Ascidian Larvae Prefer Hydrophobic Substrates for Settlement

The selection of settlement sites by larvae is a crucial event in the life cycle of sessile animals, as they are usually unable to migrate after settlement and metamorphosis into sessile forms. For example, some species prefer hydrophilic substrates, whereas others prefer hydrophobic substrates. Understanding larval preferences for substrates will provide an essential basis for controlling biofouling. In this study, we focused on the water wettability of substrates for settlement and investigated larval substrate preference in three ascidian species, Phallusia philipinennsis, Ascidia sydneiensis, and Ciona robusta (= C. intestinalis type A), using a four-choice assay with substrates of different water wettability (contact angle of a water droplet): silicone rubber (115°), polyvinyl chloride (80°), glass (31°), and hydrophilic film SH2CLHF (14°). In some regions of the world, A. sydneiensis and C. robusta are often noted as invasive species and are serious biofoulants. In all three species, larvae showed a significant preference for more hydrophobic substrates, and none settled on the most hydrophilic substrate, that is, SH2CLHF. It is difficult to explain why ascidian larvae prefer a more hydrophobic substrate, since hydrophobic materials, such as silicon, are often used as antifouling measures. This inconsistent substrate preference can be applied to mitigate biofouling.