L Journal of Experimental Marine Biology and Ecology 248 (2000) 225–238 www.elsevier.nl / locate / jembe Light induced larval release of a colonial ascidian a, b c * Richard B. Forward Jr. , James M. Welch , Craig M. Young a Duke University Marine Laboratory, School of Environment, 135 Duke Marine Lab Road, Beaufort, NC 28516, USA b Biology Department, Wittenberg University, P .O. Box 720, Springfield, OH 45501-0720, USA c Division of Marine Sciences, Harbor Branch Oceanographic Institution, 5600 US 1 North, Fort Pierce, FL 34946, USA Received 7 October 1999; received in revised form 15 January 2000; accepted 31 January 2000 Abstract Larval release and photobehavior were studied in the colonial ascidian Polyandrocarpa zorritensis. The test hypothesis was that if larval release is induced by light, then larvae should be attracted to settlement areas where light is sufficient for larval release. Light induced larval release but the time course varied with light intensity. As the intensity of either sunlight or blue-green light decreased (1) the time until the beginning of larval release (latency) became longer, (2) the mean time of larval release increased, and (3) the time interval over which larvae were released 12 increased. The threshold light intensity to induce larval release in blue-green light (8.75 3 10 22 21 13 22 21 photons cm s ) was lower than that in sunlight (3.6 3 10 photons cm s ). Light induced 21 larval release was not affected by currents up to 15 cm s . Larvae aggregate in light when given a choice between light and dark. This response did not vary with larval age. The lowest light 12 22 intensity, at which larvae could distinguish between light and dark was 5.0 3 10 photons cm 21 14 22 21 s in blue-green light and 2.9 3 10 photons cm s in sunlight. Thus, the hypothesis is supported because larvae are attracted to areas where light intensity is sufficient for larval release.  2000 Elsevier Science B.V. All rights reserved. Keywords: Ascidian; Behavior; Larvae; Light; Polyandrocarpa zorritensis; Settlement 1. Introduction Colonial ascidians release fully developed larvae upon exposure to light following a period of dark adaptation (Svane and Young, 1989). Actual larval release could result *Corresponding author. Tel.: 1 1-252-504-7610; fax: 1 1-252-504-7648. E-mail address: rforward@duke.edu (R.B. Forward Jr.) 0022-0981 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00168-4