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.G.L. Koh, H. Sweatman J. Exp. Mar. Biol. Ecol. 251 2000 141 –160
1. Introduction
Competition among sessile organisms for space on coral reefs can be intense. Competitive interactions are frequently manifested as overgrowth of adjacent individuals
and epibiosis. Apart from the fundamental requirement for space in which to grow, sessile marine invertebrates depend on unobstructed water flow to obtain food. In
addition to being filter-feeders, some also possess symbiotic algae that require access to sunlight. Hence it is necessary for these organisms to avoid encroachment and
overgrowth by neighbors. Organisms that have slow growth rates will be overgrown unless they have some mechanisms for resisting invasion by faster growing organisms.
These alternative means of competing for space include the use of spines Stebbing, 1973; Harvell, 1998, sweeper tentacles, mesenterial filaments Lang and Chornesky,
1990 and allelopathy Sullivan et al., 1983. Allelopathy has been studied in sponges Thompson, 1985; Thompson et al., 1985; Nishiyama and Bakus, 1999 and soft corals
Coll, 1992; Maida et al., 1995a,b but has received little attention in scleractinian corals.
Compounds involved in allelopathic interactions range in their action from repellents to toxins and vary in the distances over which they act. Some allelochemicals are soluble
or volatile so as to diffuse away from the source Hadfield and Scheuer, 1985; Slattery et al., 1997. Others may be deposited on the surface of the organism and act by direct
contact with potential competitors Schmitt et al., 1995. Dispersion of allelochemicals provides the obvious advantages of increasing the range of influence but these
advantages may be limited by additional costs involved in maintaining active con- centrations of molecules. If the molecules can bind to surrounding substrates and not be
easily advected away, less would need to be secreted over time. In the marine environment water-soluble compounds diffuse readily whereas compounds which are
insoluble in water would have a longer residence time Woodin, 1993. Allelochemicals allow the producer to expand or prevent neighboring organisms from encroaching on the
space it occupies. For example, tropical soft corals and sponges have been shown to release allelochemicals that kill tissue of adjacent scleractinian corals Coll et al., 1982;
Sammarco et al., 1983; Sullivan et al., 1983; Porter and Targett, 1988; Nishiyama and Bakus, 1999.
The cases so far have concerned competitive interactions among adults; allelochemi- cals are also involved in interactions with larvae. In the marine environment, larvae of
sessile organisms respond both positively and negatively to settlement cues Kato et al., 1975; Hadfield and Pennington, 1990; Pawlik, 1992; Rodriguez et al., 1993; Fleck and
Fitt, 1999. By hindering the settlement of larvae, allelochemicals can prevent surface fouling and the recruitment of potential competitors. Various benthic marine organisms
produce bioactive metabolites that inhibit larval settlement Young and Chia, 1981; Davis and Wright, 1990; Butler et al., 1996 or kill the larvae Walters et al., 1996;
Fearon and Cameron, 1996, 1997. If larvae of potential competitors are not able to settle in the vicinity, interactions with adult competitors will be avoided. The scleracti-
nian species Porites compressa is known to release a water-soluble compound that induces larvae of the nudibranch Phestilla sibogae to metamorphose Hadfield, 1977,
1978, 1984; Hadfield and Scheuer, 1985; Hadfield and Pennington, 1990. However, the
E .G.L. Koh, H. Sweatman J. Exp. Mar. Biol. Ecol. 251 2000 141 –160
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ability of scleractinian corals to inhibit the settlement of invertebrate larvae is only starting to be investigated.
The soft corals Sinularia flexibilis and Sarcophyton glaucum secrete toxic secondary metabolites into the water Coll et al., 1982. Settlement of scleractinian larvae around
these two species were inhibited, suggesting an allelopathic function for the secreted compounds Maida et al., 1995a,b. Bioactive compounds have been detected in
scleractinians Fusetani et al., 1986; Rashid et al., 1995; Koh, 1997, some of which could function as allelochemicals. In a study of the antimicrobial activity of extracts
from 100 scleractinian species, Tubastraea faulkneri inhibited the most species of microbes Koh, 1997. Tubastraea faulkneri may also produce compounds that are toxic
to coral competitors. T
. faulkneri is a non-zooxanthellate coral of the family De- ndrophylliidae and it inhabits overhangs and vertical surfaces. It has a reported depth
range of 3–7 m Wells, 1982, but it occurs from shallow reef flats down to at least 15 m personal observations. This slow-growing species shares the shallower parts of its
range with other faster growing zooxanthellate species of corals, and therefore is likely to experience interspecific competition. The use of allelochemicals that inhibit settlement
and growth of coral competitors would aid T
. faulkneri in persisting among the faster growing species.
The present study investigated the hypothesis that a coral can produce chemicals that are toxic to the larvae of other coral species that are potential competitors. We examined
the effects of natural products from Tubastraea faulkneri on its own larvae and the larvae of 11 other sympatric coral species. The specific aims of this study were: a to
determine if natural metabolites produced by T . faulkneri are deleterious to larvae of
other scleractinian coral species, b to evaluate the likelihood of larvae encountering these compounds at relevant concentrations in their natural environment, and c to
characterize the bioactive compounds in T . faulkneri.
2. Materials and methods
