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Territorial defense by the herbivorous damselfishes, Stegastes nigricans and S. lividus, benefits the hermatypic corals growing inside their territories. Coral diversity was significantly higher inside damselfish territories at 2 sites in Moorea, French Polynesia, and at 1 site in Guam, Mariana Islands. In Guam, this pattern was stable for at least 10 yr. Certain coral species, including Pocillopora damicornis, were found only inside damselfish territories at these sites. All fishes, including coral-feeding butterflyfishes, approaching territories of Stegastes spp. were vigorously chased. Colonies of P. damicornis inside territories were not preyed upon by any potential intruders. When transplanted outside of territories, colonies of P. damicornis were preyed upon rapidly by several species of butterflyfishes. Feeding rates of individual butterflyfishes on transplanted P. damicornis colonies were initially very high and decreased significantly over 30 min as accessible coral tissue was removed. In contrast, transplanted Porites rus, a major component of the reef outside of territories in Guam, attracted no predators. This study suggests that territorial damselfishes provide an associational defense for certain coral species that live inside their territories, whereas these species settling outside territories experience heavy predation, resulting in suppressed growth and negligible survival. This demonstrates the potential importance of this type of positive indirect interaction between species in structuring coral reef communities, that protection from predation by coral-feeding fishes is a mechanism by which damselfishes can enhance coral diversity on reefs, and that this association can remain stable for many years.

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Diseases of marine organisms, and corals in particular, have been reported with increasing frequency over the past 3 decades. Although little is known about resistance to disease in corals, a potential mechanism of defense is the production of antimicrobial compounds that protect corals from microbial pathogens. We assessed the antibacterial activity of crude aqueous extracts from 3 common Hawaiian reef corals, Montipora capitata, Porites lobata, and Pocillopora meandrina, against 9 strains of bacteria. The bacteria selected included known coral pathogens, potential marine pathogens found in human waste, and bacteria previously identified from the surfaces of Hawaiian corals. Extracts from all 3 coral species exhibited high levels of antibacterial activity. Overall, 54.3% of assays resulted in bacterial growth inhibition. In addition, 9.9% of extracts stimulated the growth of certain bacteria. Although levels of overall antibacterial activity did not differ significantly among coral species (ranging from 40.7% of assays for P. meandrina to 63.0% of assays for M. capitata), the 3 species exhibited a high degree of selectivity as demonstrated by significant variability in their effects toward specific bacteria. Extracts also exhibited intraspecific variability, both within and between sites, and significant site-by-species interactions were observed against 6 of 9 bacterial strains. In M. capitata, healthy corals had significantly higher levels of antibacterial activity than those affected by Montipora White Syndrome, and affected tissues had significantly higher levels of antibacterial activity than unaffected tissues. Variability in antibacterial activity of Hawaiian corals may, in part, explain differential susceptibility to disease at the colony, species, or population level.

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Scleractinian corals appear to be increasingly susceptible to pathogenic diseases, yet it is poorly understood why certain individuals, populations or species are more susceptible to diseases than others. Clearly an understanding of mechanisms of disease resistance in corals is essential to our understanding of patterns of disease incidence and virulence; this work must begin by examining the colony and population levels of organization. The Caribbean coral Siderastrea siderea exhibits variability in susceptibility to dark spot syndrome (DSS), a disease of unknown origin that can result in tissue necrosis. On the reef scale, variability in DSS prevalence in S. siderea occurred through time, but was not correlated with site, seawater temperature or depth. We monitored colonies of S. siderea affected by DSS, as well as their nearest neighbor controls, for 2 years in the Bahamas and found a marked decline in extent of DSS infection in October of both years. A preliminary survey of antimicrobial activity in S. siderea indicated selective activity against certain ecologically relevant bacteria. To assess whether changes in chemical defenses were responsible for the observed temporal variability in DSS prevalence, we sampled S. siderea for qualitative and quantitative analysis of chemical variability between resistant and susceptible colonies of S. siderea. These data suggest that phenotypic plasticity in antimicrobial activity may impact microbial settlement and/or survival.

Although hybridization is a relatively widespread phenomenon in marine ecosystems, the ecological importance of hybrids is poorly understood. As crosses between two distinct genotypes, hybrids might express fitness characteristics similar to either parent species, or they might produce unique phenotypic attributes that make them more or less fit than either parent species. We identified a potential hybrid zone among soft corals in a backreef community on Guam, where the broadcast-spawning species Sinularia maxima and S. polydactyla co-occur. Morphological and chemical traits confirmed the intermediate and unique status of the putative hybrid. Laboratory cross-fertilization experiments using S. maxima and S. polydactyla gametes demonstrated that barriers to hybridization are absent and that the laboratory-reared hybrids developed specific characteristics identified in adult hybrid field populations. Changes in the populations of each parent species and the hybrid were monitored over a decade. While populations of the parent species declined significantly, the hybrid population doubled in percent cover over this period.We assessed responses to abiotic (sedimentation) and biotic (competition and predation) factors to determine whether the two parent species or the hybrids exhibited greater fitness. While sedimentation alone could not explain the decline in field populations of the parent species, sediment effects exact a significant cost to these soft corals over narrow spatial scales. Competition between the parent and hybrid soft corals explains some of the changes in the distribution and abundance of these species through time. The extracts of S. polydactyla had no effect against congeners, whereas those of S. maxima and the hybrid caused significant tissue necrosis. Field predation was significantly lower on the hybrid than on either parent species. Laboratory feeding assays demonstrated that the extracts of the hybrid were significantly more deterrent than either S. maxima or S. polydactyla. Comparisons of the bioactive metabolites from the parent species, a hybrid compound, and a semi-purified mixture indicated that the hybrid is more deterrent than either of the parent species. Our results indicate that the hybrid Sinularia maxima 3 polydactyla exhibits greater fitness than the parent species and suggests that hybrid vigor may have implications for coral reef resilience.