Response of tropical and subtropical chthamalid barnacles to increasing substrate temperatures

Marine invertebrates inhabiting low-latitude shores are exposed to both extremely hot and highly variable conditions. Further changes in habitat temperature may pose a risk to these populations. In this study, we examined the early life stage response of foundation species from subtropical and tropical latitudes to changes in the thermal habitat. We manipulated the color of settling surfaces for barnacle species that occupy the same ecological niche, Chthamalus bisinuatus (southeastern coast of Brazil), and Chthamalus proteus (northwestern coast of Panama). Using an in situ experimental approach, we assessed: 1) the combined effects of temperature and other abiotic parameters, and 2) the significance of larval and early juvenile traits in determining post-settlement performance for the first three days of benthic life. We found that the biological outcomes differed for the two species, according to the local thermal regime experienced. C. bisinuatus juveniles responded positively to higher temperatures, through a positive interaction between larval energetic condition, post-metamorphic size and temperature on their initial growth. Conversely, C. proteus juveniles grew slower at higher temperatures, although a positive effect of size at settlement was found. Continuous air- and sea-surface temperature measurements revealed that the tropical chthamalids experienced temperatures ≈6 °C higher than their subtropical congeners, as well as harsher thermal conditions during aerial exposure. This suggests that manipulated temperatures likely exceeded the tolerance of the Caribbean population, already living on the edge of their thermal threshold. Our findings contribute to the understanding of the factors underpinning the critical post-settlement events that drive patterns of abundance and distribution of barnacles in low-latitude systems.