Chapter

Competition Among Sessile Organisms on Coral Reefs

DOI: 10.1007/978-94-007-0114-4_20

ABSTRACT Competition among sessile organisms is a major process on coral reefs, and is becoming more important as anthropogenic disturbances
cause shifts in dominance to non-reef builders such as macroalgae, soft corals, ascidians, and corallimorpharians. Long-term
monitoring and field experiments have demonstrated that competition for limited space can exert major impacts on reef biodiversity
and community composition across habitats and regions. Recent experiments also reveal increasingly important roles of allelopathic
chemicals and the alteration of associated microbes in shaping competitive outcomes among benthic space occupiers. Competition
impacts the recruitment, growth, and mortality of sessile reef organisms and alters their population dynamics. Co-settlement
and aggregation of conspecific coral colonies may lead to intense intraspecific competition, including chimera formation and
potential somatic and germ cell parasitism. The complexity of competitive outcomes and their alteration by a wide variety
of factors, including irradiance, water motion, and nutrient levels, results in mostly circular networks of interaction, often
enhancing species diversity on coral reefs. Competition is a model process for revealing impacts of human activities on coral
reefs, and will become increasingly important as alternate dominants gain space at the expense of reef-building corals.

KeywordsInterference competition-exploitation competition-competition-cnidarian-macroalgae-cyanobacteria-scleractinian-corallimorpharian-actinarian-sea anemone-ascidian-zoanthid-fungiid-hydrocoral-octocoral-soft coral-stony coral-coral-sponge-climate change-chimera-growth-mortality-reproduction-competitive network-coral–algal interaction-phase shift-feedback loop-model-allelopathy-herbivory-recruitment-antibiotic-microorganism-bacteria-abrasion-palytoxin-bleaching-disease-natural products-nematocyst-mucus-diversity-community structure-aggression-population-alternate dominant

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    ABSTRACT: Coral populations have precipitously declined on Caribbean reefs while algal abundance has increased, leading to enhanced competitive damage to corals, which likely is mediated by the potent allelochemicals produced by both macroalgae and benthic cyanobacteria. Allelochemicals may affect the composition and abundance of coral-associated microorganisms that control host responses and adaptations to environmental change, including susceptibility to bacterial diseases. Here, we demonstrate that extracts of six Caribbean macroalgae and two benthic cyanobacteria have both inhibitory and stimulatory effects on bacterial taxa cultured from the surfaces of Caribbean corals, macroalgae, and corals exposed to macroalgal extracts. The growth of 54 bacterial isolates was monitored in the presence of lipophilic and hydrophilic crude extracts derived from Caribbean macroalgae and cyanobacteria using 96-well plate bioassays. All 54 bacterial cultures were identified by ribotyping. Lipophilic extracts from two species of Dictyota brown algae inhibited >50% of the reef coral bacteria assayed, and hydrophilic compounds from Dictyota menstrualis particularly inhibited Vibrio bacteria, a genus associated with several coral diseases. In contrast, both lipo- and hydrophilic extracts from 2 species of Lyngbya cyanobacteria strongly stimulated bacterial growth. The brown alga Lobophora variegata produced hydrophilic compounds with broad-spectrum antibacterial effects, which inhibited 93% of the bacterial cultures. Furthermore, bacteria cultured from different locations (corals vs. macroalgae vs. coral surfaces exposed to macroalgal extracts) responded differently to algal extracts. These results reveal that extracts from macroalgae and cyanobacteria have species-specific effects on the composition of coral-microbial assemblages, which in turn may increase coral host susceptibility to disease and result in coral mortality. KeywordsAntimicrobial–Bacteria–Competition–Coral–algal interaction–Phase-shift
    Coral Reefs 01/2011; 30(2):309-320. · 3.66 Impact Factor
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    ABSTRACT: Benthic macroalgae, or “seaweeds,” are key members of coral reef communities that provide vital ecological functions such as stabilization of reef structure, production of tropical sands, nutrient retention and recycling, primary production, and trophic support. Macroalgae of an astonishing range of diversity, abundance, and morphological form provide these equally diverse ecological functions. Marine macroalgae are a functional rather than phylogenetic group comprised of members from two Kingdoms and at least four major Phyla. Structurally, coral reef macroalgae range from simple chains of prokaryotic cells to upright vine-like rockweeds with complex internal structures analogous to vascular plants. There is abundant evidence that the historical state of coral reef algal communities was dominance by encrusting and turf-forming macroalgae, yet over the last few decades upright and more fleshy macroalgae have proliferated across all areas and zones of reefs with increasing frequency and abundance. Ecological processes that sustain these shifts from coral- to algal-dominated tropical reefs include increases in open suitable substrate due to coral mortality, anthropogenic increases in nutrient supply, reductions in herbivory due to disease and overfishing, and the proliferation of algae with chemical defenses against herbivory. These shifts are likely to be accelerated and the algal state stabilized by the impacts of invasive species and climate change. Thus, algal-dominated tropical reefs may represent alternative stable states that are resistant to shifts back to coral domination due to the strength and persistence of ecological processes that stabilize the algal state. KeywordsMacroalgae-cyanobacteria-chemical defenses-nutrients-herbivory-climate change-invasive species-diversity-coral/algal competition
    12/2010: pages 241-272;

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