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