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Coral Reefs

Coral Reefs (1983) 1:173-178

@ Springer-Verlag 1983

Competitive Strategies of Soft Corals (Coelenterata: Octocorallia): Allelopathic Effects on Selected Scleractinian Corals P.W. Sammarco 1, J.C. Coll 2, S. La Barre 2 and B. Willis 3 1 Australian Institute of Marine Science, P. M. B. No. 3, M. S. O. Townsville 4810, Australia 2 Department of Chemistry and Biochemistry, and 3 School of Biological Sciences, James Cook University of North Queensland, Townsville 4811, Queensland, Australia Received 17 October 1981; accepted 27 July 1982

Summary. A striking retardation of growth was observed in the scleractinian coral Pavona cactus (Coelenterata: Scleractinia) growing in the vicinity of the soft coral SinulariaflexibiIis (Coelenterata: Alcyonacea). More extensive field observations of naturally occurring interactions between soft corals and scleractinian corals suggested that members of the former group can be the more effective competitors for space on hard substrate. To test this hypothesis, colonies of three soft corals, Lobophytum paucijTorurn, Sinularia pavida, and Xenia sp. aff. danae, were relocated next to stands of two hard corals, Pavona cactus and Porites andrewsi (= Porites cylindrica), and compared with undisturbed control areas. In areas where soft corals and scleractinian corals were in direct contact, significantly high levels of local mortality in the latter occurred in three of the six interaction pairs. One soft coral, L. pauciflorum, also caused extensive and significant mortality in Porites andrewsi in a non-contact situation. The scleractinian corals had no effect on the soft corals considered here. These results indicate that soft corals can effectively compete for space against hard corals. Furthermore, it is inferred that toxic exudates from the soft coral might be responsible for causing localized mortality in hard corals, since extensive mortality occurred in certain cases in the absence of contact. Competitive abilities of soft corms in interactions with hard corals varied in a species-specific manner. Susceptibility of hard corals to competitive mechanisms utilized by soft corals, particularly allelopathic ones, likewise varied species-specifically. It is commonly believed that the adaptive value of toxic compounds in soft corals stems from their effectiveness as a chemical defence mechanism in predator-prey interactions. This study has demonstrated their further role as allelopathic agents in interspecific competitive interactions. Introduction

Predation and competition for space are among the most important factors limiting populations on marine hard

substrata (Connell 1961; Paine 1966; Dayton 1971). It has been proposed that these biotic selective forces have resulted in the evolution of defense mechanisms such as toxicity in sessile or sedentary marine invertebrates, serving to reduce predation pressure on a given species (Bakus 1971). With respect to competition for space, sessile colonial marine organisms utilise various strategies for defending space and expanding into areas occupied by other organisms (Jackson 1977), some of which are aggressive (e.g. Lang 1971, 1973). Coll et al. (1982b) have provided evidence that approximately half the soft corals (Coelenterata: A1cyonacea) from the central region of the Great Barrier Reef contain toxic compounds. Bakus demonstrated that toxicity is an important defensive strategy in sessile (Bakus and Thun 1979) and sedentary (Bakus 1974, 1976; Bakus and Green 1974) organisms against predation and suggested that toxicity afforded a considerable adaptive advantage to the organisms possessing it (Bakus 1981). Several workers, notably Jackson and Buss (1975), Jackson (1977) and Sheppard (1979) have suggested a second role for toxicity in sessile organisms: they have implicated the release of toxic substances by sessile organisms into seawater as a mechanism for maintaining living space. Whittaker and Feeny (1971) have proposed that allelopathy plays an important role in niche differentiation and the control of community structure in numerous ecosystems. Here, we define allelopathy as "direct inhibition of one species by another using noxious or toxic chemicals" (Ricklefs 1979; also see Muller 1966; Muller et al. 1968; Gove 1976; McNaughton and Wolf 1979). Coll et al. (1982a) have isolated toxic terpenes in the seawater surrounding several soft corals, including Sinulariaflexibilis Quoy and Gaimard (1883) (Coelenterata: Alcyonacea). One of us (B. W.), while studying different growth forms in Pavona cactus F6rskal (1975) (Coelenterata: Scleractinia), noted growth inhibition and mortality of this coral in the vicinity of the common soft coral S. Jlexibilis. Stunting of growth in P. cactus occurred up to 30 cm away from the base of the soft coral and local mor-

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Fig. 1. Field observation showing a gradient of stunted growth in Pavona cactus (Scleractinia) surrounding a colony of SinuIariaJlexibilis (Alcyonacea), Photo by B. Willis

tality was observed at