Light induced larval release of a colonial ascidian

Larval release and photobehavior were studied in the colonial ascidian Poly androcarpa zorritensis. The test hypothesis was that if larval release is i nduced by light, then larvae should be attracted to settlement areas where light is sufficient for larval release. Light induced larval release but th e time course varied with light intensity. As the intensity of either sunli ght or blue-green light decreased (1) the time until the beginning of larva l release (latency) became longer, (2) the mean time of larval release incr eased, and (3) the time interval over which larvae were released increased. The threshold light intensity to induce larval release in blue-green Light (8.75 x 10(12) photons cm(-2) s(-1)) was lower than that in sunlight (3.6 x 10(13) photons cm(-2) s(-1)). Light induced Larval release was not affect ed by currents up to 15 cm s(-1). Larvae aggregate in light when given a ch oice between light and dark. This response did not vary with larval age. Th e lowest light intensity, at which larvae could distinguish between light a nd dark was 5.0 x 10(12) photons cm in blue-green light and 2.9 x 10(14) ph otons cm(-2) s(-1) in sunlight. Thus, the hypothesis is supported because l arvae are attracted to areas where light intensity is sufficient for larval release. (C) 2000 Elsevier Science B.V. All rights reserved.