Research Articles
letters to natureNATURE|VOL 388 | 17 JULY 1997 265Landscape ecology of algalRob Rowan*†, Nancy Knowlton*, Andrew Baker*‡& Javier Jara**Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Panama†University of Guam Marine Laboratory, Mangilao, Guam, 96923, USA‡Rosenstiel School of Marine and Atmospheric Science, Unversity of Miami,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reef-building corals are obligate, mutualistic symbioses ofSymbiodinium spp.)1. Contrary to the earlier, widely acceptedMontastraea annularisandM. faveolata can act as hosts to dynamic, multi-speciesSymbiodinium. Composition of these communities2–4 is not the only mechanism by5,6.The coralsMontastraea annularis and M. faveolata each host three7 of the dinoflagellate genus Symbiodinium,A, B and C, that are identified by restriction-fragment7. A and B are common in shallow-water corals (highirradianceC predominates in deeper corals (lowirradianceA þ C and B þ C, common atNature © Macmillan Publishers Ltd 1997letters to nature266NATURE |VOL 388 | 17 JULY 1997intermediate locations7, suggest that symbionts may actually exist as2.M. faveolata, and all but one colony of M., yielded two or three types of symbionts. As predicted,Symbiodinium Aand B dominated locations with higher, downwellingC dominated locations of lower irradiance (communities 3 andP , 0:001; x2 test). TheseM. annularis, and 6–12m in M. faveolata),Symbiodinium C is predominant overall7. As before7,Symbiodinium Awas more common in M. faveolata than in M..,1 cm) confirmed that symbiontsM. annularis create a localized gradient of low (on the side,Symbiodinium C toB, B þ C, or A (Fig. 2a–d). Analyses of shallower (1–2 m) and7 and intracolony3,8. Bleaching3,8,9. Many bleaching events exhibit intraspecificFigure 1Symbiont communities in M. annularis (a) and M.(b). Each symbol represents one core sample thatSymbiodinium A, B, C or mixtures of taxa summarizedFigure 2Symbiont zonation in columns of M. annularis. a, b, TaqI digests of srRNA genes. a,c) and standards forSymbiodinium A, B and C. b, Standards (see Methods). Lane 1, C alone; 2–10, B þ C mixturesC: B ratios); 11, B alone. c, Column sampled in a; symbols 1–11 (dark,Symbiodinium C; white, Symbiodinium B) represent the data in lanes 1–11, respectively in a.d, Zonation in 14 columns from 14 colonies sampled as in c (differences in shading are forx-axis, location relative to lower side and top centre; y-axis, genotype rangingC to all B or all A (A in one column only); z-axis, depth. e, Zonation in a column8 from vertical for 6 months, presented as in c. f, Zonation in 7 columns (alle,d; zonation in natural columns at a depth of 6m (from d) are plotted forNature © Macmillan Publishers Ltd 1997letters to natureNATURE|VOL 388 | 17 JULY 1997 267that are difficult to explain9–12. Because irradiance and temperature12–14, and the symbionts ofM. annularisand M. faveolata exhibit different associations withM. annularis and M.on 18 September 1995, and bleaching was extensive by the15. At our2. We also observed complex9,10,18 bleachingM. annularis and M. faveolata. Bleaching was,2 m) and deep (.15 m) sites; inM. annularis partitioned as in Fig. 1an ¼ 76 colonies,P , 0:05, x2 test). Some colonies exhibited a ‘ring’ ofM. faveolatacolonies are not easily partitioned into two distinct,6 m) centre. Such12 because theSymbiodinium C’s ‘adaptive9,15,19 (see Fig. 3a, b), if Symbiodinium C were,1 cm from many sites sampled the previous January (Fig. 1). AllSymbiodinium C plus either A or B (or both) werea priori (Fig. 2) thatSymbiodinium C in corals,1 cm awaySymbiodiniumsrRNA RFLPs in these replicate, pre-bleaching pairsSymbiodinium C had decreased in relative abundanceSymbiodinium C were typically close toB underwent a median decrease of 14%, and A moreSymbiodiniumtaxa as symbionts under ‘bleaching conditions’. TheA . BqC.M. annularis and M., which are dominant members of western Atlantic reefs20Figure 3Bleaching in M. annularis (a, c) and M. faveolata (b, d) ata, b) andc, d) patterns. e, Sea surface temperatures (three-week30) at the San Blas Islands,8C in 1983 and 1995 were16 (this study). Records from ourNature © Macmillan Publishers Ltd 1997letters to nature268NATURE |VOL 388 | 17 JULY 1997and are widely used as model systems in reef biology and11,13,18,19,21,22. Each coral ‘species’ encompasses one animal1, in which host taxa alone are adequate units of biodiversity,2–4, and bleaching variability is often12. We conclude that polymorphic symbiont communities20 and bleaching ecology of19. For these corals, and for mutualisms in general, the5,6.M.and M. faveolata in the Bahamas also host Symbiodinium, B and C (data not presented), and published photographs18 and9,10 of bleaching elsewhere strongly resemble our own9.We can attribute thisM. annularis andM. faveolatato symbiont polymorphism and zonation. Moreover,Symbiodinium Aand C (ref. 23; our unpublished data). For otherSymbiondinium taxonomy would be2–and genetic differences among hosts11,14.3,8,21. Alternatively,24–26. Our findingsSymbiodinium to produceM.and M. faveolata might adjust to a warmer AtlanticSymbiodinium A and less Symbiodinium BandC. However, long-term consequences of such replacementsM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MethodsCoral samples were collected at27 by coring (1.1 cm2 surface area) and,0.12cm2)M. annularis were broken off ,15 cmLaboratory analyses.Symbionts and symbiont DNA were isolated from7 and from fresh28 samples. Nuclear srRNA genes were amplified usingSymbiodinium-biased’ primer ss3Z (all data in Figs 2 and 4), andTaqI and DpnII (data were consistent in every case)7. The biasedSymbiodinium genotypes (discussed in ref. 28), as confirmed by7 and by comparing results from ‘universal’ and ‘biased’ amplificationsA þ C or B þ C) in variousFigure 4Symbiont communities before (January 1995) and during (October 1995)a–c, Lanes contain TaqI (a, b) or DpnII (c) digests ofa, Standards for B, C and B:C ratios of 8 : 1 (lane 1) and 1 : 1 (lane 2);M.(lanes 3–6) and M. faveolata (lanes 7–10). b, Standards for A, C and A:Cratios of 2 : 1 (lane 1) and 1 : 8 (lane 2); lane pairs 3 (M. annularis) and 4–7 (M.) compare symbionts as in a. c, Standards for A, B, C and equal amountsA, B and C (lane 1) and A and B (lane 2); lane pair 3 compares symbionts in M., as in a and b. The vertical bracket identifies bands that identify eachd, Densities of A (grey), B (white) and C (black) before and duringa(B þ C, communities 3–10), b (A þ C, communities 3–7) and c (ABC, community 3).e, Chlorophyll contents of the samples reported above, presented as in d.Nature © Macmillan Publishers Ltd 1997letters to natureNATURE|VOL 388 | 17 JULY 1997
symbionts creates variation
in episodes of coral bleaching
Miami, Florida 33149, USA
heterotrophic animals and phototrophic dinoflagellates
(
belief that corals harbour only one symbiont, we found that the
ecologically dominant Caribbean corals
communities of
follows gradients of environmental irradiance, implying
that physiological acclimatization
which corals cope with environmental heterogeneity. The importance
of this diversity was underlined by analysis of a natural
episode of coral bleaching. Patterns of bleaching could be
explained by the preferential elimination of a symbiont associated
with low irradiance from the brightest parts of its distribution.
Comparative analyses of symbionts before and after bleaching
from the same corals supported this interpretation, and suggested
that some corals were protected from bleaching by hosting an
additional symbiont that is more tolerant of high irradiance and
temperature. This ‘natural experiment’ suggests that temporal
and spatial variability can favour the coexistence of diverse
symbionts within a host, despite the potential for destabilizing
competition among them
distantly related taxa
denoted
length polymorphisms (RFLPs) in genes encoding small ribosomal
RNA (srRNA)
habitats), whereas
habitats). Mixed samples
complex communities that track differences in irradiance within a
colony
To test this hypothesis we sampled four locations in each of 46
colonies (Fig. 1). All
annularis
irradiance (communities 1 and 2, unshaded colony tops),
and
4, colony sides and shaded colony tops) (
patterns of intra-colony zonation largely disappear at slightly
greater depths (8–11m in
where
annularis
Analyses at a finer scale (
occupy distinct but overlapping habitats (Fig. 2). Unshaded columns
of
no downwelling) to high (on the top, full downwelling) irradiance,
which we sampled along transects. At intermediate depths (3–7 m)
this gradient coincides with the transition from
deeper (9–12 m) corals (Fig. 2d) show that depth
zonation of the symbionts occur in parallel. These consistent
patterns strongly argue that zonation is controlled by ambient
irradiance. Furthermore, experimentally toppled columns, which
experienced immediate and severe changes in irradiance gradients,
largely re-established expected patterns of symbiont zonation
during a six-month period (Fig. 2e,f). This response shows that
the patterns are maintained dynamically.
Symbioses between corals and dinoflagellates are stable mutualisms,
with the notable exception of coral bleaching, which involves
the loss of symbionts and/or photosynthetic pigments
is an ecologically important but poorly understood response to
environmental stress
variation distributed within and among habitats in ways
faveolata
contained
according to the code shown below. Columns in the data
matrices represent individual coral colonies (depth increases
from left to right), and rows represent locations of higher (rows 1
and 2) and lower (rows 3 and 4) irradiance, as defined in the
diagrams to the left. Samples were collected in January 1995.
Transect from column side to top (lanes 1–11; positions shown in
(numbers below are
clarity only);
from all
transplanted to 90
at a depth of 6 m) transplanted and sampled along two transects (white and black) as in
presented as in
comparison (grey).
act synergistically to induce bleaching
irradiance (Figs 1 and 2), we hypothesized that symbiont polymorphism
underlies this variation.
We observed ‘paling’ in several colonies of
faveolata
second week in October, both in Panama and elsewhere
study site, this event was ‘typical’: like a similar event there in 1983
(ref. 16), it followed a prolonged excursion above the mean summer
maximum of temperature (Fig. 3e); it also coincided with atypically
high water clarity (data in ref. 17), which implies increased
irradiance at depth
patterns in both
rare or slight at both shallow (
between, however, both species displayed a curious pattern, with
shallower colonies bleached preferentially in shaded places
(Fig. 3a,b) and deeper colonies bleached preferentially in unshaded
places (Fig. 3c,d). Among
(communities 1 and 2 versus 3 and 4) and by depth (above 8m
versus below 8 m), this difference was significant (
64 bleached;
bleaching at the boundary between column top and side (Fig. 3a).
irradiance microhabitats, but they clearly showed the same reciprocal
pattern (Fig. 3b,d), with a shallower (
observations have previously been hard to explain
environment is isothermal, and the associations with irradiance and
colony morphology are inconsistent.
Symbiont zonation provides a simple hypothesis to explain these
bleaching patterns. Bleaching was disproportionately common in
what seems to be the upper limit of
zone’: low-irradiance parts of corals in shallower water, and highirradiance
parts of corals in deeper water. Slight increases in
temperature and irradiance might push these symbioses, but not
others, beyond some physiological limit, resulting in bleaching. This
hypothesis accounts for our bleaching observations, including areas
of slight bleaching
expelled selectively from mixed symbiont communities.
An analysis of symbionts collected in late October supported this
interpretation of events. Post-bleaching samples were obtained
available samples from communities that had previously contained
mixtures of
identified (Fig. 1) and analysed. We reasoned
these sites accurately defined the limit of
under non-bleaching conditions. Such mixtures also allow the fates
of different symbionts to be compared directly. We also tested
archived samples taken at the same time as, and
from, the original (pre-bleaching event) samples. In every case,
were equivalent (data not shown), indicating that the small distance
between pre- and post-bleaching samples was unlikely to be
significant.
As predicted,
in all 18 communities tested (see Fig. 4a–c). Absolute
responses of different symbionts within a mixed community were
compared by using estimates of relative abundances (from RFLP
data; see Fig. 4a–c) to partition direct counts of symbionts into each
genotype (Fig. 4d). Losses of
100%, whereas
than doubled in 3 of 5 instances. The single sample that contained
all three symbionts exhibited these same trends (Fig. 4c, d). Changes
in colony chlorophyll contents and subjective assessments of
bleaching paralleled changes in symbiont numbers (Fig. 4e).
From these data we can tentatively rank the ‘fitness’ of the different
ranking obtained in this manner is:
Our study provides a fuller understanding of
faveolata
the study site on 28 October 1995 showing ‘shallow’ (
‘deep’ (
running means, from satellite data
Panama. Temperatures above 29
associated with coral bleaching
study site (at 7-m depth) since 1993 (Marine Environmental
Sciences Program, Smithsonian Tropical Research Institute)
corroborate satellite data.
geology
and dynamic, multi-species communities of symbiotic dinoflagellates.
This strongly contradicts the ‘one host, one symbiont’ view of
reef corals
environmental variability is accommodated largely by
physiological acclimatization
not understood
underlie the broad distributions
these corals. Directed shifts in symbiont populations following
extreme environmental change (Figs 2e, f and 4) suggest that similar
shifts may also occur over annual cycles of environmental
variation
ability to cope with environmental change through changes in
symbiont community composition reflects the selective advantage
of hosting several distinct symbionts, despite the potential for
destabilizing competition among them
How typical and important are the patterns documented here?
annularis
A
descriptions
(Fig. 3a–d). With respect to Caribbean corals in general, bleaching
is often predominant at intermediate depths
pattern (and its within-colony correlate) in
at least three other species of Caribbean corals host (at least) both
species, which might host multiple but not so distantly related
symbionts, refinements of
essential. However, symbiont polymorphism does not exclude the
significance of other attributes that are important features of coral
biology, such as physiological acclimatization of hosts and symbionts
4
It has been hypothesized that a global warming trend, with
increased frequencies of world wide coral bleaching induced by
increasing temperature or ultraviolet irradiation, could have catastrophic
consequences for many living coral reefs
coral communities may adjust to climate change by recombining
their existing host and symbiont genetic diversities
supply a precedent for this idea: that one species of coral can
flexibly host more than one taxon of
symbioses with distinct ecological properties. For example,
annularis
ocean by hosting more
would depend on how they affect rates of coral growth and
reproduction.
Field collections and manipulations.
Aguadargana reef in San Blas, Panama
freezing immediately in liquid nitrogen (data in Figs 1 and 4). Other colonies
(data in Fig. 2) were sampled by removing a defined circular area (
of living tissue from freshly collected colonies with an airbrush. In transplant
experiments (Fig. 2e, f), columns of
below the living tissue, turned on their sides, and cemented (at the non-living
base) back to the colony at a comparable location; this increased (new top),
decreased (new side), or did not change (side) local irradiance. All 28
transplants at a depth of 6m seemed to be normal after 6 months. Analyses
of non-transplanted (control) columns showed that natural zonation patterns
were stable over this period (data not shown).
frozen
the ‘universal’ PCR primers ss5 and ss3 (all data in Fig. 1) or a combination of
ss5 and the ‘
analysed with
primer (ss3Z) does not discriminate (within this study) against unknown,
specific
sequencing
of 30 samples that contained two genotypes (
an episode of coral bleaching.
srRNA genes.
lane pairs compare symbionts before (left) and during (right) bleaching in
annularis
faveolata
of
faveolata
symbiont.
bleaching (left and right bars of each pair, respectively) in samples reported in
Samples were scored as ‘normal’ (not marked) or ‘slightly pale’, ‘pale’, or
‘bleached’ (marked by asterisks) when collected.
Hybridization and the EvolutionSteven V. Vollmer* and Stephen R. PalumbiHundreds of coral species coexist sympatrically on reefs, reproducing in massspawningAcropora corals show that mass spawningA. cervicornis and A. palmata areAcropora prolifera are entirely F1 hybrids of these two species,1 individuals can reproduce asexually and form long-lived, potentially immortalDiverse reef-building coral assemblages have1), thereby providing2). Laboratory2, 3). Interspecific4) that some species-rich genera3).Acropora, the world’s most speciose coral5), exemplify this view (2–4). Most ofAcropora arose over the6, 7), and many2, 8). One3),Department of Organismic and Evolutionary Biology,Fig. 1.The Caribbean Acropora species: (A) A. cervicornis and (B) A. palmata, and (C) the bushy andD) palmate F1 hybrid A. prolifera morphs from Puerto Rico.RE P O R T Swww.sciencemag.org SCIENCE VOL 296 14 JUNE 20022023Downloaded fromwww.sciencemag.org on April 7, 2010hypothesis is lacking. Polyphyletic sequence8–11) withoutAcropora (Fig. 1).Acropora cervicornisand A. palmata are sister12, 13). Both have distinct morphologiesA. cervicornis occurs throughoutA. palmata occurs primarily14, 15). Both species spawn synchronously16) and canAcropora, occurs Caribbean-wide, where it7, 14, 15). It is morphologicallyA. cervicornis and A. palmata,7, 15). Pax-C intron data showing10). Morphological variation in A. prolifera isA.morphs—a thin, highly branchedAcroporaspecies at introns of the nuclear17). TheA. cervicornisandA. palmata are genetically distinctA. proliferais actually a first-generation1) hybrid. Acropora cervicornis and A. palmatawere reciprocally monophyletic at minicollagenA. prolifera (n “22) were heterozygous at minicollagen, containingA. cervicornisandA. palmata formed three distinct alleles: A,‘ (Fig. 2B). Allele A was exclusive toA. cervicornis. B alleles were exclusive to A., but the variant B‘ was shared betweenA. prolifera (n ” 28) were heterozygous” 26; B/B‘ ” 2). TheA. prolifera at these1 hybrid.A. cervicornis and hybrid A. prolifera.A. palmata, A. cervicornis, and hybridA. prolifera. All three haplotypes were foundA. prolifera, indicating that hybrid crossesA. palmata (B haplotype) or A. cervicornis(A haplotype) “mothers.”A. cervicorniscolonies possess all three haplotypeA. palmata colonies do not. The datapalmata” (B) haplotypes areA. cervicornis through backcrossingA. cervicornis with hybrid A. prolifera. IntrogressedA. cervicornis were(20%) and sampled at every site.A. cervicornisindicates the mtDNA introgression has18, 19), polyphyletic patterns inA. prolifera, i.e., the bushy and1 hybrids, they differ in which speciespalmatamaternal and mitochondrial background,cervicornis background. This suggestsFig. 2.Maximum likelihood (ML) trees for (A) minicollagen, (B) calmodulin, and (C) mitochondrial31) with estimated model32) inMODELTEST 3.06 (33). Major allele/haplotype clades are labeled. Tick marksn). Site abbreviations: Yucatan ( Y); Panama (Pa); Jamaica ( Ja); Puerto Rico (PR); St. Croix)50%) from 300 replicates are labeled on relevant nodes. The Pacific congenerAcropora nasutawas used as the outgroup. Sequences are available in GenBank (accession numbers” 654.81).” 592.86). (C)$*model (ln score ” 2014.96).A. prolifera hybrids are shown in blue; bushy hybrids are in red.RE P O R T S202414 JUNE 2002 VOL 296 SCIENCE www.sciencemag.orgDownloaded fromwww.sciencemag.org on April 7, 2010that maternal and/or cytoplasmic effects account20); however, a rarely explored alternative is21) to our data and the publishedPax-C data (10) to estimate the rate ofM) in units 2 + theNe) andm)] and test null hypotheses of noM ” 0) using likelihood ratio22). Results [Table 1 and supplemental23)] indicate that the mitochondrialM ” 0.20), roughly equivalent toNf (i.e., mtDNA effective populationPax-C data were also consistent with low levelsM ” 0.30), whereas the minicollagen‘ allele at calmodulin is a retained24), and/or the25).A. proliferashows that complete barriers to hybridizationA. palmata and A.. However, the observation that A.hybrid populations are composed1 individuals suggests thatA. proliferais severely limited or that hybrid breakdownA. proliferaare reproductive, produceA.. Yet, the limited introgression1 hybrids are26). Where F1 hybrids dominate,27). Such F1 hybrids should be common1 offspring are long-lived. Like many corals28), hybrid A. prolifera can propagate29), allowing forAcropora morphotypesAcropora show that reefbuilding30), but has never been postulated5), suggests that morphologically3) without genetic isolation.References and Notes1. P. L. Harrisonet al., Science 223, 1186 (1984).Coral Reefs16, s53 (1997).Corals in Space and Time: The Biogeography(CornellAnnu. Rev. Ecol. Syst. 25,Corals of the World (Australian Institute!!!!,R. Kelley, Assoc. Australas. Palaeontol. Mem.6, 1 (1988).Staghorn Corals of the World: A Revision(Scleractinia; Astrocoeniina;) Worldwide, with Emphasis on(Commonwealthet al., Mol. Biol. Evol. 16, 1607 (1999).Mol. Biol. Evol. 14, 465Mol. Ecol. 9, 1363 (2000).Mol. Biol. Evol. 18, 1315 (2001).J. Paleontol.68, 951 (1994).Proc. 8th Int. Coral Reef1, 423 (1997).Ecology 40, 67 (1959).Stony Corals (Cnidaria: Anthozoa:) of Carrie Bow Cay, Belize (SmithsonianBull. Mar. Sci.64, 189 (1999).A. cervicornis,, and A. prolifera, respectively) were” 3, 3, 3; Panama ” 7, 5, 0;” 3, 4, 0; Puerto Rico ” 41, 22, 19 (10 bushy,” 0, 12, 6. DNA extractions used,g), and standard phenol-chloroform‘-GAGGTTGATGCTGATGGTGAG-3‘)‘-CAGGGAAGTCTATTGTGCC-3‘). The$ bp) plus‘-GCTTAGACAGGTTGGTTGATTGCCC-‘) and CO3r (5‘-CTCCCAAATACATAATTTGAACTAA-‘), and two internal sequencing primers, CRseqf (5‘-‘) and CRseqr‘-ATAACCCAACAAAGTCTAATTCCC-3‘). AmplificationsMol. Biol. Evol. 5, 568 (1988).Evolution 55, 859Molecular Markers, Natural History and(Chapman & Hall, New York, 1994).Genetics 158, 885 (2001).