEGMEMA-r-PTMA-r-PDMSMA), were synthesized and evaluated for antifouling (AF) and fouling-release (FR) properties using diverse marine fouling organisms. In the first stage of production, the two respective precursor amine terpolymers containing (2,2,6,6-tetramethyl-4-piperidyl methacrylate) units (PEGMEMA-r-PTMPM-r-PDMSMA) were synthesized by atom transfer radical polymerization using various comonomer ratios and two initiators: alkyl halide and fluoroalkyl halide. In the second stage, these were selectively oxidized to introduce nitroxide radical functionalities. Finally, the terpolymers were incorporated into a PDMS host matrix to create coatings. AF and FR properties were examined using the alga Ulva linza, the barnacle Balanus improvisus, and the tubeworm Ficopomatus enigmaticus. The effects of comonomer ratios on surface properties and fouling assay results for each set of coatings are discussed in detail. There were marked differences in the effectiveness of these systems against the different fouling organisms. The terpolymers had distinct advantages over monopolymeric systems across the different organisms, and the nonfluorinated PEG and nitroxide combination was identified as the most effective formulation against B. improvisus and F. enigmaticus.

Anti-Barnacle Activities of Isothiocyanates Derived from β-Citronellol and Their Structure-Activity Relationships

Antifouling agents with low toxicity are in high demand for sustaining marine industries and the environment. This study aimed to synthesize 15 isothiocyanates derived from β-citronellol and evaluate their antifouling activities and toxicities against cypris larvae of the barnacle Amphibalanus amphitrite. The synthesized isothiocyanates exhibited effective antifouling activities (EC₅₀ =0.10-3.33 μg mL^-1 ) with high therapeutic ratios (LC₅₀ /EC₅₀ >30). Four isothiocyanates with an amide or isocyano group showed great potential as effective antifouling agents (EC₅₀ =0.10-0.32 μg mL^-1 , LC₅₀ /EC₅₀ =104-833). The enantiomers of the isothiocyanates only slightly differed in their antifouling activities. These results may serve as a basis for further research and development of β-citronellol-derived isothiocyanates as effective low-toxic antifouling agents. To the best of our knowledge, this study is the first to report the antifouling activities of isothiocyanates derived from accessible natural products.

[The adhesion mechanism of barnacle and its cement proteins: a review]

The adhesive protein secreted by marine sessile animals can resist the resistance of water and exert stickiness under the humid environment. It has become a candidate for the development of high-performance materials in the field of biomedicine and bionics. Barnacles are as one of the marine macrofoulers that can be firmly attached to the underwater substrate materials with different surface characteristics through its cement proteins. To date, the adhesion process of barnacle has been understood in-depth, but the specific underwater adhesion mechanism has not been elucidated and needs further exploration. This review first presented an overview of barnacle and its adhesion process, followed by summarizing the advances of barnacle adhesive protein, its production methods, and applications. Moreover, challenges and future perspectives were prospected.

Chromosome-level genome assembly, annotation, and phylogenomics of the gooseneck barnacle Pollicipes pollicipes

BACKGROUND

The barnacles are a group of >2,000 species that have fascinated biologists, including Darwin, for centuries. Their lifestyles are extremely diverse, from free-swimming larvae to sessile adults, and even root-like endoparasites. Barnacles also cause hundreds of millions of dollars of losses annually due to biofouling. However, genomic resources for crustaceans, and barnacles in particular, are lacking.

RESULTS

Using 62× Pacific Biosciences coverage, 189× Illumina whole-genome sequencing coverage, 203× HiC coverage, and 69× CHi-C coverage, we produced a chromosome-level genome assembly of the gooseneck barnacle Pollicipes pollicipes. The P. pollicipes genome is 770 Mb long and its assembly is one of the most contiguous and complete crustacean genomes available, with a scaffold N50 of 47 Mb and 90.5% of the BUSCO Arthropoda gene set. Using the genome annotation produced here along with transcriptomes of 13 other barnacle species, we completed phylogenomic analyses on a nearly 2 million amino acid alignment. Contrary to previous studies, our phylogenies suggest that the Pollicipedomorpha is monophyletic and sister to the Balanomorpha, which alters our understanding of barnacle larval evolution and suggests homoplasy in a number of naupliar characters. We also compared transcriptomes of P. pollicipes nauplius larvae and adults and found that nearly one-half of the genes in the genome are differentially expressed, highlighting the vastly different transcriptomes of larvae and adult gooseneck barnacles. Annotation of the genes with KEGG and GO terms reveals that these stages exhibit many differences including cuticle binding, chitin binding, microtubule motor activity, and membrane adhesion.

CONCLUSION

This study provides high-quality genomic resources for a key group of crustaceans. This is especially valuable given the roles P. pollicipes plays in European fisheries, as a sentinel species for coastal ecosystems, and as a model for studying barnacle adhesion as well as its key position in the barnacle tree of life. A combination of genomic, phylogenetic, and transcriptomic analyses here provides valuable insights into the evolution and development of barnacles.

Amphiphilic Nitroxide-Bearing Siloxane-Based Block Copolymer Coatings for Enhanced Marine Fouling Release

The buildup of organic matter and organisms on surfaces exposed to marine environments, known as biofouling, is a disruptive and costly process affecting maritime operations. Previous research has identified some of the surface characteristics particularly suited to the creation of antifouling and fouling-release surfaces, but there remains room for improvement against both macrofouling and microfouling organisms. Characterization of their adhesives has shown that many rely on oxidative chemistries. In this work, we explore the incorporation of the stable radical 2,2,6,6-tetramethylpipiderin-1-oxyl (TEMPO) as a component in an amphiphilic block copolymer system to act as an inhibitor for marine cements, disrupting adhesion of macrofouling organisms. Using polystyrene-b-poly(dimethylsiloxane-r-vinylmethysiloxane) block copolymers, pendent vinyl groups were functionalized with TEMPO and poly(ethylene glycol) to construct an amphiphilic material with redox active character. The antifouling and fouling-release performance of these materials was investigated through settlement and removal assays of three model fouling organisms and correlated to surface structure and chemistry. Surfaces showed significant antifouling character and fouling-release performance was increased substantially toward barnacles by the incorporation of stable radicals, indicating their potential for marine antifouling applications.

On Sea Turtle-associated Craspedostauros (Bacillariophyta), with Description of Three Novel Species

The current study focuses on four species from the primarily marine diatom genus Craspedostauros that were observed growing attached to numerous sea turtles and sea turtle-associated barnacles from Croatia and South Africa. Three of the examined taxa, C. danayanus sp. nov., C. legouvelloanus sp. nov., and C. macewanii sp. nov., are described based on morphological and, whenever possible, molecular characteristics. The new taxa exhibit characters not previously observed in other members of the genus, such as the presence of more than two rows of cribrate areolae on the girdle bands, shallow perforated septa, and a complete reduction of the stauros. The fourth species, C. alatus, itself recently described from museum sea turtle specimens, is reported for the first time from loggerhead sea turtles rescued in Europe. A 3-gene phylogenetic analysis including DNA sequence data for three sea turtle-associated Craspedostauros species and other marine and epizoic diatom taxa indicated that Craspedostauros is monophyletic and sister to Achnanthes. This study, being based on a large number of samples and animal specimens analyzed and using different preservation and processing methods, provides new insights into the ecology and biogeography of the genus and sheds light on the level of intimacy and permanency in the host-epibiont interaction within the epizoic Craspedostauros species.

Complete mitochondrial genome of Tetraclita squamosa squamosa (Sessilia: Tetraclitidae) from China and phylogeny within Cirripedia

Here we present the complete mitochondrial genome of Tetraclita squamosasquamosa, which is 15,191 bp in length with 67.20% AT content. It contains 13 protein-coding genes, 2 ribosomal-RNA genes and 22 transfer-RNA genes. All PCGs except nad4l in T. squamosasquamosa start with ATN, and terminated with a complete stop codon, except nad3. Phylogenetic analysis based on mitochondrial PCGs shows that T. squamosasquamosa is clustered with T. serrata into a branch (BP = 100). Our result is consistent with previous reports that genus Tetraclita and family Tetraclitidae are not monophyletic. This study contributes to further phylogenetic analysis within Cirripedia. 

Amphibalanus amphitrite begins exoskeleton mineralization within 48 hours of metamorphosis

Barnacles are ancient arthropods that, as adults, are surrounded by a hard, mineralized, outer shell that the organism produces for protection. While extensive research has been conducted on the glue-like cement that barnacles use to adhere to surfaces, less is known about the barnacle exoskeleton, especially the process by which the barnacle exoskeleton is formed. Here, we present data exploring the changes that occur as the barnacle cyprid undergoes metamorphosis to become a sessile juvenile with a mineralized exoskeleton. Scanning electron microscope data show dramatic morphological changes in the barnacle exoskeleton following metamorphosis. Energy-dispersive X-ray spectroscopy indicates a small amount of calcium (8%) 1 h post-metamorphosis that steadily increases to 28% by 2 days following metamorphosis. Raman spectroscopy indicates calcite in the exoskeleton of a barnacle 2 days following metamorphosis and no detectable calcium carbonate in exoskeletons up to 3 h post-metamorphosis. Confocal microscopy indicates during this 2 day period, barnacle base plate area and height increases rapidly (0.001 mm2 h-1 and 0.30 µm h-1, respectively). These results provide critical information into the early life stages of the barnacle, which will be important for developing an understanding of how ocean acidification might impact the calcification process of the barnacle exoskeleton.

Insights into the Synthesis, Secretion and Curing of Barnacle Cyprid Adhesive via Transcriptomic and Proteomic Analyses of the Cement Gland

Barnacles represent one of the model organisms used for antifouling research, however, knowledge regarding the molecular mechanisms underlying barnacle cyprid cementation is relatively scarce. Here, RNA-seq was used to obtain the transcriptomes of the cement glands where adhesive is generated and the remaining carcasses of Megabalanus volcano cyprids. Comparative transcriptomic analysis identified 9060 differentially expressed genes, with 4383 upregulated in the cement glands. Four cement proteins, named Mvcp113k, Mvcp130k, Mvcp52k and Mvlcp1-122k, were detected in the cement glands. The salivary secretion pathway was significantly enriched in the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of the differentially expressed genes, implying that the secretion of cyprid adhesive might be analogous to that of saliva. Lysyl oxidase had a higher expression level in the cement glands and was speculated to function in the curing of cyprid adhesive. Furthermore, the KEGG enrichment analysis of the 352 proteins identified in the cement gland proteome partially confirmed the comparative transcriptomic results. These results present insights into the molecular mechanisms underlying the synthesis, secretion and curing of barnacle cyprid adhesive and provide potential molecular targets for the development of environmentally friendly antifouling compounds.

Microstructure and crystallography of the wall plates of the giant barnacle Austromegabalanus psittacus: a material organized by crystal growth

In biomineralization, it is essential to know the microstructural and crystallographic organization of natural hard tissues. This knowledge is virtually absent in the case of barnacles. Here, we have examined the crystal morphology and orientation of the wall plates of the giant barnacle Austromegabalanus psittacus by means of optical and electron microscopy, and electron backscatter diffraction. The wall plates are made of calcite grains, which change in morphology from irregular to rhombohedral, except for the radii and alae, where fibrous calcite is produced. Both the grains and fibres arrange into bundles made of crystallographically co-oriented units, which grow onto each other epitaxially. We call these areas crystallographically coherent regions (CCRs). Each CCR elongates and disposes its c-axis perpendicularly or at a high angle to the growth surfaces, whereas the a-axes of adjacent CCRs differ in orientation. In the absence of obvious organic matrices, this pattern of organization is interpreted to be produced by purely crystallographic processes. In particular, due to crystal competition, CCRs orient their fastest growth axes perpendicular to the growth surface. Since each CCR is an aggregate of grains, the fastest growth axis is that along which crystals stack up more rapidly, that is, the crystallographic c-axis in granular calcite. In summary, the material forming the wall plates of the studied barnacles is under very little biological control and the main role of the mantle cells is to provide the construction materials to the growth front.

Different effects of mating group size as male and as female on sex allocation in a simultaneous hermaphrodite

Sex allocation theory predicts that the optimal sexual resource allocation of simultaneous hermaphrodites is affected by mating group size (MGS). Although the original concept assumes that the MGS does not differ between male and female functions, the MGS in the male function (MGSm; i.e., the number of sperm recipients the focal individual can deliver its sperm to plus one) and that in the female function (MGSf; the number of sperm donors plus one) do not always coincide and may differently affect the optimal sex allocation. Moreover, reproductive costs can be split into "variable" (e.g., sperm and eggs) and "fixed" (e.g., genitalia) costs, but these have been seldom distinguished in empirical studies. We examined the effects of MGSm and MGSf on the fixed and variable reproductive investments in the sessilian barnacle Balanus rostratus. The results showed that MGSm had a positive effect on sex allocation, whereas MGSf had a nearly significant negative effect. Moreover, the "fixed" cost varied with body size and both aspects of MGS. We argue that the two aspects of MGS should be distinguished for organisms with unilateral mating.

The first mitochondrial genome of Megabalanus tintinnabulum (Sessilia: Balanidae) from China: phylogeny within Cirripedia based on mitochondrial genes

Here we present the complete mitochondrial genome of Megabalanus tintinnabulum. The genome is 15,107 bp in length with a 67.35% AT content. It contains 13 protein-coding genes (PCGs), 2 rRNAs genes, and 22 tRNAs. Both rRNAs are encoded on the light strand, as in the other crustacean and barnacle mitochondrial genomes. Besides five tRNAs are encoded on the light strand (nad1, trnV, trnL1, trnC, trnQ, and trnK). Only one PCG is encoded on the light strand (nad1), whereas the other 12 PCGs are located on the heavy strand, which is consistent with M. ajax. Phylogenetic analysis based on mitochondrial PCGs shows that M. tintinnabulum is clustered with M. ajax into a branch (BP = 100), and the group with M. volcano with high support. This study contributes to further phylogenetic analysis within Cirripedia.

The expression and characterization of recombinant cp19k barnacle cement protein from Pollicipes pollicipes

Adhesive proteins of barnacle cement have potential as environmentally friendly adhesives owing to their ability to adhere to various substrates in aqueous environments. By understanding the taxonomic breath of barnacles with different lifestyles, we may uncover commonalities in adhesives produced by these specialized organisms. The 19 kDa cement protein (cp19k) of the stalked barnacle Pollicipes pollicipes was expressed in Escherichia coli BL21 to investigate its adhesive properties. Initial expression of hexahistidine-tagged protein (rPpolcp19k-his) yielded low levels of insoluble protein. Co-overproduction of E. coli molecular chaperones GroEL-GroES and trigger factor (TF) increased soluble protein yields, although TF co-purified with the target protein (TF-rPpolcp19k-his). Surface coat analysis revealed high levels of adsorption of the TF-rPpolcp19k-his complex and of purified E. coli TF on both hydrophobic and hydrophilic surfaces, while low levels of adsorption were observed for rPpolcp19k-his. Tag-free rPpolcp19k protein also exhibited low adsorption compared to fibrinogen and Cell-Tak controls on hydrophobic, neutral hydrophilic and charged self-assembled monolayers under surface plasmon resonance assay conditions designed to mimic the barnacle cement gland or seawater. Because rPpolcp19k protein displays low adhesive capability, this protein is suggested to confer the ability to self-assemble into a plaque within the barnacle cement complex. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Articulation and growth of skeletal elements in balanid barnacles (Balanidae, Balanomorpha, Cirripedia)

The morphology and ultrastructure of the shells of two balanid species have been examined, paying special attention to the three types of boundaries between plates: (i) radii-parietes, (ii) alae-sheaths, and (iii) parietes-basal plate. At the carinal surfaces of the radii and at the rostral surfaces of the alae, there are series of crenulations with dendritic edges. The crenulations of the radius margins interlock with less prominent features of the opposing paries margins, whereas the surfaces of the longitudinal abutments opposing the ala margins are particularly smooth. The primary septa of the parietes also develop dendritic edges, which abut the internal surfaces of the primary tubes of the base plates. In all cases, there are chitino-proteinaceous organic membranes between the abutting structures. Our observations indicate that the very edges of the crenulations and the primary septa are permanently in contact with the organic membranes. We conclude that, when a new growth increment is going to be produced, the edges of both the crenulations and the primary septa pull the viscoelastic organic membranes locally, with the consequent formation of viscous fingers. For the abutting edges to grow, calcium carbonate must diffuse across the organic membranes, but it is not clear how growth of the organic membranes themselves is accomplished, in the absence of any cellular tissue.

Microbiome acquisition during larval settlement of the barnacle Semibalanus balanoides

Barnacles are conspicuous members of rocky intertidal communities and settlement of the final larval stage, the cyprid, is influenced by the presence of biofilms. While modulation of cyprid settlement by biofilms has been studied extensively, the acquisition of a specific microbiome by the settling larva has not. This study investigated settlement in the field of Semibalanus balanoides in two consecutive years when the composition of the benthic bacterial community differed. In both years, settling cyprids adopted a specific sub-set of benthic bacteria that was distinct from the planktonic cyprid and the benthos. This microbiome was consistent, regardless of annual variability in the benthic community structure, and established within hours of settlement. The results imply that a natural process of selection occurs during the critical final transition of S. balanoides to the sessile form. The apparent consistency of this process between years suggests that optimal growth and survival of barnacles could depend upon a complex inter-kingdom relationship, as has been demonstrated in other animal systems.

Isotopes from fossil coronulid barnacle shells record evidence of migration in multiple Pleistocene whale populations

Migration is an integral feature of modern mysticete whale ecology, and the demands of migration may have played a key role in shaping mysticete evolutionary history. Constraining when migration became established and assessing how it has changed through time may yield valuable insight into the evolution of mysticete whales and the oceans in which they lived. However, there are currently few data which directly assess prehistoric mysticete migrations. Here we show that calcite δ¹⁸O profiles of two species of modern whale barnacles (coronulids) accurately reflect the known migration routes of their host whales. We then analyze well-preserved fossil coronulids from three different locations along the eastern Pacific coast, finding that δ¹⁸O profiles from these fossils exhibit trends and ranges similar to modern specimens. Our results demonstrate that migration is an ancient behavior within the humpback and gray whale lineages and that multiple Pleistocene populations were undertaking migrations of an extent similar to those of the present day.

First report on the complete mitochondrial genome of the deep-water scalpellid barnacle Arcoscalpellum epeeum (Cirripedia, Thoracica, Scalpellidae)

Scalpellids are one of the largest families of Scalpelliformes and reproduce either androdioeciously or dioeciously. Here, we characterized the first mitogenome of a scalpellid barnacle (Arcoscalpellum epeeum), which was 15,593 bp in length with a 71.5% AT content. In comparison with the pollicipedids Capitulum mitella and Pollicipes polymerus, the tRNA genes of A. epeeum were rearranged between ND3 and ND5, between CYTB and ND1, and between 12S rRNA and ND2. On the mitogenomic tree, the Scalpelliformes families Pollicipedidae and Scalpellidae were not monophyletic, which concurs with previous studies.

Complete mitochondrial genome of the catophragmid barnacle Catomerus polymerus (Cirripedia, Thoracica, Balanomorpha, Catophragmidae)

The family Catophragmidae is one of the lower balanomorphs from traditional and recent multiple mitochondrial and nuclear markers molecular analysis. Here, we characterized the first mitogenome of the catophragmid barnacle Catomerus polymerus, which was 15,446 bp in length with a 68.3% AT content. The mitogenome had the typical pancrustacean gene arrangement, which was identical to the mitogenome configurations of the chthamalid Octomeris sp. and pachylasmatoid Eochionelasmus ohtai. On the mitogenomic tree, the catophragmid Catomerus polymerus formed an independent branch that was basal to the members of the superfamilies Tetraclitoidea and Balanoidea, which was inconsistent with previous findings.

Competing with barnacle cement: wetting resistance of a re-entrant surface reduces underwater adhesion of barnacles

Surfaces with re-entrant topographies can repel liquids even of extremely low surface tension, almost independently of the material's inherent wettability. We show that this topography-based wetting resistance can also be applied to underwater applications, reducing the permanent adhesion of marine hardfouling organisms. Having combined a biofouling assay in the marine environment with microscopic analyses, we demonstrate how a synergistic effect of a soft silicone-based material with a re-entrant mushroom-shaped surface topography strongly increases the fouling release ability of such coatings compared with a smooth control made from the same material. Our coating inhibited the complete wetting of the solidifying glues produced by marine organisms, which resulted in a decreased contact area and, consequently, low adhesion. Our work suggests that topography-induced wetting resistance of surfaces may become a viable approach in preventing permanent adhesion of marine hardfouling organisms.

Self-Assembled Nanofibers for Strong Underwater Adhesion: The Trick of Barnacles

Developing adhesives that can function underwater remains a major challenge for bioengineering, yet many marine creatures, exemplified as mussels and barnacles, have evolved their unique proteinaceous adhesives for strong wet adhesion. The mechanisms underlying the strong adhesion of these natural adhesive proteins provide rich information for biomimetic efforts. Here, combining atomic force microscopy (AFM) imaging and force spectroscopy, we examine the effects of pH on the self-assembly and adhesive properties of cp19k, a key barnacle underwater adhesive protein. For the first time, we confirm that the bacterial recombinant Balanus albicostatus cp19k (rBalcp19k), which contains no 3,4-dihydroxyphenylalanine (DOPA) or any other amino acids with post-translational modifications, can self-assemble into aggregated nanofibers at acidic pHs. Under moderately acidic conditions, the adhesion strength of unassembled monomeric rBalcp19k on mica is only slightly lower than that of a commercially available mussel adhesive protein mixture, yet the adhesion ability of rBalcp19k monomers decreases significantly at increased pH. In contrast, upon preassembly at acidic and low-salinity conditions, rBalcp19k nanofibers keep stable in basic and high-salinity seawater and display much stronger adhesion and thus show resistance to its adverse impacts. Besides, we find that the adhesion ability of Balcp19k is not impaired when it is combined with an N-terminal Thioredoxin (Trx) tag, yet whether the self-assembly property will be disrupted is not determined. Collectively, the self-assembly-enhanced adhesion presents a previously unexplored mechanism for the strong wet adhesion of barnacle cement proteins and may lead to the design of barnacle-inspired adhesive materials.

Evaluating the occurrence of cryptic invasions of a rocky shore barnacle, Semibalanus cariosus, between the north-eastern Pacific and Japan

Many coastal barnacles are introduced to non-native regions. However, data are lacking on cryptic invasion, which is defined as an invasion that remains unrecognised because the invader is mistaken for a native or previously introduced species or clade. In this work, cryptic invasions of an intertidal barnacle, Semibalanus cariosus, between Japan and the north-eastern Pacific were evaluated based on population genetic analyses. A significant genetic differentiation was found between the Japanese and north-eastern Pacific populations, suggesting a limited introduction of non-native genotypes between these regions. Haplotype frequencies did not differ significantly between the past (museum samples collected in 1971 from Hokkaido, Japan) and present Japanese populations, implying the rare occurrence of human-mediated migration from the north-eastern Pacific to Japan. Migrate-n analysis revealed a low level of directional gene flow in S. cariosus from the north-eastern Pacific to Japan, possibly by natural stepping-stone dispersal via directional water currents or human-mediated transport.

Isocyanides Derived from α,α-Disubstituted Amino Acids: Synthesis and Antifouling Activity Assessment

Herein, we contribute to the development of environmentally friendly antifoulants by synthesizing eighteen isocyanides derived from α,α-disubstituted amino acids and evaluating their antifouling activity/toxicity against the cypris larvae of the Balanus amphitrite barnacle. Almost all isocyanides showed good antifouling activity without significant toxicity and exhibited EC₅₀ values of 0.07 - 7.30 μg/mL after 120-h exposure. The lowest EC₅₀ values were observed for valine-, methionine-, and phenylalanine-derived isocyanides, which achieved > 95% cypris larvae settlement inhibition at concentrations of less than 30 μg/mL without exhibiting significant toxicity. Thus, the prepared isocyanides should be useful for further research focused on the development of environmentally friendly antifouling agents.

Complete mitochondrial genome of the deep-sea asymmetrical barnacle Altiverruca navicula (Cirripedia, Thoracica, Verrucumorpha)

The hitherto suborder Verrucomorpha contains asymmetrical barnacles of two groups: the true Verrucomorpha (Eoverruca + Verrucidae) and the Neoverrucidae. Here, we determined the mitochondrial genome (mitogenome) of Altiverruca navicula, a true Verrucomorpha species. The mitogenome was 15,976 base pairs in length and had the typical pancrustacean gene arrangement. Its protein-coding genes were very similar to those of other thoracican species in terms of length, AT content, and start and stop codons. In phylogenetic trees constructed with 13 protein-coding genes, A. navicula was positioned at an ancestral node of sessile barnacles, consistent with the findings of previous studies.

New Marine Antifouling Compounds from the Red Alga Laurencia sp

Six new compounds, omaezol, intricatriol, hachijojimallenes A and B, debromoaplysinal, and 11,12-dihydro-3-hydroxyretinol have been isolated from four collections of Laurencia sp. These structures were determined by MS and NMR analyses. Their antifouling activities were evaluated together with eight previously known compounds isolated from the same samples. In particular, omaezol and hachijojimallene A showed potent activities (EC₅₀ = 0.15–0.23 µg/mL) against larvae of the barnacle Amphibalanus amphitrite.

Predation on transmission stages reduces parasitism: sea anemones consume transmission stages of a barnacle parasite

While parasites serve as prey, it is unclear how the spatial distribution of parasite predators provides transmission control and influences patterns of parasitism. Because many of its organisms are sessile, the rocky intertidal zone is a valuable but little used system to understand spatial patterns of parasitism and elucidate the underlying mechanisms driving these patterns. Sea anemones and barnacles are important space competitors in the rocky intertidal zone along the Pacific coast of North America. Anemones are voracious, indiscriminate predators; thus, they may intercept infectious stages of parasites before they reach a host. We investigate whether a sea anemone protects an associated barnacle from parasitism by Hemioniscus balani, an isopod parasitic castrator. At Coal Oil Point, Santa Barbara, California USA, 29% of barnacles were within 1 cm from an anemone at the surveyed tidal height. Barnacles associated with anemones had reduced parasite prevalence and higher reproductive productivity than those remote from sea anemones. In the laboratory, anemones readily consumed the transmission stage of the parasite. Hence, anemone consumption of parasite transmission stages may provide a mechanism by which community context regulates parasite prevalence at a local scale. Our results suggest predation may be an important process providing parasite transmission control.

Incorporation of silicone oil into elastomers enhances barnacle detachment by active surface strain

Silicone-oil additives are often used in fouling-release silicone coatings to reduce the adhesion strength of barnacles and other biofouling organisms. This study follows on from a recently reported active approach to detach barnacles, which was based on the surface strain of elastomeric materials, by investigating a new, dual-action approach to barnacle detachment using Ecoflex®-based elastomers incorporated with poly(dimethylsiloxane)-based oil additives. The experimental results support the hypothesis that silicone-oil additives reduce the amount of substratum strain required to detach barnacles. The study also de-coupled the two effects of silicone oils (ie surface-activity and alteration of the bulk modulus) and examined their contributions in reducing barnacle adhesion strength. Further, a finite element model based on fracture mechanics was employed to qualitatively understand the effects of surface strain and substratum modulus on barnacle adhesion strength. The study demonstrates that dynamic substratum deformation of elastomers with silicone-oil additives provides a bifunctional approach towards management of biofouling by barnacles.

Cochliomycin A inhibits the larval settlement of Amphibalanus amphitrite by activating the NO/cGMP pathway

Cochliomycin A is a compound with anti-barnacle settlement activity and low toxicity, but the molecular mechanism of the compound is unknown. Here, isobaric tags for the relative or absolute quantitation (iTRAQ) labeling proteomic method were applied to analyze changes in the proteome of Amphibalanus (=Balanus) amphitrite cyprids in response to cochliomycin A treatment. Cochliomycin A affected the cytochrome P450, glutathione S-transferase (GST) and NO/cGMP pathways, among which the NO/cGMP pathway was considered to play a key role in barnacle larval settlement, while the cytochrome P450 and the GST pathways are mainly for detoxification. The results of real-time PCR further suggested the NO/cGMP pathway was activated in response to cochliomycin A. Larval settlement assays revealed that S-methylisothiourea sulfate (SMIS) and 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) rescued cyprids from cochliomycin A-induced inhibition of larval settlement. The findings supported the hypothesis that cochliomycin A inhibited barnacle larval settlement by stimulating the NO/cGMP pathway.

Shell Structure and Growth in the Base Plate of the Barnacle Amphibalanus amphitrite

The base plate of the acorn barnacle Amphibalanus amphitrite (equivalent to Balanus amphitrite) is composed of hierarchically scaled, mutually aligned calcite grains, adhered to the substratum via layered cuticular tissue and protein. Acorn barnacles grow by expanding and lengthening their side plates, under which the cuticle is stretched, and adhesive proteins are secreted. In barnacles with mineralized base plates, such as A. amphitrite, a mineralization front follows behind, radially expanding the base plate at the periphery. In this study, we show that the new mineralization develops above the adhesion layers in a unique trilayered structure. Calcite crystallites in each of the layers have distinct sizes, varying from coarse-grained (>1 μm across) in the upper layer, to fine-grained (∼1 μm) in the middle layer, to nanoparticulate (∼40 nm) in the basal layer. The fine-grained crystallites dominate the growth front, comprising the bulk of the shell at the periphery, with later coarse grain development on the top of the base plate (toward the barnacle interior) and nanocrystalline calcite templating underneath in contact with the cuticle/protein layer. While the coarse-grained calcite on the upper surface contains a range of crystal orientations, the underlying fine-grained and nanocrystalline calcite are mutually oriented to within a few degrees of each other. Electron diffraction and X-ray absorption spectroscopy confirm that all of the crystallites are calcite, and metastable aragonite or amorphous calcium carbonate (ACC) phases are not observed. The complex morphology of the leading edge of the base plate suggests that crystallization initiates with the emplacement of mutually aligned fine-grained calcite, followed by the accumulation of coarser grains above and nucleation of highly oriented nanocrystalline grains below.

Chemistry-specific surface adsorption of the barnacle settlement-inducing protein complex

Gregarious settlement in barnacle larvae (cyprids) is induced by a contact pheromone, the settlement-inducing protein complex (SIPC). The SIPC has been identified both in the cuticle of adult barnacles and in the temporary adhesive secretion (footprint) of cyprids. Besides acting as a settlement inducer, the presence of the SIPC in footprints points to its additional involvement in the adhesion process. SIPC adsorption behaviour was therefore investigated on a series of self-assembled monolayers (SAMs) by surface plasmon resonance at the pH of seawater (8.3). Fibrinogen and α2-macroglobulin (A2M) (blood complement protease inhibitors with which the SIPC shares 29% sequence homology) were used in the adsorption experiments as positive and negative standards, respectively. The mass uptake of the SIPC was comparable to that of fibrinogen, with adsorption observed even on the protein-resistant oligo(ethylene glycol) surface. Notably, on the positively charged SAM the SIPC showed a kinetic overshoot, indicating a metastable configuration causing the amount of adsorbed protein to temporarily exceed its equilibrium value. A2M adsorption was low or negligible on all SAMs tested, except for the positively charged surface, indicating that A2M adsorption is mainly driven by electrostatics. Evaluation of SIPC non-specific adsorption kinetics revealed that it adsorbed irreversibly and non-cooperatively on all surfaces tested.

The chemistry of stalked barnacle adhesive (Lepas anatifera)

The results of the first chemical analysis of the adhesive of Lepas anatifera, a stalked barnacle, are presented. A variety of elements were identified in scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) of the adhesive, including Na, Mg, Ca, Cl, S, Al, Si, K and Fe; however, protein-metal interactions were not detected in Raman spectra of the adhesive. Elemental signatures from SEM-EDS of L. anatifera adhesive glands were less varied. Phosphorous was mostly absent in adhesive samples; supporting previous studies showing that phosphoserines do not play a significant role in adult barnacle adhesion. Disulfide bridges arising from Cys dimers were also investigated; Raman analysis showed weak evidence for S-S bonds in L. anatifera. In addition, there was no calcium carbonate signal in the attenuated total reflectance Fourier transform infrared spectra of L. anatifera adhesive, unlike several previous studies in other barnacle species. Significant differences were observed between the Raman spectra of L. anatifera and Balanus crenatus; these and a range of Raman peaks in the L. anatifera adhesive are discussed. Polysaccharide was detected in L. anatifera adhesive but the significance of this awaits further experiments. The results demonstrate some of the diversity within barnacle species in the chemistry of their adhesives.

Affinity states of biocides determine bioavailability and release rates in marine paints

A challenge for the next generation marine antifouling (AF) paints is to deliver minimum amounts of biocides to the environment. The candidate AF compound medetomidine is here shown to be released at very low concentrations, ie ng ml(-1) day(-1). Moreover, the release rate of medetomidine differs substantially depending on the formulation of the paint, while inhibition of barnacle settlement is independent of release to the ambient water, ie the paint with the lowest release rate was the most effective in impeding barnacle colonisation. This highlights the critical role of chemical interactions between biocide, paint carrier and the solid/aqueous interface for release rate and AF performance. The results are discussed in the light of differential affinity states of the biocide, predicting AF activity in terms of a high surface affinity and preserved bioavailability. This may offer a general framework for the design of low-release paint systems using biocides for protection against biofouling on marine surfaces.

The quick extraction of chitin from an epizoic crustacean species (Chelonibia patula)

Chitin was isolated from the shells of Chelonibia patula (barnacle, Crustacea), which lives on blue crab epizoically, following a 10-min demineralisation process through HCl and a 20-min deproteinisation process through NaOH. Due to the low-crystalline structure, and mineral-rich and low-protein content of the shells, chitin isolation was convenient. It was observed that the shell structure of C. patula contains 3.11% chitin per its dry weight. Following characterisation of the isolated chitin by using Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffractometry, elemental analysis and scanning electron microscopy, it was determined that there was close similarity with the α-chitin isolated from crabs, shrimps and insects in various studies. It was observed that chitin was composed of nanofibres with a width of 10-20 nm. It was concluded that this was an economically advantageous chitin resource compared with crustaceans such as shrimp, crayfish and crab, because it is possible to isolate chitin in a significantly shorter time.

3D imaging provides a high-resolution, volumetric approach for analyzing biofouling

A volumetric approach for determining the fouling burden on surfaces is presented, consisting of a 3D camera imaging system with fine (5 μm) resolution. Panels immersed in an estuary on the southwest coast of Florida, USA were imaged and the data were used to quantify seasonal changes in the biofouling community. Test panels, which were submerged in seawater for up to one year, were analyzed before and after gentle scrubbing to quantify the biovolume of the total fouling community (ie soft and hard organisms) and the hard fouling community. Total biofouling ranged from 0.01 to 1.16 cm(3) cm(-2) throughout the immersion period; soft fouling constituted 22-87% of the total biovolume. In the future, this approach may be used to inform numerical models of fluid-surface interfaces and to evaluate, with high resolution, the morphology of fouling organisms in response to antifouling technologies.

Homogeneous nuclear background for mitochondrial cline in northern range of Notochthamalus scabrosus

A mitochondrial cline along the Chilean coast in the barnacle Notochthamalus scabrosus suggests a species history of transient allopatry and secondary contact. However, previous studies of nuclear sequence divergence suggested population genetic homogeneity across northern and central Chile. Here, we collect single-nucleotide polymorphism data from pooled population samples sequenced with restriction site−associated DNA sequencing procedures, confirm these data with the use of a GoldenGate array, and identify a discordance between population genetic patterns in the nuclear and mitochondrial genomes. This discordance was noted in previous work on this species, but here it is confirmed that the nuclear genome exhibits only slight phylogeographic variation across 3000 km of coastline, in the presence of a strong and statistically significant mitochondrial cline. There are nevertheless markers (approximately 5% of nuclear single-nucleotide polymorphisms) exhibiting cytonuclear disequilibrium relative to mitotype. Although these data confirm our previous explorations of this species, it is likely that some of the nuclear genomic diversity of this species has yet to be explored, as comparison with other barnacle phylogeography studies suggest that a divergence of similar magnitude should be found in the nuclear genome somewhere else in the species range.

Growth and development of the barnacle Amphibalanus amphitrite: time and spatially resolved structure and chemistry of the base plate

The radial growth and advancement of the adhesive interface to the substratum of many species of acorn barnacles occurs underwater and beneath an opaque, calcified shell. Here, the time-dependent growth processes involving various autofluorescent materials within the interface of live barnacles are imaged for the first time using 3D time-lapse confocal microscopy. Key features of the interface development in the striped barnacle, Amphibalanus (= Balanus) amphitrite were resolved in situ and include advancement of the barnacle/substratum interface, epicuticle membrane development, protein secretion, and calcification. Microscopic and spectroscopic techniques provide ex situ material identification of regions imaged by confocal microscopy. In situ and ex situ analysis of the interface support the hypothesis that barnacle interface development is a complex process coupling sequential, timed secretory events and morphological changes. This results in a multi-layered interface that concomitantly fulfills the roles of strongly adhering to a substratum while permitting continuous molting and radial growth at the periphery.

Confocal microscopy-based goniometry of barnacle cyprid permanent adhesive

Biological adhesives are materials of particular interest in the fields of bio-inspired technology and antifouling research. The adhesive of adult barnacles has received much attention over the years; however, the permanent adhesive of the cyprid - the colonisation stage of barnacles - is a material about which very little is presently known. We applied confocal laser-scanning microscopy to the measurement of contact angles between the permanent adhesive of barnacle cyprid larvae and self-assembled monolayers of OH- and CH3-terminated thiols. Measurement of contact angles between actual bioadhesives and surfaces has never previously been achieved and the data may provide insight into the physicochemical properties and mechanism of action of these functional materials. The adhesive is a dual-phase system post-secretion, with the behaviour of the components governed separately by the surface chemistry. The findings imply that the cyprid permanent adhesion process is more complex than previously thought, necessitating broad re-evaluation of the system. Improved understanding will have significant implications for the production of barnacle-resistant coatings as well as development of bio-inspired glues for niche applications.

Characterization of terpenoids from the root of Ceriops tagal with antifouling activity

One new dimeric diterpenoid, 8(14)-enyl-pimar-2'(3')-en-4'(18')-en-15'(16')-endolabr- 16,15,2',3'-oxoan-16-one (1) and five known terpenoids: Tagalsin C (2), Tagalsin I (3), lup-20(29)-ene-3β,28-diol (4), 3-oxolup-20(29)-en-28-oic acid (5) and 28-hydroxylup- 20(29)-en-3-one (6) were isolated from the roots of the mangrove plant Ceriops tagal. Their structures and relative stereochemistry were elucidated by means of extensive NMR, IR and MS analysis. The antifouling activity against larval settlement of the barnacle Balanus albicostatus were evaluated using capsaicin as a positive control. All these terpenoids exhibited antifouling activity against cyprid larvae of the barnacle without significant toxicity. The structure-activity relationship results demonstrated that the order of antifouling activity was diterpenoid (Compound 2) > triterpenoid (Compounds 4, 5 and 6) > dimeric diterpenoid (Compounds 1 and 3). The functional groups on the C-28 position of lupane triterpenoid significantly affect the antifouling activity. The diterpenoid dimmer with two identical diterpenoid subunits might display more potent antifouling activity than one with two different diterpenoid subunits. The stability test showed that Compounds 2, 4, 5 and 6 remained stable over 2-month exposure under filtered seawater.

Uptake of precursor and synthesis of transmitter in a histaminergic photoreceptor

As a first step in understanding how the supply of the neurotransmitter histamine is maintained in a photoreceptor, we followed the uptake and metabolism of the immediate precursor of histamine, histidine. [3H]Histidine taken up into photoreceptors and glia was detected using autoradiography, and synthesis of [3H]histamine from [3H]histidine was assayed with thin-layer chromatography. Photoreceptors from barnacles were pulsed (15 min) with [3H]histidine (0.2-200 microM), then maintained in normal saline for up to 24 hr. Autoradiography showed that photoreceptor somata, axons, and presynaptic arbors were labeled, but only weakly, like (nonhistaminergic) ganglion cells. Label instead was concentrated over surrounding glia. Stimulating preparations with light did not increase photoreceptor labeling. Grain counts from photoreceptor axons showed uptake of [3H]histidine into these neurons by a Na+-dependent mechanism with a Km of approximately 50 microM. Over 24 hr only 1% of the [3H]histidine taken up by preparations was converted to [3H]histamine either in the dark or in the light. Injections of [3H]histidine directly into photoreceptors established that synthesis takes place within the photoreceptors and confirmed that stimulation with light did not measurably affect the rate of conversion of [3H]histidine to [3H]histamine. These results suggest that de novo synthesis of transmitter is unlikely to be as important as its reuptake in maintaining neurotransmitter supply in these photoreceptor terminals. In support of this conclusion, photoreceptors accumulated more label when transmitter release was stimulated with high K+ and histamine uptake was antagonized with chlorpromazine.

Selective, activity-dependent uptake of histamine into an arthropod photoreceptor

The synapses made by many arthropod photoreceptors are disinhibitory and use histamine as their transmitter. Because decreases and not increases in the cleft concentration of transmitter constitute the important event at these synapses, a transporter to clear the cleft of histamine would seem particularly crucial to signal transfer. We report here that 3H-histamine is taken up selectively into barnacle photoreceptors by a Na+-dependent mechanism, presumably a transporter. Using light microscopic autoradiography, we observe heavy label over axons and presynaptic terminals of these neurons when they are stimulated during uptake. The radioactivity taken up was identified as 3H-histamine by thin layer chromatography; no metabolites were detected, even after 5 hr. Radiolabeled 5-hydroxytryptamine and GABA are not taken up by the photoreceptor. 3H-histamine uptake into photoreceptors is decreased markedly by an excess of unlabeled histamine and by chlorpromazine and phenoxybenzamine. Unexpectedly for uptake dependent on the NA+ gradient, photoreceptor terminals label more intensely in the light (when depolarized) than in the dark (when hyperpolarized). Glia label more strongly than photoreceptors in dark-incubated preparations. The presence of presynaptic uptake strengthens the evidence that histamine is the neurotransmitter of arthropod photoreceptors and provides a mechanism by which this synapse could recycle transmitter, control its steady-state cleft concentration, and clear it from the cleft in response to decreases in its release from the photoreceptors.