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First published online January 25, 2005
Journal of Experimental Biology 208, 433-438 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01420

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Pigmentation in the sensory organs of the ascidian larva is essential for normal behavior

Di Jiang^1,*, Jason W. Tresser^1,*, Takeo Horie², Motoyuki Tsuda² and William C. Smith¹

¹ Department of Molecular, Cellular and Developmental Biology, University of California, Santa Barbara, California 93106, USA
² Department of Life Science, Graduate School of Life Science, University of Hyogo, 3-2-1 Kouto, Kamigori, Ako-gun, Hyogo 678-1297, Japan

Author for correspondence (e-mail: w_smith{at}lifesci.ucsb.edu)

Accepted 1 December 2004

Free-living animals and their larvae utilize light and gravity as cues to navigate in open space. Detection and response to these environmental stimuli are important for the dispersal and settlement of ascidian larvae. Two pigmented structures in the brain of the ascidian larva, the ocellus and the otolith, have been shown to function as the photoreceptive and gravity sensitive organs, respectively. Here, we show that pigmentation is essential for proper phototactic and geotactic behavior in larvae of the ascidian species Ciona savignyi. Two recessive and complementing mutant lines of C. savignyi, immaculate and spotless, that specifically disrupt the pigmentation of the sensory organs during larval development are described. Homozygous mutant larvae are unable to respond properly to gravity and illumination cues while settling. Genetic analysis shows that spotless is caused by a point mutation within the tyrosinase gene that creates a premature stop codon, while the molecular nature of immaculate is unknown. Although the role of pigmentation in the ocellus of C. savignyi is similar to that in vertebrate visual systems, our results demonstrate a novel use of melanin in geotactic behavior.

Key words: ascidian, pigmentation, melanin, settlement, behavior, ocellus, otolith

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