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