Reference: Biol. Bull. 196: 94-104. (February, 1999) Ultraviolet Radiation and Distribution of the Solitary Ascidian Core&z ivtflata (Huntsman) BRIAN L. BINGHAM’.” AND NATHALIE B. REYNS2 ’ Huxley College of Environmental Studies, Western Washington University, Bellingham, Washington 98225; and 2 Marine Sciences Research Center, State University of New York, Stony Brook, New York 1 I794 Abstract. The solitary ascidian Corella injutu is a com- mon fouling organism in many areas of Puget Sound and the San Juan Archipelago, Washington, USA. Despite its abun- dance, it is conspicuously absent from areas that receive direct sunlight. Previous work suggests that ascidians in unshaded habitats can be overgrown and killed by algal overgrowth. In this study, we tested the hypothesis that UV irradiation contributes to C. inflatu distribution by killing individuals exposed to direct sunlight. To test this, we exposed C. infuta embryos, larvae, juveniles, and adults to UV irradiation and measured the responses. We also tested for UV-absorbing compounds in larvae, juveniles, and adults. In the laboratory, UV significantly damaged all life stages; the earliest stages were most vulnerable. A 3-week UV exposure significantly shortened adult life span. Juve- niles suffered 100% mortality after only 3 days. Tadpole larvae decreased settlement and metamorphosis after I day of UV exposure, and embryos exhibited developmental abnormalities after only 30 minutes of exposure. None of the life-history stages had apparent UV-absorbing com- pounds. Given the vulnerability of this species to UV, we suggest that its unique life-history traits (i.e., time of spawn- ing, brooding behavior, length of larval life) help it persist in its preferred habitat and avoid dispersal into inappropri- ate, UV-exposed areas. Introduction Corella inflata (Huntsman) is a solitary ascidian common throughout Puget Sound, Washington, and in waters off the west coast of British Columbia. It occurs from the intertidal zone to 45 m (Van Name, 1945) but is most abundant on Received 26 November 1997; accepted 6 November 1998. * To whom correspondence should be addressed. E-mail: bingham@ cc.wwu.edu docks and pilings (Young, 1982). Adults, which may reach 5 cm in length, have a thin, transparent outer tunic. This is in marked contrast to the tough, opaque tunic that protects most other solitary ascidians. Lambert (1968) studied a population of C. inflatu in a Puget Sound marina for 12 months and observed mass mortality in the early spring. The mortality coincided with a period of heavy diatom growth, and Lambert suggested that smothering diatom mats were responsible for the ascidian deaths. This conclusion was supported by observations that mass mortality occurred only in areas exposed to full solar radiation; C. inflatu in shaded habitats survived. Recent research suggests that another factor may contrib- ute to mortality of C. injlatu in exposed areas. Increasing interest in the status of the stratospheric ozone layer has led to intense study of the deleterious effects of ultraviolet radiation (UV). Jokiel (1980) first demonstrated the damag- ing effects of UV radiation on tropical marine invertebrates (including ascidians). More recent work has shown that the UV-B portion of the spectrum (280-320 nm) is particularly lethal to marine bacteria, plankton, invertebrates, and fish (reviewed by Worrest, 1982; Hardy and Gucinski, 1989; see also Shick et al., 199 1; Karentz et al., I99 1; Karentz, 1994a, b). There are indications that marine invertebrate embryos and larvae may be particularly sensitive to solar radiation (Damkaer et al., 1980; Pennington and Emlet, 1986; Bier- mann et al., 1992; Adams and Shick, 1996). The habitat (primarily shallow water) and anatomy (thin, transparent tunic) of C. inflata may make it particularly vulnerable to UV damage, and the effects may not be limited to adult animals. Unlike most solitary ascidians, C. inflata holds its embryos in a spacious brood chamber. The eggs are buoyant and float to the top of the chamber where there is potential for UV-induced damage during develop- ment. 94