Diversity and Age Patterns in Hermatypic Corals

DIVERSITY AND AGE PATTERNS IN HERMATYPICCORALS

FRANCIS G. STEHLI1 AND JOHN W. WELLS

Abstract

Stehli, Francis G. and John W. Wells (Dept. of Geology, Case Western Reserve Univ.,Cleveland, Ohio 44106 and Dept. of Geol. Sciences, Cornell Univ., Ithaca, N.Y. 14850)1971. Diversity and patterns in hermatypic corals. Syst. Zool., 20:115-126.—Diversity andaverage generic age distributions for hermatypic corals show that while the Atlantic andIndo-Pacific faunas have evolved at different rates and in some respects have long beenisolated systems, many responses to environment are the same. In each ocean, the diversityis highest north of the equator and in the western portion of the ocean. Positive anomaliesin mean annual sea surface temperature are closely associated with maximum diversityand with minimum average generic age and a general relationship between both param-eters and the area suitable for hermatypic coral growth is also evident. The data suggestthat evolution of new genera is proceeding most rapidly in the regions of warmest water,but it is not possible to separate from the temperature relationship the affect of area offavorable bottom which cannot yet be quantified. [Species diversity; generic age; hermatypiccorals.]

DIVERSITY

The global distribution of stations whichwe believe to be reasonably well sampledfor hermatypic coral genera (Wells, inStehli, 1968, revised to include new informa-tion) is shown in Fig. 1. The raw data forgeneric diversity of these stations includedetails not requisite to a search for generalrelationships and is doubtless partly spuri-ous as a result of imperfect collecting andstudy (Fig. 2). This problem can be over-come if we compute best fitting simple sur-faces for the data. These surfaces provideno new information, but allow us to ex-amine generalities without being undulydisconcerted by the detail seen in the rawdata (Fig. 3).

It is evident that the Atlantic and Indo-Pacific Oceans are behaving as separatesystems in terms of diversity. Each is seenfrom Fig. 3 to exhibit a distinct diversitycenter, but maximum diversity in the Indo-Pacific is 50% higher than that in the At-lantic and the raw data show that an evengreater difference appears actually to exist.A further difference is seen in the raw data(Fig. 2) which suggest that the Indo-Pacificdiversity center may really represent two

1 Contribution No. 62, Department of Geology,Case Western Reserve University.

centers imperfectly separated by lower di-versity around the islands of Sumatra, Java,and Borneo. This feature may be an artifacthowever, because Wells (1954) noted thatthe Sumatra-Java-Flores arc has been littlestudied.

In many respects the response of diversityto environmental factors is the same in bothoceans and similar patterns exist in them.In both, for instance, the areas of maximumdiversity occur north of the actual equatorwhether one examines raw data or fittedsurfaces, though the effect is most pro-nounced in the Atlantic. This phenomenonis of interest because it suggests that despitethe importance of zooxanthellae in the meta-bolic economy of hermatypic corals, diver-sity is not uniquely tied to capture of radiantenergy or depth of light penetration, bothof which should be optimal at the equator.

The relationship between diversity andtemperature appears to be quite strong. Itwas shown by Bohnecke (1936) that thethermal equator in the Atlantic lay well tothe north of the geographic equator, a factevident in both raw data for mean annualsea surface temperature and in the fittedsurface (Figs. 4 and 5). Comparison ofFigs. 3 and 5 shows the close relationshipbetween temperature and diversity in

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116 SYSTEMATIC ZOOLOGY

FIG. 1.—Distribution of sample stations for which reasonably reliable data regarding hermatypic coralgenera are available. Pacific and Indian Ocean stations are shown by black dots and Atlantic stations byopen circles. Three Atlantic stations yielding only one genus are distinguished by open circles combinedwith crosses. All stations are numerically keyed to the list in Appendix A.

FIG. 2.—Raw data for generic level diversity of hermatypic corals. The Indo-Pacific and the Atlanticare contoured as separate distributions.



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HERMATYPIC CORALS 117

FIG. 3.—Second order nonorthogonal polynomial surfaces separately fitted to the Indo-Pacific andAtlantic generic diversity distributions for hermatypic corals seen in figure 2. The coefficient of deter-mination for the Pacific surface is 0.580 and that for the Atlantic 0.813.

terms of displacement north of the equator.Bohnecke (1936) noted that positive ther-mal anomalies in the Atlantic were con-centrated in its western reaches, and in thisrespect also, coral diversity reflects thesurface temperature pattern, not only in theAtlantic, but to a lesser degree in the Indo-Pacific.

The thermal gradient from west to eastacross the oceans is relatively small, but ineach ocean the corresponding diversity de-crease is large and it appears likely thatfactors in addition to temperature are in-volved. Except in the Equatorial CounterCurrents, the movement of surface waterswithin the region most favorable for her-matypic coral growth is principally east towest, which doubtless hampers the east-ward spread of planktonic coral larvae. Amore important factor may be the area ofsea floor suitable for coral growth. Thereare no large areas of shallow water betweenthe Indonesian-Australian area and the

east Pacific coastline. An "area affect," orthe influence of the relative area of favor-able habitat in different localities, has beenfrequently mentioned in connection withevolutionary processes (e.g., Darlington,1957; Preston, 1962), and it may be operat-ing in this case.

The problem of accumulating quantita-tive data concerning the area of sea floorfavorable for hermatypic coral growth inany region is insurmountable. A glance atthe map will quickly reveal, however, thatconcentrations of islands occur in eachocean in the regions characterized by highdiversity. It should be mentioned that sincecorals perpetuate, increase or even developshallow water regions suitable for their owngrowth, the amount of suitable sea floorcould hardly be independent from the cen-ters of maximum development and diver-sity.

Before leaving the matter of diversity, itis of interest to consider the manner in



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FIG. 4.—Mean annual sea surface temperature for the oceans derived from the U. S. Navy, MarineClimatic Atlas of the World.

which diversity decrease away from centersis effected. Wells (1954) examined thisquestion for the Indo-Pacific and extractedfrom the data the general answer that re-gardless of direction away from the diver-sity center, the same genera seem to dropout in the same sequence. This gradient iswell exemplified by the latitudinal distribu-tion of reef coral genera along 1200 milesof the Great Barrier Reef (Wells, 1955).The question may be examined in some-what greater detail in a cluster analysis(Bonham-Carter, 1967) of the distributiondata available to us using Jaccard's Co-efficient and unweighted pair groups in theQ mode. Clustering is influenced prin-cipally by the similarity of the assemblagescompared, but is affected to some extentby the number of items (genera) compris-ing the assemblage, so that clustering be-comes less significant as very small assem-blages are compared. This problem isapparent in some isolated Atlantic stations,but does not seriously affect our conclu-

sions. Fig. 6 shows the results of the clusteranalysis in terms of the degree of associa-tion between generic assemblages at var-ious sample stations. A long recognizeddichotomy separates the Atlantic and theIndo-Pacific hermatypic coral fauna, sug-gesting that their isolation has been pro-found and has endured for a considerableperiod of time.

In the Indo-Pacific, cluster analysis sug-gests the existence of three subprovinces.Predominant among them is the main Indo-Pacific sub-province, considered by Wells(1954) which remains homogeneous butbecomes depauperate toward the north andthe southeast (Fig. 7). A somewhat dis-tinct fauna can be recognized in the east-ern Pacific sub-province beyond a line ex-tending from Midway Island southeastwardto Easter Island, where diversity abruptlydecreases. The low diversity stations in theeastern Pacific tend to be characterized bythe persistence of the genera Pavona, Fun-gia and Leptoseris which are, of course,



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FIG. 5.—Second order nonorthogonal polynomial surfaces for the mean annual sea surface tempera-tures data (°F) of figure 4 fitted separately for the Atlantic and the Indo-Pacific. The coefficient ofdetermination for the Pacific is .9366 and that for the Atlantic is .8185.

present in the main Indo-Pacific fauna aswell, but are not particularly characteristicof other marginal areas. A second regionalsub-province can be distinguished in thesouthwestern Pacific and southern IndianOcean. This southwestern sub-province,like that of the eastern Pacific and marginalareas, in general is characterized by a de-pauperate Indo-Pacific fauna. In this case,however, the genera which persist are dif-ferent and include Favites, Platygyra,Turbinaria, and Favia. The southwesternsub-province includes the regions south ofan east-west line running from Natal to theKermadec Islands (Fig. 7). The PersianGulf also appears to fall in the sub-province,but as Wells (1954) noted, the hermatypiccorals of the Persian Gulf are not wellknown and further study may reveal thatthis station belongs with the main Indo-Pacific sub-province rather than with thesouthwestern sub-province.

As a first generalization, Wells (1954)

apparently was correct in concluding thatdecrease in diversity follows the same path-ways into all marginal areas. Viewed inmore detail, however, there appear to beminor differences in the order in whichgenera drop out as environmentally stressedmarginal areas are reached, because per-sistent genera differ in the two marginalsub-provinces. This fact suggests that thenature of the environmental stresses tend-ing to limit hermatypic coral diversity dif-fer somewhat in different directions awayfrom the diversity center. This is not sur-prising, but our knowledge of coral ecologyis too imperfect to allow us to suggest thenature of these differences.

AVERAGE GENERIC AGE

Diversity data for hermatypic corals canbe interpreted to suggest that centers of di-versity are also evolutionary centers where,new forms evolve and from which theyspread to marginal areas. If such a model



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FIG. 7.—Geographic distribution of stations belonging to each of the principal clusters recognized infigure 6. In the Atlantic, stations 47, 48, and 49 yielded only one genus each and their association withother Atlantic stations is very low. In the Indo-Pacific, the main sub-province, C, characterizes the larg-est area, while sub-province B is restricted to the eastern Pacific and sub-province D comprises the south-west Pacific and the Persian Gulf.

is correct, areas of high diversity must ex-hibit many young genera, while areas of lowdiversity must show the presence of manyold genera. This consequence of the modelcan be tested by combining generic listsfor our various control stations with data onthe known geological range of each genus.Such a procedure allows the calculation of"an average generic age" for each stationand an examination of these data for sys-tematic variations. Fig. 8 shows raw datafor average generic age and Fig. 9 showsthe second order surfaces best fitting thedata for the Atlantic and the Indo-Pacific.For the Atlantic, the picture is simple withyoung average generic ages concentratedin the Caribbean center of high diversity,and a progressive increase in average ge-neric age with decrease in diversity as pe-ripheral stations are reached. The Atlanticdata strongly support the concept that evo-lution is proceeding rapidly in regions of

high diversity and that newly evolved gen-era extend their ranges over a considerableperiod of time into peripheral regions. Inthe Indo-Pacific, a similar pattern is evidentand average generic age tends to increasein all directions away from the diversitycenter so that the model receives additionalsupport.

The form of the surfaces best fitting theaverage generic age data, like those bestfitting generic diversity data, suggests thatevolutionary activity in each ocean is cen-tered north of the equator, in close associa-tion with positive thermal anomalies andin association with the region that appearsqualitatively, to offer the maximum area ofsea floor suitable for hermatypic coralgrowth. It is interesting to note as a furtherindication of the isolation of the Atlantic andIndo-Pacific faunas, that the oldest averagegeneric age observed in the Indo-Pacific(44 million years at Natal) is younger than



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FIG. 8.—Average generic age data for hermatypic corals at our control stations shown as raw data.Omitted from the data are stations 47, 48, and 49 in the Atlantic which because they have each yieldedonly one genus do not give useful average age information.

FIG. 9.—Second order nonorthogonal polynomial surfaces fitted to raw data for average generic ageseen in figure 8. Atlantic and Indo-Pacific distributions have been fitted separately. The coefficient ofdetermination for the Pacific is 0.502 and that for the Atlantic 0.528.



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FIG. 10.—Percent of genera at each of our control stations which have "0 age," that is have no fossilrecord. The "0" contour in the main Atlantic Ocean Basin reflects the existence of very small faunas (3of the 4 stations yielded only 1 genus) all members of which have a fossil record.

FIG. 11.—Contour map of average generic age hermatypic corals when all genera having "0 age" (nofossil record) are omitted from the calculations.



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ATLANTIC

PACIFIC

10 20 30 40 50GENERIC DIVERSITY

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FIG. 12.—Relationships between diversity andaverage generic age shown separately for the Indo-Pacific and for the Atlantic (for which three pointshaving only one genus each and thus not yieldinga useful average generic age have been omitted).

the youngest average generic age encoun-tered for an Atlantic station (Barbados, 49million years). In the Indo-Pacific, theyoungest average generic age encountered(Celebes-Philippines, 23-24 million years)is about 50% that of the youngest in the At-lantic. One may conclude that if survivor-ship is indeed the reciprocal of evolutionaryrate, as Simpson (1953) suggested, then theevolution of hermatypic coral genera hasbeen proceeding about twice as fast in theIndo-Pacific as in the Atlantic.

The examination of average generic agedistributions provides support for a generalrelationship between diversity and rate ofevolution, and it is perhaps well to considerthe reliability of the observed age pattern.If one examines the age-structure of ahighly diverse station, such as the MaldiveIslands, one discovers that twelve of the

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JAMAICA

BERMUDA

AGE IN MILLION YEARS

FIG. 13.—Age-structure of the generic assem-blage of hermatypic corals at Jamaica, a station inthe high diversity-low average generic age centerin the Caribbean, and at Bermuda a marginal sta-tion well outside the Caribbean center.

fifty-six reported genera (21.5%) do nothave a fossil record (that is have "0" age).It is possible that this condition reflectspoor sampling of the fossil record in trop-ical regions, and is caused by an unusuallylarge number of genera for which no fossilrecord has been found rather than by anunusually large number of very young gen-era. The time of first occurrence fromwhich the age of each genus has been de-termined is not dependent on first occur-rence of a fossil at each individual samplingstation, but rather on the first known oc-currence anywhere in the world. Thisshould tend to alleviate the possible prob-lem of poor sampling in the present tropicsbecause during the Cenozoic the tropicsextended far into regions which are tem-perate today. The most satisfactory dem-onstration that the concentration of "0"age genera in centers of diversity does notseverely alter the average age picture isobserved in the well controlled CaribbeanRegion of the Atlantic when one maps thepercentage of genera at each station whichhave "0" age (Fig. 10). It may be seen inthis figure that in the region shown in Fig.9 to exhibit the lowest average generic ages,the percentage of genera with "0" age isactually lower than it is in the peripheralCaribbean area with much higher averageages. One can, in fact, omit from the cal-



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culations of average generic age all of thegenera having "0" age and when this isdone, as Fig. 11 shows, a strong trend ofincreasing generic age away from theCaribbean center remains. In the Indo-Pacific, the situation is less clear, but thesimilarity of the trend to that found in theAtlantic suggests that it is real.

In the Atlantic and Indo-Pacific, there isa general relationship between diversity andaverage generic age (Fig. 12) which in-dicates that high levels of diversity are cor-related with the presence of a large pro-portion of relatively young (i.e., recentlyevolved) genera. In more detail the phe-nomenon may be seen by comparing the per-centages of genera falling into each ageclass in a station from the Caribbean di-versity center (Jamaica) with similar datafor a peripheral station (Bermuda) as seenin Fig. 13. The same ancient genera tendto be present in both stations while generain the 0 to 50 million year age class aredominant in Jamaica but unimportant atBermuda. This kind of age structure pat-tern suggests that genera evolve in thehigh diversity regions and, through time,extend their ranges in the peripheral regionsof higher environmental stress. If such amodel is correct, it has implications for therelative age scale so widely used in geologybecause "time planes," as defined by thefirst occurrences of certain index genera,would inevitably transgress time as onedealt with sections increasingly distant fromthe center of origin. There are as yet nodata that allow us to determine accuratelythe magnitude of this time transgression ofbiostratigraphic "time planes." It could belarge enough to be significant, becausespread of organisms has to be regarded notmerely as migration, which could be geolog-ically instantaneous, but as migration acrossenvironmental stress barriers, which may re-quire considerable adaptation.

CONCLUSIONS

The general distribution pattern seenhere in generic diversity and in average ge-neric age suggests that the capture of solar

energy is not the principal control of coraldiversity or evolution, despite the depen-dence of hermatypic forms on zooxanthellaefor vigorous growth, because in both the At-lantic and Indo-Pacific the diversity centers,as seen in simple form, are north of theequator. It is possible, however, that thisfactor becomes important only when criticalthresholds are exceeded and that the di-versity centers observed are close enoughto the equator so as not to exceed thesecritical thresholds. A strong relationship be-tween diversity, evolutionary rates and tem-perature exists in both oceans, not only interms of the displacement to the north of theequator, but also in terms of displacementtoward the western sides of oceans. Theaffect of area of sea floor suitable for her-matypic coral growth is more difficult toconsider because accurate area data are notavailable, but a relationship between di-versity, evolutionary rate and estimatedfavorable area seems to exist. The observa-tion that evolutionary rates in the Indo-Pacific, with a presumed greater area offavorable sea floor, are about twice thoseencountered in the Atlantic, while tempera-ture in the two cases varies only by a fewdegrees, suggests a strong "area affect" inevolutionary rates.

REFERENCES

BOHNECKE, G. 1936. Temperate, Salzgehalt undDichte an der Oberflache des Atlantic Ozeans,Meteor-Werk, 5: L. 1.

BONHAM-CARTER, G. F. 1967. Fortran IV pro-gram for Q-mode cluster analysis of nonquantita-tive data using IBM 7050/7094 Computers,Kansas Geol. Survey, Comp. Contr. 17:1-28.

DARLINGTON, P. J. 1957. Zoogeography: theGeographical Distribution of Animals, JohnWiley and Sons, Inc.

PRESTON, F. W. 1962. The canonical distribu-tion of commonness and rarity, Ecology 43:185-215.

SIMPSON, G. G. 1953. The major features of evo-lution, Columbia University Press, N. Y.

WELLS, J. W. 1954. Recent corals of the Mar-shall Islands, U.S.G.S., Prof. Pap. 260 1:385-459.

WELLS, J. W. 1955. A survey of the distributionof reef coral genera in the Great Barrier Reefregion: Repts. Great Barrier Reef Committee, 6:pt. 2, 21-29, chart.

WELLS, J. W. 1968. Data in Stehli, F. G., Taxo-



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nomic Diversity Gradients i1—The Recent Model, invironment, Drake ed., Yale

n Pole Location: Pt.EvolutionL and En-

University Press.

(Received June 11,1970.)

APPENDIX A

Key to numerically code sample stations used inthis study and shown in figs.

Location Generic1 = Philippines2 = Celebes3 = Palau Island4 = China Sea5 = Maldive Islands6 = Marshall Islands7 = Red Sea8 = Ryukyu Island9 = Singapore

10 = Fiji Island11 = Caroline Islands12 = Formosa13 = Batavia14 = Somoa Island15 = Kyushu-shi16 = New Caledonia17 = Ceylon18 = Ogasawara Island19 = Mascarene Island20 = Honshu21 = Zanzibar22 = Mergui Arch23 = Seychelles24 = Mariana Is.25 = Tonga Is.26 = Line Is.

1 and 6.

AverageDiversity Generic Age5755555456525451474746454446424940363734373148312923

23.623.424.224.224.826.326.125.128.325.928.028.225.627.927.926.426.726.832.228.632.329.527.533.437.029.7

27 = Cocos-(Keeling) Is.28 = Tuamotu29 = Wake30 = Hawaii31 = Lord Howe Is.32 = Persian Gulf33 = Natal34 = Moreton Bay35 = S.W. Australia36 = Johnston37 = Marquesas38 = Kermadec Is.39 = Panama40 = Sydney41 = Galapagos42 = S.W. Madagascar43 = Easter Is.44 = Midway45 = Guam46 = Tahiti47 = Madeira Is.48 = So. Trinidad49 = Azores50 = Cape Verde Is.51 = Gulf of Guinea52 = Mexico53 = Brazil54 = Bermuda55 = Venezuela56 = Bahama57 = Panama58 = Curacao59 = Barbados60 = Puerto Rico61 = Florida62 = Cuba63 = Jamaica

231917141315131212779

1076

394

114627

111689

1015151921202023232324

30.034.136.533.938.334.843.735.033.837.630.942.233.831.137.328.742.827.828.628.56747

1279589597571655357494753535352



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Diversity and Age Patterns in Hermatypic Corals