Ocean acidification impairs olfactory discrimination and homing ability of a marine fish Philip L. Mundaya,b,1, Danielle L. Dixsona,b, Jennifer M. Donelsona,b, Geoffrey P. Jonesa,b, Morgan S. Pratchetta, Galina V. Devitsinac, and Kjell B. Døvingd aAustralian Research Council Centre of Excellence for Coral Reef Studies, bSchool of Marine and Tropical Biology, James Cook University, Townsville, QLD 4811, Australia; cIchthyology Department, Faculty of Biology, Moscow MV Lomonosov State University, Moscow 119992, Russia; and dPhysiology Program, Institute of Molecular Bioscience, University of Oslo, N-0316 Oslo, Norway Edited by David M. Karl, University of Hawaii, Honolulu, HI, and approved December 29, 2008 (received for review October 6, 2008) The persistence of most coastal marine species depends on larvae olfactory signals from preferred microhabitats (12, 20), resident finding suitable adult habitat at the end of an offshore dispersive conspecifics (21–23), or symbiotic partners such as anemones stage that can last weeks or months. We tested the effects that (24–25). Any disruption to the ability of larvae to detect and ocean acidification from elevated levels of atmospheric carbon discriminate between olfactory cues that guide them to reefs, or that dioxide (CO2) could have on the ability of larvae to detect olfactory enable them to select preferred settlement habitat, would have cues from adult habitats. Larval clownfish reared in control sea- far-reaching implications for the sustainability of adult populations. water (pH 8.15) discriminated between a range of cues that could We tested if elevated CO2 and reduced seawater pH consistent help them locate reef habitat and suitable settlement sites. This with ocean acidification predictions could affect the ability of discriminatory ability was disrupted when larvae were reared in orange clownfish (Amphiprion percula; Pomacentridae. Fig. 1) conditions simulating CO2-induced ocean acidification. Larvae be- larvae to respond to olfactory cues that are used to locate reef came strongly attracted to olfactory stimuli they normally avoided habitat and distinguish preferred settlement sites. Specifically, when reared at levels of ocean pH that could occur ca. 2100 (pH 7.8) we tested the ability of settlement-stage larvae to respond to and they no longer responded to any olfactory cues when reared olfactory cues that are preferred during the settlement process at pH levels (pH 7.6) that might be attained later next century on compared with olfactory cues that are likely to be avoided when a business-as-usual carbon-dioxide emissions trajectory. If acidifi- searching for reefs and settlement sites. Orange clownfish mostly cation continues unabated, the impairment of sensory ability will live on oceanic reefs surrounding vegetated islands and recent reduce population sustainability of many marine species, with research has shown that the larvae can discriminate between potentially profound consequences for marine diversity. seawater from reefs surrounding vegetated islands versus sea- water from reefs without islands (26). Furthermore, the larvae climate change ͉ larval sensory mechanisms ͉ population connectivity ͉ are positively attracted to water-borne cues from tropical rain- population replenishment forest trees (26) that should provide a reliable cue to the presence of vegetated oceanic islands. We tested the response of cean acidification caused by the uptake of additional car- larval clownfish to olfactory cues from a range of tropical vegetation types when reared in seawater simulating 2 future Obon dioxide (CO2) at the ocean surface is now recognized as a serious threat to marine ecosystems (1–4). At least 30% of CO2-induced acidification scenarios (seawater pH 7.8 and 7.6) compared with current-day controls (pH 8.15). For larvae reared the anthropogenic CO2 released into the atmosphere in the past 200 years has been absorbed by the oceans, causing ocean pH to in each treatment we tested preference or avoidance of olfactory decline at a rate Ϸ100 times faster than at any time in the past cues from the leaves of 3 vegetation types: (i) a tropical rainforest 650,000 years (1, 4). Global ocean pH is estimated to have tree (Xanthostemon chrysanthus) that is a positive cue for settling dropped by 0.1 units since preindustrial times and is projected to clownfish (26), (ii) a swamp tree (Melaleuca nervosa) that fall another 0.3–0.4 units by 2100 because of existing and future contains pungent oils in the leaves and is avoided by settling clownfish (26), and (iii) a tropical savannah grass (Megathyrsus CO2 emissions (1, 5–6). Considerable research effort has focused on predicting the impact that reduced carbonate-ion saturation maximus) that is not expected to provide a reliable cue for the states that accompany ocean acidification will have on calcifying presence of trees on islands. marine organisms, particularly corals and other invertebrates It is well known that anemonefishes are positively attracted to that precipitate aragonite skeletons (2–3, 6). However, the olfactory cues of host anemones (24–25). Therefore, we also effects that ocean acidification will have on other marine or- tested the ability of larval clownfishes to respond to anemone ganisms, including fishes, remain almost completely unknown, olfactory cues when reared at the 3 pH levels. Finally, the especially for conditions of atmospheric carbon dioxide and presence of adult populations should be a good signal of seawater pH that could occur in the near future (4, 7–9). favorable habitat and previous studies have found that larvae of The persistence of most coastal marine species depends on the some reef fishes are attracted to olfactory cues from conspecific adults (21–22). However, in species such as the orange clownfish ability of larvae to locate suitable settlement habitat at the end where many larvae recruit to natal reefs (19), it would also be of a pelagic stage that can last weeks or months. Accumulating advantageous for juveniles to be able to discriminate between evidence for reef fishes suggests that both reef sounds (10) and their parents and other adults to avoid inbreeding that could result 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 Author contributions: P.L.M., D.L.D., J.M.D., G.P.J., and M.S.P. designed research; P.L.M., that larvae of some species can discriminate the smell of water D.L.D., J.M.D., and G.V.D. performed research; K.B.D. contributed new reagents/analytic from their natal reef compared with water from other reefs (13), tools; P.L.M. analyzed data; and P.L.M., D.L.D., J.M.D., G.P.J., M.S.P., G.V.D., and K.B.D. which provides a mechanism to explain high levels of self- wrote the paper. recruitment in some reef fish populations (16–19). It is well The authors declare no conflict of interest. known that coral reef fish larvae can use olfactory cues to This article is a PNAS Direct Submission. identify suitable settlement sites once they are in the vicinity of 1To whom correspondence should be addressed. E-mail: [email protected]. reef habitat. Settling larvae have been shown to respond to © 2009 by The National Academy of Sciences of the USA 1848–1852 ͉ PNAS ͉ February 10, 2009 ͉ vol. 106 ͉ no. 6 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0809996106 Downloaded by guest on September 29, 2021 10 100 control 26 A 16 B 90 7.8 46 80 46 46 70 60 20 40 20 50 40 30 20 10 Percent time treatment of water in 20 0 Seawater Xanthostemon Melaleuca Grass Anemone Fig. 2. Response of larval clownfish to olfactory cues from tropical plants and anemones when reared at current-day seawater pH (control, open bars) and in seawater, where the pH had been reduced using CO2 to simulate the effect 1cm of ocean acidification (pH 7.8, filled bars). The first pair of columns shows the mean percentage of time that larvae spent of one side of a 2-channel flume chamber when neither stream of water in the chamber contained a test cue. C Subsequent columns show the mean percentage of time that larvae spent in the stream of water containing the cue when one stream contained the cue and the other stream did not. Numbers above bars are the number of repli- cates for each test. stream of seawater in the flume contained an additional olfactory cue (Fig. 2). Larvae exhibited a strong preference for Xanthostemon (Fig. 2, P Ͻ 0.001) spending Ͼ93% of their time in the stream of water in which leaves of this rainforest tree had 10cm been soaked. In contrast, all larvae completely avoiding the stream of water in which Melaleuca leaves had been soaked (Fig. 2, P Ͻ 0.001). Avoidance of Melaleuca might be expected Fig. 1. Clownfish larvae use olfactory cues to locate adult habitat at the end because the leaves of these trees contain pungent oils. Larvae of their pelagic stage. (A) Adult orange clownfish A. percula form breeding showed no preference or avoidance for olfactory cues from pairs on host anemones. (B) A settlement stage (11 days posthatching) larva of grass, spending approximately equal amounts of time in the A. percula beside an Australian 5 cent coin for scale. (C) The Atema flume stream of water in which grass leaves had been soaked and in chamber used to test the responses of larvae to olfactory cues from leaves, the stream of water without olfactory cues from grass (Fig. 2, anemones, and conspecifics in control and acidified water. Water from 2 P Ͼ 0.1). As expected, larvae also exhibited a strong prefer- different sources flowed through the chamber in the direction of the arrows.