Comparative Assessment of Shell Structural, Mechanical, and Elemental Properties in Adult Acorn Barnacles

Balanomorph (acorn) barnacles are found throughout the world’s coastal oceans, and their success is dependent on a hard, mineralized, outer shell. Although macro-scale morphology of barnacle shells has been studied extensively, relatively little is known about shell properties at the micron-scale and if such properties vary among species. We assessed shell structure, mechanics, and composition in seven species of balanomorph barnacles from five genera. Three species, <i>Amphibalanus amphitrite</i>, <i>Amphibalanus improvisus</i>, and <i>Austrominius modestus</i>, were laboratory-reared, enabling direct comparison of shell properties of barnacles grown under the same conditions for the same duration. Four other species, <i>Semibalanus balanoides</i>, <i>Amphibalanus eburneus</i>, <i>Chthamalus stellatus</i>, and <i>Tetraclita rubescens</i>, were field-collected. At the macro- and meso-scales, shell properties varied markedly among species, with differences in the number of shell plates, the presence of canals within the plates, mineralization of the base, and shell plate thickness. At the micron-scale, however, structure was remarkably similar among species. Plates of all species were constructed of irregular micron-scale crystallites, with a broad range of crystallite dimensions observed within the same shell. Similarly, micromechanical properties did not vary among species, regardless of testing orientation. Calcium carbonate was identified as calcite in all species assessed with no other mineral phases present, and calcium content did not vary among species. Hence, despite variation in the overall macro- and meso-scale morphology of barnacles, all appear to be built using the same, evolutionarily conserved, mineralization pathway.