Larval behaviour and choice of settlement site: Correlation with environmental distribution pattern in an erect bryozoan

Summary This paper concerns the preferred environmental conditions for settlement and metamorphosis of larvae of the erect, rigidly calcified cheilostome bryozoanSchizotheca serratimargo and compares the settlement preferences with in situ distribution of adult colonies. A review of bryozoan larval behaviour and responses to environmental parameters is given in order better to understand larval settlement and metamorphosis in this species. Release of larvae ofS. serratimargo (Hincks) was induced so that each travelled down a path with a limited set of microenvironments in which it could choose to settle. Four different flow velocities, three different depths below the water surface, two different light intensities, three different orientations of plexiglas surfaces, and subdivision of distance of travel into three segments, yielded 216 microenvironments. 466 larvae successfully settled and metamorphosed within the experimental chamber. They metamorphosed in significantly higher numbers in microenvironments with low flow velocity, in the shallowest of the three possible depths, in darker areas, and on nonoverhanging surfaces with little or no sediment. The most important stimulus for settlement was darkness, especially where there was a strong gradient in light intensity.S. serratimargo grows as erect bilaminate bushes on protected flanks of boulders, in small caves, and just within the entrances of larger caves at depths of 5–30 m, as well as on hard debris on open muddy sand floors at 30–35 m depth in the Adriatic and Mediterranean Seas. The experimentally-determined settlement patterns suggest that recruitment is more important than post-settlement interactions or exposure to physical damage in determining local distribution of the species.