Wettability and Substrate Selection in the Larval Settlement of the Solitary Ascidian Phallusia philippinensis (Phlebobranchia: Ascidiidae)

For marine benthic animals, the selection of substrate by larvae is important for their survival, with early post-settlement mortality often being affected by the microenvironment where they settle. We tested the substrate preference of the larvae of the ascidian Phallusia philippinensis toward nine commercially available substrates. In the present assay, the larvae settled on one of four substrates for seven substrate combinations with different wettabilities; we counted the number of settled larvae on each of the four substrates, and Manly's selection indices were compared to determine the preference rank of each substrate. Larvae significantly preferred more hydrophobic and oleophilic substrates to hydrophilic and oleophobic ones. While it is uncertain how the larvae detect the properties of the substrate surface, they might be able to sense the physical force, such as stickiness and repellent force. Although a hydrophobic surface is not common in a natural marine environment, the use of hydrophobic materials (as flypaper-like tools) for ascidian larvae might help to prevent the settlement of non-indigenous ascidians in aquaculture facilities.