RESEARCH ARTICLE◥

CORAL REEFS

Breakdown in spawning synchrony:A silent threat to coral persistenceTom Shlesinger* and Yossi Loya

The impacts of human and natural disturbances on coral reefs are typicallyquantified through visible damage (e.g., reduced coral coverage as a result ofbleaching events), but changes in environmental conditions may also causedamage in less visible ways. Despite the current paradigm, which suggestsconsistent, highly synchronized spawning events, corals that reproduce bybroadcast spawning are particularly vulnerable because their reproductivephenology is governed by environmental cues. Here, we quantify coral spawningintensity during four annual reproductive seasons, alongside laboratory analysesat the polyp, colony, and population levels, and we demonstrate that, comparedwith historical data, several species from the Red Sea have lost their reproductivesynchrony. Ultimately, such a synchrony breakdown reduces the probabilityof successful fertilization, leading to a dearth of new recruits, which may driveaging populations to extinction.

Many of the coherent ecological impactsof climate change are reflected in shiftsin animal and plant phenology (i.e., thetiming of periodic life cycle events, suchas reproduction, migration, flowering,

etc., mediated by the environment) (1–4). Theseshifts have been shown to result in populationdeclines, ecosystem alterations, and maladap-tions (1–4). Studies of phenological responsesto changing environments, however, have beenheavily biased toward terrestrial ecosystems(3–6). In the marine domain, climate change–induced ecological effects, such as species rangeexpansion, decline in sea ice extent, and massbleaching of tropical coral reefs (6–8), are wellstudied and receive global attention, whereasphenological changes remain poorly understood.Additionally, most of the studies on phenologicalchanges have related temporal shifts to mis-matched interactions or mismatched synchronybetween species or trophic levels (e.g., consumer-prey, hatchling–food source, plant-pollinator)(1–4). Here, we focus onmismatches of population-level reproductive-phenology of reef-buildingcorals in the Gulf of Eilat (also referred to as theGulf of Aqaba) in the Red Sea (fig. S1) and showthat the once highly synchronized, iconic spawn-ing events of certain corals have lost synchrony,and we examine the demographic consequences.Coral spawning is often presented as a prom-

inent example of synchronized phenomena innature. Coral colonies spanning vast areas re-lease their reproductive material simultaneouslyinto the water column (fig. S2 and movie S1),

where the gametes remain viable for only afew hours (9, 10). Successful fertilization, whichonly takes place within this narrow time frameand is further challenged by gamete dilution(9–12), has led to the evolution of precise spawn-ing synchrony within populations. Such synchro-nicity relies on environmental cues operating ondifferent scales (13, 14). For example, the exactmonth of spawning has been found to be cor-related with temperature (15–17), solar irradiance(18), andwind (19), while lunar cycles are believedto cue the exact night (20–23) and sunset cues theexact hour (11, 20, 24). The widely acceptedparadigm regarding coral reproduction sug-gests a precise within-population spawningsynchrony, with a brief spawning period aroundspecific lunar phases, which for most speciesoccurs during the warmer months of the year.Given that global warming is intensifying massbleaching and mortality events of coral reefs(6, 8) and that temperature strongly influencescoral reproductive phenology (15–17), severalauthors have raised concerns about the possibledeleterious effects of ocean warming on coralspawning phenology and reproductive success(13, 14, 17, 25). However, the possibility of phe-nological mismatches affecting coral reefs hasremained unexplored.Whenwe revisited the historical coral commu-

nity reproductive phenology in the Gulf of Eilat(21, 26), we discovered that in some coral spe-cies the reproductive phenology has dramaticallychanged. Can spawning events, therefore, still beconsidered highly precise, synchronized, and sta-ble through time? By combining intense night-ly field observations with meticulous samplingand cascading analyses at the oocyte, polyp, col-ony, and population levels, we investigated thetemporal patterns of reproductive traits at differ-

ent levels over four annual reproductive seasons(27). To assess the possible demographic conse-quences, we established a long-term populationand community dynamics study (27). We focusedon five of the most abundant coral species in theRed Sea (fig. S2) that belong to four families withdifferent life history traits and growth morphol-ogies (27). In comparison with the historical data(21, 26), our observations indicated spawningsynchrony breakdown in some of these species,and this led us to question whether high coralcover or abundance necessarily indicates healthyand functional communities.

Coral spawning phenology

The majority of reef-building corals are simul-taneous hermaphrodites (i.e., individuals pro-duce both sperm and oocytes) that reproduceonce a year by broadcast spawning within one orseveral consecutive nights (13, 14). In some coralspecies, split or protracted spawning events mayoccur in different years or habitats (15, 23, 28).Such populations are still highly synchronized,but spawning takes place over two consecutivemonths and occurs during the same lunar phase.For the Gulf of Eilat, extensive studies from theearly 1980s that incorporated histological studiesof weekly or monthly samples, in conjunctionwith nightly in situ and ex situ spawning obser-vations performed throughout May to Septemberon the exact same reef (21, 26), showed that themajor coral breeding season occurs betweenJune and September, with many species havingdiscrete spawning periods corresponding to dif-ferent lunar phases. To determine whether thereproductive phenology of corals in the Gulf ofEilat has changed fromwhat was described fromthe 1980s studies (Fig. 1A), we initiated long-term monitoring of coral spawning in 2015. Weperformed nightly field surveys during the ma-jor annual coral reproductive season (June toSeptember) throughout 2015 to 2018 (total of225 night surveys, lasting 2.5 to 5.5 hours pernight) and recorded the number of spawningindividuals of each species (Fig. 1B and data S1)(27). In contrast with the widely accepted para-digm of highly synchronous coral spawning, wefound that, in some species, thewithin-populationspawning synchrony had become “out of tune.”During 2015 to 2018, three species displayed

no consistent multiannual pattern (Fig. 1B) rel-ative to the lunar phase, sea temperature, orwind speed (Fig. 1C). These species spawnedannually, in an irregular way, along severalweeks, with individual colonies spawning ondifferent nights. For Acropora eurystoma, weobserved only one spawning event in both 2015and 2016, which in 2015was closer to a full moonnight and in 2016 was closer to a new moon.Furthermore, dissections of tissue samples col-lected during those years indicated that additionalspawning events of this species had probablyoccurred (figs. S3 and S4). In both 2017 and2018 we observed five to six spawning events ofA. eurystoma, occurring at different lunar phaseseach season. For both Galaxea fascicularis andPlatygyra lamellina, numerous spawning events

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School of Zoology, George S. Wise Faculty of Life Sciences,Tel-Aviv University, Tel-Aviv 69978, Israel*Corresponding author. Email: tomshlez@gmail.com

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occurred almost every night over periods of 1.5 to2.5 months (Fig. 1B), whereas two other speciesdisplayed a precise spawning synchrony within abrief period, as expected (Fig. 1B). For example,Dipsastraea favus displayed a discrete and brief

spawning period matching the lunar phase re-ported from the 1980s, whileAcanthastrea echinataalso had a discrete and brief spawning periodicitybut with a shift along the lunar cycle comparedto the periodicity reported from the 1980s.

Scale of breakdown in spawningsynchronyDuring the spawning periods of the four yearsof this study, we repeatedly sampled small frag-ments for laboratory dissection analyses, either

Shlesinger et al., Science 365, 1002–1007 (2019) 6 September 2019 2 of 6

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Fig. 1. Breakdown in coral spawning synchrony in the Gulf of Eilat,Red Sea. (A) During the 1980s, the studied coral species demonstrateddistinct and concise spawning time frames (21, 26). (B) However, in recentyears a marked breakdown in spawning synchrony has been evident inseveral species: multiple spawning events within one season were observedregardless of the lunar phase and with no consistency between years.(C) Mean daily temperature and wind speed data for our study area wereacquired from the Israel National Monitoring Program (27). (D) To compare

data among years, the exact dates of nightly spawning surveys weretransformed to lunar age (with the new moon as lunar day 1). For the 1980sdata (A), only the nights in which spawning was observed are indicated,i.e., “no spawning” was recorded for all other nights. *Spawning periodicityduring the 1980s for this species was inferred by the complete disappear-ance of gametes in histological sections; it was not recorded in situ (26).Some data points indicating no spawning in (B) are not visible in the figuredue to overlapping zero values in different years (for full details see Data S1).

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from random or individually tagged coloniesof A. eurystoma at different sites or times (n =259 colonies) (data S2) (27). We quantified fourparameters reflecting coral reproductive stateand degree of synchrony on four different levels(Fig. 2): (i) at the cell level we quantified sizesof oocytes; (ii) at the single coral polyp level,fecundity (i.e., number of oocytes per polyp);(iii) at the colony level, colony fertility (i.e., pro-portion of gravid polyps within a colony); and(iv) at the population level, population fertility(i.e., proportion of gravid colonies). To examinepossible local effects on reproductive traits andtemporal patterns, in 2015 we performed col-lections at two sites in the Gulf of Eilat (fig. S1B)representing two contrasting levels of local dis-turbances to the reefs. One site, located in thenorthernmost part of the Gulf of Eilat and closerto urban areas, is subjected to stressors such ashigher levels of nutrients, sedimentation, lightpollution, etc. (29–31). The second site, located~7 to 8 km farther south, is a nature reserve with

limited public access and fewer environmen-tal and human perturbations (29, 30, 32). Ouranalyses indicated that coral populations at bothsites lacked precise within-population synchrony(fig. S3). We concluded that local stressors alonecannot explain this breakdown in synchrony,and in the following years we concentrated oursampling in the southern, more protected site(fig. S1).The analyses of all samples between 2015 and

2018 revealed that multiple spawning eventshad occurred each reproductive season regard-less of lunar phase and that the breakdown inspawning synchrony exists on all four levelsnoted above (Fig. 2 and figs. S3 to S5). To furtherunderstand the exact temporal trajectories andlevels of breakdown, in 2018 we tagged 25 col-onies of A. eurystoma and repeatedly sampledthem (five times during the spawning period)for tissue dissection (Fig. 2). Traditional ap-proaches, describing gametogenesis dynamicsas a function of changes in gamete sizes (e.g.,

Fig. 2A), and simple observations scoring thepresence or absence of gametes as a basis forpredicting their spawning time, can easily over-look other spawning events that may have oc-curred. For example, in our study, from thepresence alone of gametes in some samples,or their sizes (Fig. 2A), one might conclude thatspawning had occurred only between the lasttwo sampling dates (30 June and 10 July). How-ever, this approach ignores the full ecologicalpicture, which can be unveiled only by quantify-ing reproductive traits on other levels (Fig. 2, Band C). Our examination of polyp fecundity in-dicated a gradual decrease (Fig. 2B), suggestingthat some spawning had taken place and thatsome polyps may have partially spawned. Addi-tionally, the increasing variability found in fe-cundity along the spawning period also indicateda lack of synchrony between polyps (within col-ony), i.e., more polyps possessing either no or justa few oocytes, alongside other polyps that stillrevealed high fecundity at different times (Fig.2B). The within-colony lack of synchrony be-came even clearer when we examined colonyfertility (Fig. 2C), which also showed a gradualdecrease. Thus, at different times, some coloniescontained high proportions of gravid polypsalongside colonies without or with very fewgravid polyps. Lastly, population fertility (Fig. 2C),reflecting the population-level (within-species)breakdown in synchrony, revealed a similar grad-ual decrease.The overall evaluation of these different levels

(Fig. 2, A to C) led us to conclude that a clearlack of synchrony was occurring at all the ex-amined reproduction levels. In many spawningevents, some colonies spawned all their repro-ductive products, some colonies did not spawnat all, and others spawned partially, either withonly somepolyps spawning orwith polyps spawn-ing incompletely. These patterns were repeatedlyevident throughout the 4 years of study (Fig. 2and figs. S3 to S5).

Connecting mismatched phenology topopulation trajectories

Does a breakdown in coral spawning synchronytranslate into reproductive failure and gametewastage? To answer this question, we examinedcoral community and population dynamics usingtwo approaches (27). First, in 2015, we establishedlong-term monitoring of permanent plots at adepth of 4 to 5 m (3 m2 each; n = 10). All coralswithin these plots were mapped, and monitor-ing of dynamics of coral recruitment and mor-tality was carried out annually by visual surveysand high-resolution photography (Fig. 3, figs. S6and S7, and table S1). Second, in 2017, we mea-sured the size frequency distributions of speciesthat demonstrated a decline in synchrony (n =589 for P. lamellina, n = 112 for G. fascicularis,and n = 59 for A. eurystoma) (Fig. 4 and tableS2). Coral size can be related to processes suchas elapsed time since larval settlement, in ad-dition tomany environmental factors that affectgrowth and survival. Healthy and stable coralpopulations generally display size distributions

Shlesinger et al., Science 365, 1002–1007 (2019) 6 September 2019 3 of 6

Fig. 2. Temporal dynam-ics in reproductivetraits of A. eurystomaalong the 2018spawning season withmultiple sporadicspawning events.A breakdown in spawningsynchrony is evident fromthe level of an individualpolyp to the populationlevel. While oocytes thatare not spawned continueto grow (A), they maygive the false impressionthat spawning did notoccur. In contrast, otheraspects such as fecundity(i.e., the number ofoocytes per polyp)(B) and colony fertility(i.e., the proportion ofgravid polyps within acolony) (C), as well aspopulation fertility(i.e., the proportion ofgravid colonies within thepopulation) [which ispresented as the shadedarea in (C)], provide abetter indication ofwhether and whenspawning actually tookplace. Dashed red linesrepresent the dates onwhich spawning wasobserved in situ. Violin-shaped plots (the coloredareas behind the boxplots) show the distribution of the data (i.e., probability density) and emphasizethe changing variability in all levels, thus indicating a lack of synchrony. Letters above plotsindicate significant grouping based on Tukey post hoc test after repeated-measures analysis ofvariance using a permutation approach (P < 0.001 for all).

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that are extremely skewed to the right (positiveskewness), with a wide range of size classesrepresented (33). By contrast, a negatively skewedsize distribution indicates a population witha lack of recruitment and the dominance oflarge-sized colonies, implying a population atrisk of decline (33).Over the course of this study we recorded a

total of 657 newly recruited corals in the per-manent plots, of which 526 (80%) were iden-tifiable to at least the genus level (Fig. 3 andfig. S7). Despite a high annual recruitment rateof 7.3 ± 4.35 (mean ± SD) new recruits per squaremeter of the overall coral community (Fig. 3B),for both A. eurystoma and G. fascicularis noteven one recruit (Fig. 3A) or juvenile (Fig. 4)from the previous few years could be detected.Notably, the two genera with the highest num-ber of recruits [Stylophora and Leptastrea(Fig. 3)] include species that are not broadcastspawners but brooders (i.e., employ internalfertilization). The population of A. eurystomahad a strong negatively skewed distributiontoward large (old) colonies, with a clear lackof juvenile (small) new generations. Similarly,G. fascicularis also suffered from a lack of newrecruits. Finally, as external fertilization successin the sea is density dependent (9–12), the highabundance of P. lamellina at our study site ap-parently still resulted in limited reproductivesuccess and a subsequent limited recruitment.However, since this species is also dominated

by the largest-size class (>25 cm), the smallproportion of recruits may prove insufficient tosustain a stable population in the longer term.Indeed, among the five focal species of this study,only D. favus and A. echinata, which demon-strated precise spawning synchrony (Fig. 1), ex-hibited high recruitment leading to populationgrowth,while those species that exhibited abreak-down in synchrony and no recruitment were indecline (fig. S7).

Discussion

The mass bleaching and mortality of coral reefshave become prominent manifestations of thedestructive impacts of human activities andclimate change on marine environments. Theseimpacts are generally quantified using macro-scopic attributes of the perceivable visible dam-age, such as declines in live coral coverage andabundance (6, 8, 29, 34). Nonetheless, changesin the environmental drivers that underpin thereproductive phenology and success of marinespecies may challenge these species’ viabilityin less visible ways. Here, we identify an over-looked threat to marine species that reproduceby broadcast spawning. We show that coralreproductive phenology has not merely shiftedto a different time period relative to what washistorically known but also has lost its pro-nounced synchrony, reducing the probability ofsuccessful fertilization. Ultimately, the break-down in spawning synchrony leads to a dearth

of new recruits, creating aging populations thatlack vital juvenile replenishment.Coral reefs in the Gulf of Eilat have under-

gone conspicuous decline for several decades,mostly due to anthropogenic disturbances (29).However, in the last decade they have shownencouraging signs of recovery, including in-creased live coral coverage and high recruitmentrates (30, 32). Our present findings emphasizethat the assessment of a coral reef’s state basedsolely on broad coral community attributesmasksthe trajectories of underlying species. Moreover,it appears that the current proportion of re-cruited brooding versus broadcast-spawningspecies (Fig. 3 and fig. S7) is higher than itwas a few years earlier (32), providing furtherevidence that the breakdown in spawning syn-chrony may be altering reproductive success.Different species possess different tolerancesto environmental conditions; for example, corals

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Fig. 4. Size frequency distributions. Domi-nance of the larger size classes can be seen inall three species that exhibited a decline inspawning synchrony, together with an absenceof recruits from recent years in A. eurystoma(A) and G. fascicularis (B) and minute recruit-ment in P. lamellina (C). These populationstructures suggest aging populations withoutsufficient juvenile replenishment. Size frequencydistributions are presented as the mean(±SD) number of colonies per transect (n = 44;10-m by 1-m belt transects). Numbers belowbars indicate the total number of colonies foreach group.

Fig. 3. Absent or minute recruitment ofspecies with declining spawning synchrony,in contrast with high overall coralrecruitment rates. (A) Total number ofrecruits for the highest-recruiting genera andfor the species that showed a decline inspawning synchrony. (B) Annual recruitment ratedid not differ between the years (average 7.3 ±4.35 newly recruited corals per m2 per year).

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demonstrate species-specific responses to globalwarming and subsequent bleaching and mortal-ity (25, 34). Throughout the years of this study,some species at the same area still demonstratedwell-synchronized spawning, as expected (28, 35)(Fig. 1B). Therefore, the species studied may ac-tually represent an array of coral species that aremore sensitive in their phenological responses tochanging environments and may be just the firstto show early warning signs of disruptive phe-nological mismatch, with other species potentiallyfollowing this trend. Given the long life span ofcorals, while some species may appear to be sur-viving under certain environmental conditions,creating an impression of thriving populations,certain vital processes for species persistence, suchas sexual reproduction,might be undergoing silentcompromises.Several possible mechanisms may be driving

the breakdown in spawning synchrony reportedhere. Because coral spawning is tightly relatedto lunar cycles and inmost species occurs duringor after sunset (13, 14), a growing concern re-garding spawning phenology has to do withlight pollution (22, 24). However, our resultswith populations from two sites (fig. S1B) withhigh versus low light pollution (31) indicated aclear breakdown in spawning synchrony in both,irrespective of such local effects (fig. S3). More-over, as it appears that the precise hour of eachspecies’ spawning was not affected but rather themonth and night differed, the possible causesof variation in spawning may originate in theearlier stages of gametogenesis and gamete mat-uration. Temperature has a strong influence oncoral gametogenic and spawning cycles (13–17, 36).In our study region, sea temperatures are risingfast, at a rate of 0.31°C per decade (Fig. 5), andwetherefore posit that the breakdown in spawning

synchrony reported here may reflect a potentialsublethal effect of ocean warming (which mayresult not only in higher temperatures but also inchanges to the timing and pace of seasonal cool-ing and warming). Indeed, several studies haveprovided evidence that coral gametogenic cyclesmay be initiated asynchronously, presumably asa result of the temperature regimen (28, 36, 37).Lastly, another plausible driver that may poten-tially extend over larger areas, encompassingboth our study sites, may be related to hormonal(endocrine-disrupting) pollutants. As in manyother vertebrates and invertebrates, steroids (e.g.,progesterone, testosterone, estradiol) play animportant role in coral reproduction (38, 39).Anthropogenic chemicals that can disrupt theendocrine system accumulate in the marine en-vironment as a result of ongoing fluxes of her-bicides, pesticides, plastics, sewage, and manyother contaminants (38–40). Nonetheless, theadverse impacts of both human polluting activ-ities and climate change on the marine environ-ment are rapidly increasing worldwide (5, 6, 8).Therefore, regardless of the exact cause leadingto declines in spawning synchrony, our findingshere serve as a wake-up call to start consideringthese subtler challenges in coral survival, whichlikely also impact additional species in otherregions but remain unnoticed. The major rea-sons for such oversights could be (i) the lack ofhistorical baseline data against which currentday findings can be compared and evaluated;(ii) the rarity of long-term or appropriate tem-poral resolution studies of coral phenology; and(iii) the widely accepted paradigm regardingcoral reproductive phenology (i.e., discrete,brief, and fixed spawning periodicities withhigh within-population synchrony) that resultsin neglect of the possibility that it might change

over time. Thus, although a recent study fromthe Philippines found unusual reproductive pat-terns (similar to those described here), in theabsence of any historical data those authorsconcluded that the patterns might be a localcharacteristic of the studied species (41). Re-searchers in the Caribbean noted a marked mul-tiannual inconsistency of Acropora spp. spawningperiodicities (42) relative to those observed duringthe 1980s (37).An important implication of our findings for

future studies of marine broadcast-spawningspecies is that the common assumption or con-clusion of well-synchronized spawning eventsshould not be reached without long-term oradequate sampling. Identifying these early warn-ing signs in species that show such reproductivemismatch will contribute to directing our futureresearch and conservation efforts toward thosespecies that are at risk of decline. Regardless ofwhether the within-population breakdown inreproductive synchrony reported here is a sub-lethal effect of global warming or a consequenceof any other adverse human impact, there is aclear need for global quantification of the extentof this change and the identification of its un-derlying mechanisms. Without proper assess-ment of what constitutes reproductive success,we could easily overlook the possibility that spe-cies that appear to be abundant may actually benearing extinction through reproductive failure.We call for further studies on the potential phe-nology and reproductive mismatches in marinespecies aimed at the early identification of pop-ulations that are at possible risk of collapse, longbefore they display any visual signs of impendingmortality.

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Fig. 5.Warming trend of sea temperature in the Gulf of Eilat. The warming rate in thenorthern part of the Gulf of Eilat during the last 4 decades is 0.31°C per decade (calculatedby multiplying the slope of the regression line by 365 days and then by 10 years). Mean dailysea surface temperatures from 1981 to 2018 were acquired from the NOAA OI SST V2high-resolution dataset (27), which is based on data from NOAA Pathfinder V5.2 (AVHRR4-km-resolution satellite data).

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ACKNOWLEDGMENTS

We thank D. Akkaynak, A. Heyward, P. Harrison, Y. Nozawa, andthree anonymous referees for constructive comments on anearlier version of the manuscript. We are grateful to theUnderwater Observatory Marine Park and the InteruniversityInstitution for Marine Sciences (IUI) at Eilat for their ongoingsupport. We thank H. Rapuano, I. Kaufman, and Y. Ahouvi fortheir help in the lab work and N. Paz for further editorialassistance. Funding: This study was funded by Israel ScienceFoundation (ISF) grant no. 1191/16 to Y.L. and doctoral grantsfrom the Israel Taxonomy Initiative, Rieger Foundation, PADI

Foundation, and IUI to T.S. Author contributions: The studywas conceived and designed by T.S. and Y.L. The field andlaboratory work were conducted by T.S., and both authorsanalyzed and interpreted the data. T.S. wrote the firstmanuscript draft, and both authors contributed to writing thefinal version. Competing interests: The authors declare nocompeting interests. Data and materials availability: All dataare available in the main text or the supplementary materials.Sea surface temperature data for 1981 to 2018 providedby the NOAA/OAR/ESRL PSD are available at www.esrl.noaa.gov/psd/data/gridded/data.noaa.oisst.v2.highres.html. Windspeed and sea temperature data for 2015 to 2018 provided bythe Israel National Monitoring Program are available at iui-eilat.huji.ac.il/Research/NMPMeteoData.aspx.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/365/6457/1002/suppl/DC1Materials and MethodsFigs. S1 to S7Tables S1 and S2Movie S1Data S1 and S2

14 February 2019; accepted 16 July 201910.1126/science.aax0110

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Breakdown in spawning synchrony: A silent threat to coral persistenceTom Shlesinger and Yossi Loya

DOI: 10.1126/science.aax0110 (6457), 1002-1007.365Science 

, this issue p. 1002; see also p. 987Sciencebut no less insidious threat to coral reefs.are likely occurring globally. Such a loss of spawning synchrony could result in reproductive failure, a much less obviousshifts in the timing of gamete release in several species of broadcast-spawning corals in the Red Sea. Similar changes synchrony (see the Perspective by Fogarty and Marhaver). They found that environmental changes have resulted inwith life. Shlesinger and Loya alert us to an equally dangerous yet nearly invisible hazard to coral: loss of breeding

Our changing climate is a threat to corals, causing disfiguring bleaching and mortality to reefs that once teemedInvisible threat

ARTICLE TOOLS http://science.sciencemag.org/content/365/6457/1002

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REFERENCES

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