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Article: Coral


Coral Taxonomy and Classification

  • Corals have been classified as species of animals within the sub-classes Hexacorallia and Octocorallia of the class Anthozoa in the phylum Cnidaria.
  • Hexacorallia includes stony corals with polyps that generally have a 6-fold symmetry.
  • Octocorallia includes blue coral and soft corals with polyps that have an eightfold symmetry.
  • The group of corals is paraphyletic because sea anemones are also in the sub-class Hexacorallia.
  • There are two main classifications for corals: hard coral (scleractinian and stony coral) and soft coral (Alcyonacea and ahermatypic coral).
  • Hard corals form reefs with a calcium carbonate base and have polyps with 6 stiff tentacles.
  • Soft corals are pliable and formed by a colony of polyps with 8 feather-like tentacles.
  • Differentiation in gene expressions in branch tips and bases contribute to the two classifications.
  • Acropora genus is commonly used as a research model due to its diversity and dimorphic polyps.

Coral Colony Differentiation and Reproduction

  • Colony differentiation is the most accurate distinguisher between coral species.
  • Growth and gamete synthesis occur at different polyps within the Acropora genus.
  • Asexual reproduction and skeleton deposition contribute to growth at radial polyps.
  • Differential expression of signaling genes plays a role in colony differentiation.
  • Soft coral species face challenges in systematic studies due to a lack of taxonomic knowledge and low mutation rate of mitochondrial DNA.
  • Corals can reproduce sexually and asexually.
  • Reproduction allows corals to settle in new areas.
  • Reproduction is coordinated by chemical communication.
  • Corals can be gonochoristic (unisexual) or hermaphroditic.
  • Approximately 67% of corals are simultaneous hermaphrodites.
  • Corals can reproduce asexually through budding or division.
  • Asexual reproduction offers benefits such as high reproductive rate and geographical distribution.
  • Whole colonies can reproduce asexually through fission, bailout, or fragmentation.

Coral Conservation and Ecology

  • Approximately 33% of coral species are on the International Union for Conservation of Nature (IUCN) Red List as threatened or endangered.
  • Coral species are at risk of bleaching events due to stressors such as high water temperature or toxins.
  • Environmental factors, including rising temperatures and acid levels, contribute to coral species loss.
  • Coral species have heat shock proteins (HSP) to combat increased temperatures.
  • Conservation efforts are crucial to curb coral extinction and protect coral reefs.
  • Coral polyps feed on a variety of small organisms, immobilizing or killing prey using stinging cells called nematocysts.
  • Many corals form a symbiotic relationship with dinoflagellate algae called zooxanthellae.
  • Zooxanthellae can make up to 30% of the tissue of a coral polyp.
  • Zooxanthellae provide corals with products of photosynthesis, such as glucose and amino acids.
  • Microbiomes are found in the mucus and skeleton of corals, with the skeleton showing the greatest microbial richness.
  • Coral bleaching occurs when stressed corals eject their zooxanthellae, reducing their chance of survival.

Anatomy of Coral Polyps

  • Coral polyps are sessile animals that form colonies of genetically identical individuals.
  • The polyps vary in size, ranging from millimeters to centimeters in diameter.
  • Stony coral polyps produce a skeleton made of calcium carbonate to strengthen and protect themselves.
  • The polyps sit in cup-shaped depressions in the skeleton called corallites.
  • The body of the polyp consists of two layers of cells: the outer ectoderm and the inner endoderm.
  • Soft corals lack a solid exoskeleton but have tissues reinforced by small calcium carbonate structures called sclerites.
  • The polyps of soft corals have eight-fold symmetry.
  • Soft corals can vary in form, with most being colonial and some encrusting other sea objects.
  • Stony corals have six-fold symmetry in their polyps.
  • The tentacles of stony corals are cylindrical and taper to a point.
  • Coral skeletons are biocomposites of calcium carbonate, either calcite or aragonite.
  • The organic matrices in coral skeletons are acidic and species-specific.
  • The soluble organic matrices in coral skeletons differentiate between zooxanthellae and non-zooxanthellae specimens.

Holobionts and Coral-Microbial Interactions

  • Reef-building corals are holobionts consisting of the coral, zooxanthellae, bacteria, and viruses.
  • Co-evolutionary patterns exist between coral microbial communities and coral phylogeny.
  • The corals' microbiome and symbiont influence host health.
  • Scleractinian corals have species-specific microbiomes.
  • The bacterium Endozoicomonas exhibits codiversification with its host.
  • Coral microbiome composition and richness reflect coral phylogeny.
  • Interactions between bacterial and eukaryotic coral phylogeny influence the abundance of Endozoicomonas.
  • Host-microbial cophylogeny influences only a subset of coral-associated bacteria.
  • The coral holobiont is influenced by top-down and bottom-up control of microbiota structure.
  • Stable microbes can be introduced to the holobiont through horizontal or vertical transmission.
  • Transient microbes enter the holobiont from environmental sources.
  • Transient and stable populations compete for resources.
  • Resource-based competition ultimately determines population growth rate.
  • Stable and transient populations depend on the timeframe considered.

Coral Data Sources

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