Observations on the settlement of Phallusia mammillata larvae: effects of different lithological substrata

In the sessile marine tunicates, the selection of a suitable substratum by the larvae is an important and critical factor determining the distribution of species. The present paper investigated, under laboratory conditions, the role played by mineral content of the substratum in settlement of the ascidian Phallusia mammillata, using siliceous and carbonaceous stones. Individuals that attached to the different substrata and metamorphosed were scored. The data indicate that larvae could discriminate between the substrata on the basis of their silica content. Under the same laboratory conditions, the larvae that attached to siliceous stones grew faster and had a wider area of contact with the substratum than those that grew on carbonaceous stones. The present study concludes that silica is a mineral factor that can be discriminated by chemosensory palps of ascidian larvae during the choice of substratum. It is suggested that the mineral composition of the habitat can contribute, with other environmental factors, to regulate the spatial distribution of tunicate communities.