scientific correspondence that bacterial lysis in the stomach could have fewer parasites) is essential for testing gnathiids is probably due to cleaner fish. release L1-derived cecropin-like peptides such models. Similar quantitative studies, in which the that are active against faster-growing I compared the number of parasitic cleaner fish were removed for periods from microorganisms found there. Our data are gnathiids on caged H. melapterus on reefs one month to two years, found no effect of also consistent with the idea that cecropins with and without cleaner fish. Measure- cleaners on parasite abundance3–5. This may have evolved from an early rpl1 gene in a ments were taken after 12 days at sunset to be due to the different client and cleaner prokaryote that passed from being an intra- examine the long-term effect of cleaner fish, fish species used, the presence of other cellular parasite to a symbiont, ending up as and after 12 and 24 hours, at dawn and sun- unidentified cleaners, movement between an organelle. When the rpl1 gene moved set, respectively, to determine whether the reefs by clients, and spatial or temporal from the organelle to the host nucleus, a observed decline in gnathiid abundance8 variation in parasite loads. duplicated sequence could have begun to over the day was caused by cleaner fish. My results show that cleaner fish have evolve towards a specialized antimicrobial Six cages, each containing three H. an effect both within 12 days and within 12 peptide. melapterus, were placed on each of five hours. Cleaner fish are known to benefit Katrin Pütsep, Carl-Ivar Brändén, small coral reefs adjacent to Lizard Island, from cleaning7, but my data show that they Hans G. Boman, Staffan Normark Great Barrier Reef (reefs 7, 8, 14, 15 and 16 greatly reduce the abundance of gnathiids, Microbiology and Tumour Biology Centre, on map in ref. 4), of which three reefs had which can be deleterious to fish11. This is Karolinska Institutet, Box 280, all cleaner fish, L. dimidiatus, removed. consistent with the hypothesis that cleaning SE-171 77 Stockholm, Sweden Cages were put out at dawn on a different behaviour is mutually beneficial to both e-mail: [email protected] reef, randomly selected, each day, and participants, and paves the way to using

1. Blaser, M. Br. Med. J. 316, 1507–1510 (1998). prawns were fed to fish daily. Fish on reefs 7 a 1. 0 2. Mattson, A., Lönroth, H., Qiuding-Järbrink, M. & and 8 were cleaned regularly after cages had Svennerholm, A.-M. J. Clin. Invest. 102, 51–56 (1998). been on the reefs for 9 and 8 days, respec- 0.8 3. Hultmark, D. et al. EMBO J. 2, 571–576 (1983). tively. In the first experiment, after 12 days, 0.6 4. Boman, H. G. Scand. J. Immunol. 48, 15–25 (1998). 0.4 5. Sahl, H. G. & Bierbaum, G. Annu. Rev. Microbiol. 52, 41–79 the fish were recovered at sunset and 0.2 (1998). gnathiids counted. The effect of cleaner fish 6. Holak, T. A. et al. Biochemistry 27, 7620–7629 (1988). on gnathiid abundance was tested (Fig. 1). 0.0 7. DeLucca, A. J., Jacks, T. J. & Brogden, K. A. Mol. Cell Biochem. In the 24-hour experiment, the above b 1. 0 151, 141–148 (1995). Cleaner fish 8. Suda, Y. et al. J. Biochem. 121, 1129–1133 (1997). fish were returned to the holding tanks 0.8 No cleaner fish 9. Phadnis, S. H. et al. Infect. Immun. 64, 905–912 (1995). until the following sunset, when they were 0.6 placed on the same reef; fish from the first 0.4 experiment that were missing or dead were 0.2 substituted with other fish from the labo- 0.0 Cleaner fish really ratory. Half the cages on each reef were fish per Gnathiids c 1. 0 recovered the following dawn, and the rest 0.8 do clean were recovered the next sunset, and 0.6 gnathiids were counted again. Gnathiid The cleaning of client fish by cleaner fish is abundance was tested for the effects of 0.4 one of the most highly developed inter- cleaner fish and sampling time (Fig. 1). 0.2 0.0 specific communication systems known. The effect of replacing the missing fish on 78141516 But even though it is a seemingly obvious gnathiid abundance was also tested and Reef 1,2 3–5 mutualism , several quantitative studies was not significant (F1,52ǃ1.97, Pǃ0.166). have failed to show any benefit for the Variability in gnathiid abundance between Figure 1 Gnathiids on caged fish on reefs with and clients, leading to the hypothesis that cleaner reefs from dawn to sunset, within a treat- without cleaner fish. Data are least-square means fish are ‘behavioural parasites’ that exploit ment, was found to change signifi- and standard errors for fish sampled at the follow- 6 the sensory system of the clients to obtain cantly (analysis of covariance, F3,19ǃ4.10, ing times: a, at sunset after 12 days (F1,3ǃ17.64, food, rather than to increase the client’s fit- Pǃ0.021), probably because there were Pǃ0.0246); b, at dawn after 12 hours (F1,9ǃ1.80, ness. The cleaner fish Labroides dimidiatus high gnathiid loads at both dawn and sun- Pǃ0.213; reef (treatment), F3,9ǃ2.15, Pǃ0.164.); c, at eats parasitic gnathiid isopods, which set at reef 16, so I explored the effect of sunset after 24 hours (F1,3ǃ11.56, Pǃ0.042; reef decline in number on the client fish Hemi- time on gnathiid abundance by reanalys- (treatment), F3,3ǃ0.11, Pǃ0.950). Variation in gnathiid gymnus melapterus daily between dawn and ing each time separately. abundance in a was tested with a nested analysis 7,8 sunset . I find that the cleaner fish reduces Cleaner fish had a clear effect on the of covariance (ANCOVA) with presence of cleaners parasite abundance, resulting in a 4.5- abundance of parasites. After 12 days and at as the main effect and reefs nested within treat- fold difference within 12 hours, supporting sunset, fish on reefs without cleaner fish ments; the error term testing for an effect of treat- the hypothesis that cleaning behaviour is had on average 3.8 times more gnathiids ment was the type III mean square for reef mutualistic. than fish on reefs with cleaners (Fig. 1a). In (treatment). In the 24-hour experiment, a multifactor Various models have been proposed to the 24-hour experiment, gnathiid abun- nested ANCOVA was used; the effect of using explain cooperative interactions among dance did not differ between treatments at replaced fish was tested by including it as a factor; 9 unrelated individuals . The Iterated Prison- dawn (Fig. 1b); in contrast, gnathiids on the error term used to test the main effects and er’s Dilemma is the generally accepted reefs without cleaner fish and sampled at interactions was the type III means square for 10 2 model , although it has been criticized . sunset had 4.5 times more gnathiids than cage (treatmentǂtimeǂreef). Because there was a The behaviour of cleaner fish has been used fish on reefs with cleaner fish (Fig. 1c). significant interaction of timeǂreef (treatment), sep- to highlight the limitations of repeated Cleaner fish eat 1,200 parasites per day arate ANCOVAs were used for each time with fish games, and may be useful for developing (mostly gnathiids) and feed only during the size as the covariate. Gnathiid abundance and fish 7 alternative models, because if the client day , whereas gnathiids infest fish during size were log10 (x+1) and log10 (x) transformed, 8 cheats by eating the cleaner, the game is both day and night . The rapid reduction in respectively. Least-square means were calculated to 2 over, so it is not a repeated game . Under- gnathiid abundance between dawn and show gnathiid abundance while accounting for fish standing the benefits of cleaning (clients sunset indicates that the daily decline in size.

672 © 1999 Macmillan Magazines Ltd NATURE | VOL 398 | 22 APRIL 1999 | www.nature.com scientific correspondence cleaning behaviour to test models of non- evidence for loudness enhancement (or 4. Schlauch, R. S. J. Acoust. Soc. Am. 92, 758–765 (1992). kin cooperation. “upcruitment”) at the end of the sweep. 5. Teghtsoonian, R., Teghtsoonian, M. & Canévet, G. in Fechner Day 95 (ed. Possamaï, C.-A.) 309–314 (International Society Alexandra S. Grutter However, the effect, where it occurs at all, is for Psychophysics, Cassis, 1995). Department of Microbiology and Parasitology, much smaller than the accelerated softening 6. Teghtsoonian, R., Teghtsoonian, M. & Canevet, G. Percept. University of Queensland, in downsweeps. It should be noted that the Psychophys. (in the press). Brisbane, Queensland 4072, Australia earlier work concerned sweep durations e-mail: [email protected] much longer than the 1.8 seconds used by Neuhoff replies — Canévet and his col- 5,6 1. Trivers, R. L. Q. Rev. Biol. 46, 35–57 (1971). Neuhoff. A more recent study has shown leagues suggest that their findings address 2. Hammerstein, P. & Hoekstra, R. F. Nature 376, 121–122 (1995). that, although decruitment is diminished dynamic loudness change and are inconsis- 3. Losey, G. S. Symbiosis 4, 229–258 (1987). when duration is as short as one second, a tent with my recent discovery of a bias for 4. Grutter, A. S. J. Exp. Mar. Biol. Ecol. 196, 285–298 (1996). 1 5. Grutter, A. S. Oecologia 111, 137–143 (1997). 30-decibel downsweep for 1- and 2.5-second rising intensity tones . However, the two 6. Losey, G. S. Anim. Behav. 27, 669–685 (1979). durations covers a bigger range of loudness sets of experiments address fundamentally 7. Grutter, A. S. Mar. Biol. Prog. Ser. 130, 61–70 (1996). than the comparable upsweep. different questions. Canévet et al.’s listeners 8. Grutter, A. S. Mar. Biol. (in the press). 9. Clements, K. C. & Stephens, D. W. Anim. Behav. 50, 527–535 It would therefore be prudent to limit were asked to make judgements about loud- (1995). Neuhoff’s conclusions to his own test con- ness, whereas my listeners were asked to 10.Dugatkin, L. A. Cooperation Among Animals (Oxford Univ. ditions and method of measuring loudness make judgements about the amount of Press, 1997). 11.Paperna, I. & Por, F. D. Rapp. Comm. Int. Mer. Medit. 24, change: the larger loudness change recorded dynamic change. Essentially, their listeners 195–197 (1977). earlier for downsweeps under a range of answered the question “How loud is it conditions and with a variety of measure- now?” by assigning a number to the loud- ment techniques contrasts with Neuhoff’s ness of a changing intensity sound at vari- finding based on a single duration, a single ous times throughout the stimulus Perception of changes intensity range, and a single measurement duration. This provided a discrete measure procedure. of loudness at various snapshots in time. in loudness How can this apparent conflict be Listeners in my experiments were specifical- explained? Perhaps, as Neuhoff argues, ly asked to ignore the overall loudness of Neuhoff1 reported that “rising level tones… direct judgement of “perceived change” taps the sounds and to make summary judge- change (in loudness) more than falling level a fundamentally different process from ments about the amount of loudness tones despite having the same actual change judgements of loudness obtained at the change, essentially answering the question in level… indicating that direction of beginning and end of a sweep. But listeners “How much did it change in loudness?”. change is an important (and previously have been known to make their own deci- From static judgements of loudness at unaddressed) factor in the perception of sions about what to attend to in an experi- discrete points in time, Canévet and his col- dynamic loudness change”, and speculated mental setting: although Neuhoff cautioned leagues infer the perceived amount of that: “In a natural environment this over- his subjects to avoid making a judgement of dynamic loudness change. But they do not estimation could provide a selective advan- overall loudness, they may have done so; measure perceived dynamic change directly. tage, because rising intensity can signal the direction of change may have an effect There can be inherent shortcomings in movement of the source towards an organ- on such judgements that is quite different inferring characteristics of dynamic percep- ism.” Leaving aside the question of why it from its effect on loudness judgements tion by extrapolating from static judge- may not be as important for survival to made at discrete stages of a sweep. Close ments. Their experiments are important detect the movement of a sound source attention should also be paid to procedural and have implications in areas from physi- away from an organism, we dispute the differences that may turn out to be decisive. ology to auditory display. However, once assertion that there is no prior evidence Neuhoff’s results are of great interest the difference between the perception of about the influence of direction of change but, in our view, they need to be considered loudness and the perception of dynamic on the degree of change in perceived loud- in the context of what is already known loudness change is made clear, perhaps the ness. This evidence does exist and shows, in about loudness perception for signals that discrepancy between our results is not so contrast to the result reported by Neuhoff1, are continuously changing in intensity. It unexpected. that declining signal intensity covers a remains to be seen whether his finding is of Judgements of dynamic change may be greater range of loudness than does rising sufficient generality to warrant speculation mediated by different mechanisms from signal intensity. about its role in the process of evolutionary static judgements of loudness. Because Twelve years earlier it was reported2 that selection. intensity change is the important factor in tones continuously decreasing in intensity Georges Canévet*, Bertram Scharf†, specifying the arrival time of a source2, a from moderate to very low levels follow a Robert S. Schlauch‡, Martha Teghtsoonian§, direct judgement about the amount of course of accelerated “softening” until the Robert Teghtsoonian§ change might be more useful for localizing end of the downsweep, when the loudness is *Laboratoire de Mécanique et d’Acoustique, CNRS, a source than a series of snapshot judge- very much less than it would be if that final 13402 Marseille, France ments of loudness. A bias for rising inten- intensity had been presented alone. The †Auditory Perception Laboratory, sity when judging loudness change is underlying mechanism for this phenome- Northeastern University, consistent with studies that show that lis- non, which was subsequently named Boston, Massachusetts 02115, USA teners systematically err on the side of “decruitment”3, is unknown, but arguments ‡Department of Communication Disorders, safety when estimating the arrival time have been made for both central and University of Minnesota, of a sound source3,4. The bias for rising peripheral factors. For example, it was later Minneapolis, Minnesota 55455, USA intensity may therefore provide a selective shown4 that at least some part of the effect §Department of Psychology, Smith College, advantage. occurs at the receptor site, as a test in the Northampton, Massachusetts 01063, USA Canévet et al. suggest that their findings contralateral ear at the end of the down- e-mail: [email protected] undermine this claim. However, keeping in sweep showed little or no decruitment. mind the differences between loudness and 1. Neuhoff, J. Nature 395, 123–124 (1998). In these earlier studies, the loudness of 2. Canévet, G. Acustica 61, 256–264 (1986). loudness change, their work is not necessar- sounds sweeping up, instead of down, over 3. Canévet, G. & Scharf, B. J. Acoust. Soc. Am. 88, 2136–2142 ily inconsistent with the evolutionary posi- the same ranges of intensity yielded some (1990). tion. They have shown for tones that

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