Chapter 8

COMPETITION BETWEEN SCLERACTINIAN REEF CORALS ­A REVIEW OF MECHANISMS AND EFFECTS

JUDITH C. LANG and ELIZABETH A. CHORNESKY

"I rcturncd, and saw undcr the sun, that the race is not to the swift, nor the battle to thc strong ... but timc and chancchappcncth to them all."

Ecclesiastes 9: II

INTRODUCTION

Tropical coral reefs are dominated physicallyby long-lived, sedentary organisms. The predomi­nantly colonial, zooxanthellate, scleractinian reefcorals assume many shapes, ranging from upright"trees", "tables" or "shrubs", to sprawling"crusts" and "plates" (Dana, 1846). Maximumsize clearly differs among species, as do rates ofgrowth, reproduction, other life history character­istics and the range of habitats occupied (Dana,1846; Connell, 1973; Bak and Luckhurst, 1980;Hughes and Jackson, 1985; Barnes and Chalker,

Ch. 6; Harrison and Wallace, Ch. 7). Despitethese interspecific innate differences among spe­cies, for any given species the successful functionand survival of an individual (sensli solitary polypor separate colony) generally varies with its size.For example, larger colonies potentially are morefecund and less susceptible to whole-colonymortality (Mayer, 1918; Loya, 1976a; Bak andEngel, 1979; Rinkevich and Loya, 1979; Hughesand Jackson, 1980; Kojis and Quinn, 1981;Woodley et aI., 1981; but see Hughes, 1985;Holloran, 1986). Colony size, in turn, is related toa reef coral's ability to acquire and maintainliving space.

Reef corals initially capture space by thesettlement of planula larvae or by the attachmentof fragments or "buds" (see reviews by Connell,1973; Highsmith, 1982; Harrison and Wallace,Ch. 7). By differentially oriented growth, theyexpand two-dimensionally across the substratumor in three dimensions into the overlying watercolumn. The potential longevity and large sizes ofmany reef corals, together with rapid growth bysome species and the input of recruits (Dana,

209

1846; Buddemeier and Kinzie, 1976; Hughes andJackson, 1985; Potts et aI., 1985; Barnes andChalker, Ch. 6; Harrison and Wallace, Ch. 7),eventually produces crowding in some reef com­munities.

Neighbouring reef corals which are in closeproximity may affect one another. Some re­sponses (such as intraspecific filling or fusion)cause no obvious harm, and perhaps have neutralor even beneficial effects (but see below, p. 232).Interactions which are deleterious to one or both

reef corals usually are considered to be a form ofcompetition for space (as in Birch, 1957). Theextent to which such reactions include competi­tion for light or nutrients associated with space onor above the substratum remains to be established

(Johannes, 1974; Porter, 1976; Wellington, 1982;Goodall, 1986).

Connell's (1973) distinction between what hetermed "direct" and "indirect" spatial competi­tion has been widely adopted in the literature.During direct competition, the soft tissues ofadjacent reef corals come into physical contact.Indirect competition occurs when an individualwith an overtopping morphology (Porter, 1974)expands above an underlying reef coral withouttouching the latter. Some reef corals benefit fromboth forms of competition: for instance, thespecies of Pocillopora which overtop and directlyattack those of Pavona (Wellington, 1980). Ingeneral, the spatial requirements of two reefcorals necessarily overlap only under conditionsof direct competition.

For many years, overgrown skeletons alongcoral-coral interfaces and skeletal regions lackingsoft tissues, either beside or beneath other reefcorals, have been interpreted as resulting from

210

spatial competition (Briiggemann, 1877; Bernard,1897; Gravier, 1910), especially in crowded reefhabitats (Mayer, 1918; Manton and Stephenson,1935). At present, the role of such competition instructuring reef coral communities is being vigor­ously questioned (compare, for example, Connell,1978; Bak et aI., 1982; Bradbury and Young, 1982;Sheppard, 1982a; Glynn and Wellington, 1983;Huston, 1985a; Rinkevich and Loya, 1985a;Wellington and Trench, 1985). Various semanticand conceptual difficulties complicate this debate.Confusion exists, in part, because the descriptionsof some mechanisms by which reef corals arepresumed to compete are incomplete; variousdifferences among species sometimes are ignoredwhen the effects of competition are assessed innature; data from particular habitats or biogeo­graphic regions sometimes are extrapolated to allreefs, regardless of local variations; and the realcosts of competing in terms of individual fitness(sellSu Jackson, 1983; Chornesky, 1984, 1989; Buss,1986) are unknown for any reef coral.

We hope to clarify these problems by reviewingthe literature on competition between scleractinianreef corals. In turn we discuss:

Methods oj studying competition-at the levelof individual reef corals and at the communitylevel;Mechanisms oj competition-how reef corals di­rectly or indirectly harm neighbouring reef corals;Effects oj competition on individuals-injured softtissues, altered growth, or reduced fecundityresulting from direct and indirect competition;Factors a.ffecting competition between individuals­biotic and abiotic variables which influence the

course and outcome of individual competitiveinteractions;Potential efj'ects oj competitioll ill reeJ coral CO/llmu­nities-complexities in the combined effects ofdirect and indirect competition.

Reef corals also compete with algae and withother sessile animals (Sammarco et aI., 1985;Bakus et aI., 1986; Porter and Targett, 1988).Numerous, direct and indirect mutualistic, preda­tor-prey and host-parasite relationships furtherconnect them with the other biotic components ofreef communities (see many chapters in thisvolume). Our consideration of these diverse ecolo­gical activities is restricted to their impact on thecompetitive interactions of reef corals.

J.e. LANG AND E.A. CIIORNESKY

METHODS OF STUDYING COMPETITION BETWEEN

REEF CORALS

Various techniques have been used to studyeither the mechanisms by which competitionoccurs between individual reef corals or its effects

on the organization of reef communities. Eachmethod described in this section yields someunique information. In general, robust interpreta­tions are likely to be based on the results of severaldifferent approaches (see review by Diamond,1986).

Individual-level studies

In nature, reef corals whose skeletons lack softtissues, or which have been overgrown or over­topped by others, are often described as inferiorspatial competitors (see, for instance, Glynn et aI.,1972; Lang, 1973; Porter, In4; Sheppard, 1979,1982b; Wellington, 1980; Bak et aI., 1982; Cope1982; Rinkevich and Loya, 1983a; Logan, 1984;Chadwick, 1988). The potential inaccuracies ofthese "instantaneous" diagnoses may have beenignored in some studies. For example, woundscaused by scavengers, predators, other groups ofsedentary organisms (see p. 243 and Glynn,Ch. 13), or by pathogens and parasites (Jokiel andTownsley, 1974; Peters, 1984a) in error can beattributed to nearby reef corals. Moreover, reefcorals overgrowing the exposed skeletal surfacesof their neighbours have not necessarily injured.the latter. When observations are based on

standardized distances (usually up to 5 cm fromthe margins for direct interactions), differencesamong species in the spatial scales over whichcompetition occurs (p. 226) may be overlooked.Similarly, using the natural gaps between reefcorals to infer their competitive activity or"interactive reach" may be unjustified in theabsence of independent evidence that interactionsare possible under these conditions. Errors ofextrapolation to long-term effects are likely tooccur, especially when the observable outcomes ofcompetition change over time (Chornesky, 1984,1989).

Such difficulties are minimized when repetitive,time-series observations (as in Lang, 1971, 1973;Connell, 1973, 1976, 1979; Bak and Engel, 1979;Porter et aI., 1981, 1982; Bothwell, 1983; Chor-

i

!

~

••

COMPETITION BETWEEN SCLERACTINIAN REEF CORALS

nesky, 1983, 1989; Rinkevich and Loya, 1983a;Sheppard, 1985) are combined with an understand­ing both of the mechanisms by which reef coralscompete in nature and of other potential sources ofnatural mortality. Nevertheless, when data arebased on plan-view photographs taken fromabove, direct competition occurring along the sidesand bases of erect reef corals may go unrecorded,and presumptions concerning the fate of indi­viduals which have been overtopped may beincorrect. In general, small (several mm/year)changes along the margins of competitors arelikely to be overlooked (Chornesky, 1989). Obser­vations may need to be extended over years todecades for those individuals whose interactions

are either gradual or intermittent (as in Connell,1973, 1976, 1979).

Some descriptions of the mechanisms by whichreef corals compete come from nocturnal observa­tions of natural encounters (Lang, 1973; Dustan,1975; Chornesky, 1983). Most information, how­ever, is derived from manipulative experiments.Direct interactions are initiated by bringing tworeef corals into close proximity under conditions ofeither intermittent or continuous contact, or bytying their intact surfaces together to form para­biotic grafts. Ensuing events are observed with thenaked eye and, sometimes, at higher levels ofmagnification in the laboratory. In the onlymanipulative field study of indirect competitionpublished to date, Stimson (1985) placed reef coralfragments beneath canopy-forming colonies ofAcropora.

The distances separating certain individuals candetermine how each responds to the other or thespeed of any subsequent reactions (p. 243). Hence,manipulations (especially graftings) may fail tosimulate nature, where many interactions betweenestablished reef corals probably are initiated asthey gradually grow into contact (Chornesky,1983; Rinkevich and Loya, 1983a). Abrupt en­counters do occur when one reef coral falls onto

another (Collins, 1978; Bothwell, 1983; Rinkevichand Loya, 1983a; Tunnicliffe, 1983; Logan, 1984;Hughes and Jackson, 1985; Chadwick, 1988), andthese conditions sometimes are approximated ingrafting experiments.

Few studies have been made with adequatereplication of treatments (either observational orexperimental - see Hulbert, 1984) or carried out

211

long enough to demonstrate any temporal rever­sals that may occur (p. 227, 239). Manipulationscan even contain built-in biases facilitatingcertain competitive responses: for example, diges­tively-dominant reef corals may be favoured inshort-term experiments and/or when contact iscontinuous (Wellington, 1980; Bak et aI., 1982;Chornesky, 1983).

Due partly to the effects of modes of observa tionor of experimental design, the competitive re­sponses of some reef corals may differ betweennatural interactions and manipulative experiments(Lang, 1973; Dustan, 1975; Bak et aI., 1982; Cope,1982; Rinkevich and Loya, 1983a; Logan, 1984;Nakaya and Yamazato, 1985). Compare, forexample, the results of Glynn et al. (1972) andPorter (1974) with those of Glynn (1974) andWellington (1980) for contacts between specimensof Pocillopora and Pavona in the eastern Pacific.Reactions may also vary under different conditionsof experimental manipulation (Collins, 1978; Wel­lington, 1980; Bak et aI., 1982; Chornesky, 1983),as illustrated by differences among Lang (1973),Richardson et al. (1979) and Chornesky andWilliams (1983) for Montastraea cavernosa, and byRinkevich and Loya (1983a, 1985b) for Stylophora

pistillata. As in observations of natural inter­actions, data should be collected repeatedly during

experimental manipulations, and at the temporaland. spatial scales which are appropriate for anygiven pair of competitors.

Community-level studies

Sometimes insight into the role of competition instructuring reef communities is sought by compar­ing specific rankings of observed or inferredcompetitive abilities with species abundance and/or diversity patterns (Lang, 1973; Porter, 1974;Connell, 1976, 1979; Maguire and Porter, 1977;Sheppard, 1979, 1980, 1981; Wellington, 1980;Porter et aI., 1981, 1982; Bothwell, 1983; Logan,1984; Sakai et aI., 1986; Yamazato and Yeemin,1986).

Potential difficulties associated with this ap­

proach include all of the reasons enumeratedabove for the occurrence of flaws in estimates of

the competitive capabilities of individual reefcorals. In addition, occasionally species rankingsderived from one, experimentally-determined,

212

~competitive mechaniStyt are contrasted with thosebased on all inferred, natural competitive effects, asthough equivalent sets of data were represented.Comparisons of reef corals from different habitatsor geographic regions rarely include a discussion offaunal variations or other factors which may affectcompetitive outcomes (pp.241-244). Unresolvedconceptual problems include ambiguities as to howcertain interactions (fusing, filling, stand-offs, evensome instances of overgrowth - see p. 232) shouldbe scored and ranked. Moreover, there is, as yetfor reef corals, no information on the energeticcosts of competition nor of its effects on naturalgrowth and reproduction (Chornesky, 1984). Pre­sent correlations between competitive ability andabundance may well be spurious, especially whenbased on a limited number of interactions for any. .gIven specIes.

The likelihood, or consequences, of competitionbetween reef corals sometimes is inferred from

detailed considerations of their spatial distribu­tions in nature (Reichelt et aI., 1986). During afour-year study, Bothwell (1983) found thatrecruitment of reef coral larvae and attachment of

coral fragments was considerably lower around themargins of digestively dominant (see below) mas­sive corals than in other areas of an Australian reef

crest. According to Sheppard (1985), unoccupiedsubstratum is proportionately more commonwithin the "interactive reach" of individual reefcorals.

Because "neighbour events" (? = potential inter­actions) between reef corals on various Indo­Pacific and western Atlantic reefs were determined

to occur randomly with respect to species, Brad­bury and Young (1982, 1983), followed by Reicheltand Bradbury (1984), have argued that competi­tion has few effects at community level. Unfortu­nately, the intervals used to indicate neighbourevents exceeded the spatial distances over whichmost reef corals are likely to interact (Sheppard,1985). In addition, "overlapping colonies" (sensll

Loya, 1972), which are especially likely to competewith each other, appear not to have been included;nor were estimates given of the statistical power ofthe tests used (see Toft and Shea, 1983; Colwelland Winkler, 1984).

J.e. LANG AND E.A. CHORNESKY

MECHANISMS OF COMPETITION IN REEF CORALS'

In this section we describe how reef corals either

directly or indirectly harm one another. Tenta­tively included here are the results of parabioticgraftings and some poorly-documented mecha­nisms of putative competition. Our focus is ondetails, in part to emphasize the diverse competi­tive behaviours of different reef corals, and becausemuch of the available information is so specificthat generalizations across taxa currently would bepremature.

Kinds of mechanisms

When physical contact occurs during directencounters, many reef corals utilize one or morecompetitive structures or secretions to injure the .soft tissues of certain opponents. In addition, somereef corals overgrow the skeletons of their neigh­bours, whose soft tissues they mayor may not havepreviously damaged (Table 8.1).

The ability to use these various mechanisms ofdirect competition differs greatly among taxa. Forexample, the mesenterial filaments which normallyare held within the coelenteric cavities of reef

corals playa vital role in digestion. Mesenterialfilaments can be extended either through tempo­rary openings in the soft tissues or through thepolyp mouths and may engage in feeding activitiesoutside the coelenteron (see review by Muscatine,1973). Many reef corals temporarily extrude theirmesenterial filaments to destroy the soft tissues ofcertain others in an extracoelenteric digestiveresponse (Fig.8.IA). Most digestively dominant(sensll Porter, 1974) reef corals are slowly growing,massive or encrusting species in the suborderFaviina (Lang, 1973).

Other soft-tissue lesions are created by thespecialized sweeper tentacles (Fig. 8.2A, B) ofcertain reef corals. Species currently known topossess these structures are distributed erraticallyamong the Scleractinia. Sweeper tentacles willdevelop after a reef coral's soft tissues have comeinto direct contact with, or are injured by, a

IGiven the lack of a slandardized terminology, the lerms which

we have employed to characterize competitive mechanisms andcompetitive outcomes conform to the most common usage as ofthe mid-1980's.

TABLE 8.1

Reported mechanisms of competition between reef corals

(A) Direct competition

Mesenterial many reef corals infilaments the families

FaviidaeMeandrinidaeM ussidae

minutes-months 0-5 cm

Scales of responseduring manipulations

tv\;.>

n0~m...,=i0z~r.1...,:Er.1r.1ZennrtT1;:0>njz>z;:0tT1tT1'":1n0;:0>ren

Hildemann et al. (l975a. b'o. 1977a); Bigger et al.(1984)

Chadwick (19867,19888)

Chornesky (1983); Hidaka and Yamazato (1984);Hidaka (l985b)

Richardson et al. (1979); Wellington (1980); Baket al. (1982); Chornesky (1983)

Chadwick (19867, 19888)

References

Bak et al. (1982)

Lang (1971, 1973); Loya (1976b); Wellington(1980); Bak et al. (1982); Cope (1982);Chornesky (1983); Logan (1984)

Lang (1970, 1971, 1973); Glynn (1974);Richardson et al. (1979); Bak et al. (1982);Logan (1984, 1985)

Sheppard (1979, 1982a)

Chornesky (1983, 1984, 1989); Sebens in: Lang(1984)

Wellington (1980); Bak et al. (1982); Chornesky(1983); Chornesky and Williams (1983)

Lang (1971, 1973); Dustan (1975); Bothwell(1983); Chornesky (1984); Logan (1985,1986a4, b4)

Wellington (1980); Bak et al. (1982); Chornesky(1983); Chornesky and Williams (1983)

laboratory manipulations;continuous polypalcontact

laboratory manipulation;intermittent polypalcontact

laboratory manipulations;parabiotic grafts9

field; naturally occurring

field manipulations;continuous polypalcontact

field manipulations;intermittent polypalcontact

laboratory manipulations;intermittent polypalcontact

field manipulations;intermittent polypalcontact

field; naturally occurring

laboratorymanipulations;continuous polypalcontact

field manipulations;continuous polypalcontact6

field; naturally occurring

Observationalconditions3

field manipulations;intermittent polypalcontact5

0-10 cm

Spatial

week-months(induction)

minutes (onceformed)

2-4 days 1-15 mm(unrestrained)

1-12 days 1-6 mm(parabiotic grafts)

Temporal

sweepertentacles

Identityof inducedcomponentl

OculinideGalaxea

fascicularis

PocilloporidaeMadracis spp.,Pocillopora spp.

PoritidaeGoniopora spp.

FungiidaeFungia scutaria,other Fungia spp.?

diverse reef corals inmany families

AgariciidaeAgaricia spp.

CaryophylliidaeEuphyllia (Fimbria­

phyllia) an coraFaviidae

MOnlastraeacavernosa

Shown to occur in J

"Sweeperpolyps"

Mucussecretions

Mechanism

Sweepertentacles

TABLE 8.1 (continued) N""'"

Scales of responseduring manipulations

~

Mechanism

Histoincom­patibility

Overgrowth 18

Shown to occur in1

reef corals in manyfamilies, e.g.

AcroporidaeAcropora spp.,Montipora spp.

AgariciidaePavona spp.

PocilloporidaePocil/opora spp.,SeriaLOpora hystrix,Stylophora pistil/ala

PoritidaePorites spp.

reef corals in manyfamilies, e.g.,

AcroporidaeAgariciidaeFaviidaeFungiidaeOculinidaePocilloporidaePoritidaeThamnasteriidae

Identityof inducedcomponent2

Temporal

days-months

weeks-months

Spatial

0-5 mm

Ocm

Observationalconditions3

field manipulations;continuous polypalcontact

field manipulations;parabiotic grafts

laboratory manipulations;parabiotic grafts

field; naturally occurring

field manipulations;continuous polypalcon tact

field manipulations;parabiotic grafts

laboratory manipulations;intermittent polypalcontact

laboratory manipulations;parabiotic grafts

References

Willis and Ayre (1985)11

Collins (1978); Rinkevich and Loya (1983a, b);Heyward and Stoddard (1985); Hunter (1985);Resing and Ayre (1985); Hunter and Kehoe(1986)12

Hildemann and Reddy (1973)13; Hildemann(197413, 197714, 197914);Hildemann et a!.(1974, 1975a, blo, 1977a14,b, C14, 197914,1980a, b1S); Raison et a!. (1976); Johnstonet a!. (1981); Bigger and Hildemann (198216);Jokiel et al (1983); Hidaka (l985a, 198617).

?Briiggemann (1877); Bernard (1897, PI. XX);Gravier (1910, 1911, figs. 20, 29, 30); Lang(1973); Hildemann et a!. (1975a, fig. 5); Potts(1976, 1978); Cope (1982); Sammarco (1982,fig. 8); Bothwell (1983); Rinkevich and Loya(1983a); Tunnicliffe (1983); Logan (1984, 1986b,figs. IC, IE); Sheppard (198519.20);Yamazatoand Yeemin (1986)

Potts (1976, 1978); Bothwell (198221); Rinkevichand Loya (1983a); Willis and Ayre (1985)

Collins (1978); Bak and Criens (1982); Rinkevichand Loya (1983a); Hunter (1985); Hunter andKekoe (198612)

Hidaka and Yamazato (198422); Hidaka(1985a2o.23, 198617)

Hildemann et a!. (1974); Bigger et a!. (19847)

(1r>Zo>zCi

rn

>ois"zm[/).;r:-<

--------------------- .

'The classification system used here is based on that proposed by Vaughan and Wells, as modified by Wells (1956).2Restricted to components which have been demonstrated to be induced; other induced competitive structures or chemicals may well exist, but their existence has not beenproven.3See pp. 211 and 243 for descriptions of how conditions of contact can affect competitive outcomes between reef corals.4lncludes a summary of data in Logan (1985).5lntermittent poJypal contact - specimens placed or cemented near each other; should touch only when their soft tissues are expanded.6Continuous polypal contact - specimens placed or cemented close together; soft tissues should be in continuous contact.7Conditions- of contact were clarified by personal communication with the author, or with the senior author of joint publications.8lncludes data in Chadwick (1986).9Parabiotic grafts - specimens tied together; soft tissues should be in continuous contact, and their skeletons may abrade each other.!Olncludes a summary of data in Hildemann et al. (l975a).11As "tissue necrosis ... incompatibility".12lncludes data in Hunter (1985).13lncludes a summary of data in Hildemann et al. (1974).14lncludes a summary of data in Hildemann et al. (I 977b).15lncludes a summary of data in Hildemann et al. (l977b, 1980a).16lncludes a summary of data in Hildemann et al. (1974, 1975a, b, 1977a, b, 1979, 1980a, b) and in Raison et al. (1976).17lncludes data in Hidaka (l985a).!80vergrowth occurring between certain corals may represent a non-competitive interaction, as discussed on p. 232.'9Within "continuing interactions ... dominant species had advanced over the subordinate".20Assignment to a particular competitive mechanism clarified by correspondence with the author.2! As unilateral "fonnation of a holdfast morphology".22As "the growing edge of one polyp partially covered the growing edge of the other polyp".23As "a small thin skeletal ridge ... it is likely that the tissue of one branch grows over the exposed skeleton of the other branch".

(B) Indirect competition

TABLE 8.1 (continued)

Scales of responseduring manipulations

no3:"Cm...,

:Jozc;,m...,~mmZUJnr­m;:0;..n::jz;;z;:0mm'"!1

no;:0;..r­UJ

References

Connell (1973,1976,1979); Shinn (1976); Porteret al. (1981); Cope (198220); Hughes andJackson (1985); Glynn (1987)

Stimson (1985)

Sheppard (1979); Rinkevich and Loya (1983a)

Lamberts (1973)

field; naturally occurring

field; naturally occurring

laboratory manipulations;no contact

field manipulations;no contact

Observationalconditions3

to 40 cm

Spatial

months

Temporal

Identityof induced

component2

Shown to occur in1

branching, foliaceousand massive/flattenedreef corals in manyfamilies, including

AcroporidaeAgariciidaeFaviidae

PocilloporidaePoritidae

implicated in reefcorals, especially inthe families

AcroporidaePocilloporidae

Mechanism

Inferred,water-bornechemicals

Overtopping(sometimescalled

overgrowth)

N'-"

216 J.e. LANG AND EA CIIORNESKY

Fig. 8.1. (A) Mesenterial filaments temporarily extruded from a polyp of the solitary Sco[)'lIIio [aceral (to left) attack the soft tissues of

its digestive subordinate. the solitary S. crt/Jellsi,,! (to right). in the laboratory at Discovery Bay. Jamaica. Similar injuries occur when

polyps of these two species grow beside each other in nature. (Photo from Lang. 1971) (13) A colony of Porites astreoides (mid-right),

which recently had been injured (white area of exposed skeletal tissues above the ruler) beside a large M)'celol'Ir)'lIialerox' (to left). at 7metres on a vertical escarpment at Rio Bueno, Jamaica. The M.lerox. which is digestively dominant to P. aslreoides during

experimental manipulations, also appears to be overgrowing this colony in the area of contact. (Photo by T.F. Goreau.)

COMPETITION BETWEEN SCLERACTINIAN REEF CORALS 217

Fig. 8.2. (A) Close-up of sweeper tentacles of Agaricia agariciles1; in the laboratory at Discovery Bay, Jamaica. Note that the sweepertentacles in this species lack a conspicuous acrosphere. (B) Sweeper tentacles, each with an enlarged, terminal (nematocyst-containing)acrosphere (arrow), in a colony of Madracis sp. (to left) which had been placed within reach of the tentacles of an Agaricia agaricile,· (toright), and have abraded the soft tissues of the latter; in the laboratory at Discovery Bay, Jamaica. (Photos by J. Cripps.)

IZlatarski (1982) has synonymized many of these species of Agaricia, Scolymia and MycelOphyilia.

218

competitor (Wellington, 1980; Bak et aI., 1982;Chornesky, 1983; Hidaka and Yamazato, 1984);hence, their use against other individuals usually isdelayed. Even MOI//as/mea eal'emosa, which rou­tinely has sweeper tentacles that do not correspondto ongoing competitive encounters, develops addi­tional ones when experimentally damaged bycongeners (Chornesky and Williams, 1983). In atleast some species, sweeper tentacles regress afterdestroying opponent tissues within their reach(Wellington, 1980; Chornesky, 1983, 1989).

Unusually long polyps are limited to relativelyfew genera of reef corals. Species of GOl/iopora

(Fig. 8.3) appear to use elongate "sweeper polyps"

J.e. LANG AND E.A. CHORNESKY

to damage their neighbours (Sheppard, 1982a).Expanded polyps of large specimens of FIII/gia

sell/aria, which in nature are not motile, deposit alayer of mucus on the adjacent surfaces of at leastcertain colonial reef corals (Fig. 8.4A, B, C). Theunderlying soft tissues of the latter subsequentlyshow visible signs of decay and slough away(Chadwick, 1988). Superficially similar responsesoccur in xenografts between FIII/gia spp. and otherreef corals (Hildemann et aI., 1975a, 1977a; Biggeret aI., 1984).

Most descriptions of histoincompatibility re­actions are based on grafting experiments. Afterbeing tied together, the soft tissues of some reef

Fig. 8.3. Expanded "sweeper polyps" (arrow) in a colony of GOl1iopora .stokesi (to left) beside an injured Meru/il1a amp/iata (to right) at

10 metres on the lagoon slope at Peros Banhos Atoll in the Chagos Archipelago. (Photo from Sheppard, 1982a.)

Fig. 8.4. (A) A colony of Pocillopora damicomis (to right) which has been injured as a result of mucus (remnants at arrows) deposited

by a polyp of FUl1giaseu/aria (to left) when it is expanded at night, after seven weeks in the laboratory at Kaneohe Bay, lIawaii. The

distance separating these reef corals is less than 5 mm. (B) A colony of MOl1tipora \wrueosa (to left), the marginal soft tissues of which

have been killed for a distance of 5 to 10 mm by mucus originating in a polyp of FUl1giasell/aria (to right) emplaced within several mm

of its border; after seven weeks in the laboratory at Kaneohe Bay. Hawaii. A narrow strand of mucus (arrow) is visible in the gap

separating the two reef corals. (C) Another colony of MOlltipora I'errueosa (to right) which was positioned at a distance in excess of

5 ml11 from a polyp of FI/Ilgia soi/aria (to left), and which has not been damaged by the latter, although a small deposit of mucus

(arrow) is adhering to its marginal soft tissues; in the laboratory at Kaneohe Bay, Hawaii. (Photos by N.E. Chadwick.)

COMPETITION BETWEEN SCLERACTINIAN REEF CORALS 219

220 J.e. LANG AND E.A. CIIORNESK Y

Fig. R.5. Unilateral histoineompatihility reaction which was developed in an allogrart of MOlllil'ol"ll I"err/lco.m, in the laboratory atKancohc Bay. Ilawaii. Arrows indicate the area or sort tissue destruction. which was restricted to a rew millimetres or the contactinlcrracc in Ihc lowcr spccimcn. (From Johnston et aI., I<JRI.)

•

corals gradually bleach (that is. lose zooxanthcllae)and then die back from the contact area (rig. 8.5).Eventually border-line gaps may develop at thegra ft interfaces wh ich, in Stylophora pist il/ata. arcup to 3() pm wide (Rinkcvich and Loya. 1983a).These graft rejection responses are particularly welldocumented within (allografts) and between (xeno­grafts) certain species of acroporid. agariciid,pocilloporid and poritid reef corals.

Lesions at parabiotic graft interfaces commonlyarc attributed to induced cytotoxic effects resultingfrom contact with the soft tissues of another reef

coral (lliidemann, 1977). The underlying bioche-

mical basis of this presumed cytotoxicity is notunderstood (Bigger and Hildemann, 1982). Analternative mechanism hypothesized for gorgo­nians is that contact stimulates an induced "auto­

destruction" response. during which the affectedcells destroy themselves (Theodor, 1970; Theodorand Senelar. 1975). According to Miiller et al.(1984) the enzyme carbonic anhydrase has a role inthese activities. Occasionally mesenterial filamentsare noted at graft interfaces [allografts of Poeil/o­

pora l'errlleosa in Hawaii (Hidaka, 1985a); allo­grafts and xenografts of Jamaican Acropora (Lang.pel's. observ.)]. Moreover. within hours of initiat-

COMPETITION BETWEEN SCLERACTINIAN REEF CORALS 221

Fig. H.6. (A) Close-up of a branch of Acropo/"il I'll/lilli/II (to left) overgrowing the corallites of another colony of A. plI/lIIlIW (to right) without

causing conspicuous soft-tissue lesions to the lalter, at about Hmetres in the mixed zone at Discovery Bay, Jamaica. Sev.:ral appendages of asmall crustacean (arrow), probably [)olllecill w'wuhopllOl"tI, are barely visible below the area of overgrowth. (Photo by J.e. Lang.) (II) The

foliaceous margin of an Agllricill /£'IIlIilidillI (to right) growing just abovc a colony of Pori/es l/.v/reoitle.\' (to left), al about 6 metres on the innerfore reef al Carrie Bow Cay, Belize. Small wounds in the soft tissues of the 1'. lIs/reoitl"s (arrows) may have been caused by sweeper tentacles,

which have bcen noted on colonies of A. {£'IIlIiji,!i1l in the lield (Chornesky, pers. observ.). (l'ho(o by E.A. Chornesky.)

I Refer 10 footnote on p. 217.

222

ing contact in allografts of Stylophora p;stillata orAcropora formosa, photosynthetically derived me­tabolites pass from dislocated branches to intactcolonies (Rinkevich and Loya, 1983b; c.J. Cros­sland, pers. commun., 1986).

Overgrowth occurs whenever one reef coralgrows on (Fig. 8.6A) or, in the case of foliaceouscolonies, immediately above (Fig. 8.6B) the skeletonof another. The overgrowth response, which occursboth within and between species, is common innature and during grafting experiments. Some reefcorals naturally encroach over. the exposed skele­tons of neighbours whose soft tissues they haveinjured previously, presumably by extracoe\entericdigestion (Lang, 1973) or with some other competi­tive mechanism (Yamazato and Yeemin, 1986).Overgrowth also follows the appearance of soft­tissue lesions in certain allografts (Hunter, 1985)and xenografts (Collins, 1978). In other encounters,the reef corals being overgrown either show no; oronly minute, amounts of soft-tissue damage at theovergrowth interface (Potts, 1976, 1978; Bak andCriens, 1982; Rinkevich and Loya, 1983a; Yam a­zato and Yeemin, 1986) and possibly die as a resultof being smothered.

By definition, indirect encounters occur at adistance. Certain "shade-intolerant" (Stimson,1985) reef corals gradually lose soft tissues whenindividuals with overtopping morphologies expandabove them. Overtoppers are comparativelyrapidly growing reef corals with branching, foliace­ous or massive-flattened shapes (Table 8.1). Theirpresence clearly imposes a physical constraint onthe vertical growth of any underlying reef corals(Loya, 1972), which may also suffer from reducedlevels of ambient light, circulating water andexogenous food particles, or from increasedamounts of sediment (Connell, 1973; Dustan et al.in Tunnifcliffe, 1983; Glynn and Wellington, 1983).

We will distinguish between the erect "canopy

J.e. LANG AND E.A. CIIORNESKY

formers" (Stimson, 1985) which invariably haveovertopping morphologies and what we are infor­mally terming "low overtoppers". The former(Fig. 8.7 A, B, C) include certain acroporid, agari­ciid, faviid, pociHoporid and poritid reef coralswhich are common in many habitats, and similarlyshaped individuals in diverse other families (see,for instance, many of the iI1ustrations in Veron andPichon, 1980). "Low overtoppers", which havefoliaceous (Fig. 8.70) and other flattened shapes,sometimes appear to assume an overtoppingmorphology in response to direct competition(Chornesky, pers. observ.; Lang, pers. observ.).

There is inferential evidence for another mech­

anism of indirect competition. Some acroporid,pocil1oporid and other reef corals perhaps releasechemicals into the surrounding sea water whichinjure individuals beyond the reach of their ownsoft tissues (Lamberts, 1973; Sheppard, 1979, andpers. commun., 1986; Rinkevich and Loya 1983a).To date only alcyonaceans and a sponge have beenproven to release potential toxins under experi­mental conditions which approximate those innature (see, for instance, Coli et aI., 1982; Porterand Targett, 1988).

Initiation

Direct competitive interactions generally appearto be initiated by contact between the soft tissuesof reefcorals. For many individuals, initial contactprobably consists of the sporadic brushing ofexpanded tentacles or polyps. When, however, onereef coral tumbles onto another (p.211), theepidermal surfaces of either or both may beroughly abraded or injured. Following contact(however initiated), each competitor potentiallyreceives both mechanical (= tactile) and chemicalinformation about the other (Chornesky, 1983).

How these stimuli are utilized by reef corals is

•Fig. 8.7.(A) Tabular colonies or Acropora cythera (mid-Iert and lower right) and or A. hyacilllhus (mid-right) with overtopping morphologies,

at 15 metres on the windward, rore-reer terrace ofT Jinimi Island at Enewetak Atoll. (Photo by L.S. Land.) (B) Branches or Acropora

cen'icomis partially shading massive colonies or MOllta~traea a/Illu/aris, at 10 metres on the inner rore-reerterrace at Discovery Bay, Jamaica.(Photo by P. Dustan.) (C) Intraclonal overtopping and resultant selr-shading or a roliaceous colony or Agaricia telluifolial, at about 8 metres

on the inner rore reer at Carrie Bow Cay, Belize. (Photo by E.A. Chornesky.) (D) A roliaceous Leptoseri.~ (= llelioseris) ClIcul/ata (to right)which is partially overtopping (upper centre) a recumbent colony or Agaricia /amarckil at about 18 metres on an incised river gorge at west

Rio Bueno, Jamaica. Elsewhere (right or centre), the margin or the L. cUClil/ata is arching up, perhaps to initiate rurther overtopping or the

A. /amarcki. Sweeper tentacles have been observed on colonies or A./amarcki in the field (Chornesky. pers. obs.), and perhaps were

responsible ror the loss or soft tissues along the border or the L. cucul/ata in this area. (Photo by J.e. Lang.)

1 Rerer to rootnote on p. 217.