The persistence of most coastal marine species depends on larvae finding suitable adult habitat at the end of an offshore dispersive stage that can last weeks or months. We tested the effects that ocean acidification from elevated levels of atmospheric carbon dioxide (CO2) could have on the ability of larvae to detect olfactory cues from adult habitats. Larval clownfish reared in control seawater (pH 8.15) discriminated between a range of cues that could help them locate reef habitat and suitable settlement sites. This discriminatory ability was disrupted when larvae were reared in conditions simulating CO2-induced ocean acidification. Larvae became strongly attracted to olfactory stimuli they normally avoided when reared at levels of ocean pH that could occur ca. 2100 (pH 7.8) and they no longer responded to any olfactory cues when reared at pH levels (pH 7.6) that might be attained later next century on a business-as-usual carbon-dioxide emissions trajectory. If acidification continues unabated, the impairment of sensory ability will reduce population sustainability of many marine species, with potentially profound consequences for marine diversity.climate change ͉ larval sensory mechanisms ͉ population connectivity ͉ population replenishment O cean acidification caused by the uptake of additional carbon dioxide (CO 2 ) at the ocean surface is now recognized as a serious threat to marine ecosystems (1-4). At least 30% of the anthropogenic CO 2 released into the atmosphere in the past 200 years has been absorbed by the oceans, causing ocean pH to decline at a rate Ϸ100 times faster than at any time in the past 650,000 years (1, 4). Global ocean pH is estimated to have dropped by 0.1 units since preindustrial times and is projected to fall another 0.3-0.4 units by 2100 because of existing and future CO 2 emissions (1, 5-6). Considerable research effort has focused on predicting the impact that reduced carbonate-ion saturation states that accompany ocean acidification will have on calcifying marine organisms, particularly corals and other invertebrates that precipitate aragonite skeletons (2-3, 6). However, the effects that ocean acidification will have on other marine organisms, including fishes, remain almost completely unknown, especially for conditions of atmospheric carbon dioxide and seawater pH that could occur in the near future (4, 7-9).The persistence of most coastal marine species depends on the ability of larvae to locate suitable settlement habitat at the end of a pelagic stage that can last weeks or months. Accumulating evidence for reef fishes suggests that both reef sounds (10) and olfactory cues (11-13) are used by larvae to locate reefs. The olfactory organs of many reef fishes are well-developed by the end of the larval phase (14-15), and it has recently been shown that larvae of some species can discriminate the smell of water from their natal reef compared with water from other reefs (13), which provides a mechanism to explain high levels of selfrecruitment in some reef fish populations (16)(...

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