Coral Reefs: Structure and Types

• Coral reefs are large deposits of calcium carbonate (CaCO3).
• These deposits are produced by corals, algae, and other organisms that secrete calcium carbonate.

General Coral Information

• Coral animals are found worldwide.
• Reef-building corals are located in tropical areas.
• They are typically near the equator, where water temperatures are warmer.
• A coral is essentially a "calcified sea anemone" with a polyp structure belonging to the cnidarians/anthozoans group.
• Reefs are collections of coral species.
• Their "hard part" is a calcium skeleton secreted by coral polyps.
• Coralline red algae also deposit CaCO3
• This builds an algal ridge that protects shallow reefs from wave action via sedimentation/cementation, acting as "glue".

Coral Types

• Hermatypic corals: reef-building corals, such as elkhorn coral.
• Ahermatypic corals: solitary or colonial corals, such as black coral.
• Coral is the most important reef builder.

Coral Structure

• All corals share a similar structure: a body with three layers.
• Epidermis: the outer "skin".
• Mesoglea: the "nerve net".
• Endodermis: the inner gastrodermis.
• Tentacles are armed with nematocysts (stinging cells).
• Corallite: The "cups" that contain the polyps

Coral Nutrition

• Corals are carnivorous, feeding on zooplankton captured with tentacles.
• They eat dissolved organic matter (DOM).
• Zooxanthellae (dinoflagellate) live inside the coral polyp in a symbiotic relationship.
• They facilitate faster CaCO3 deposition, are photosynthetic, provide glucose and oxygen, and give coral its color.
• Cyanobacteria "fix" nitrogen for the zooxanthellae.
• Coral is the "house", zooxanthellae is the "garden", and cyanobacteria is the "fertilizer".

Coral Growth

• Corals grow at different rates.
• Young colonies experience quick growth, while old colonies grow slowly.
• Sunlight is a key requirement for growth, needed for photosynthetic zooxanthellae.

Coral Reproduction

• Corals can be hermaphroditic but do not self-fertilize.
• Asexual reproduction involves budding, which expands the growing colony.
• Sexual reproduction happens through spawning, generating a new colony.

Coral Competition

• Corals compete for space and light.
• Defense and competition adaptations include overgrowing and blocking light to other corals.
• Corals attack with mesenterial filaments to digest neighbors, or sting neighbors with sweeper tentacles.

Coral Predation

• Many organisms prey on coral.
• Examples include parrotfish and crown-of-thorns seastars.
• Corals have various defense mechanisms, including spicules, chemicals, toxins, and nematocysts.

Reef Growth Requirements

• A hard bottom is needed.
• Light and temperature are needed.
• Shallow and clear water is needed.
• Warm water, specifically 86-95 degrees F.

Coral Bleaching

• When ocean temperatures rise, zooxanthellae become stressed and are expelled from coral tissue.
• The coral loses its source of food and oxygen, dies, and turns white.
• Rising water temperatures are due to El Nino and climate change.

Types of Coral Communities

• Fringing reefs: common and simplest, close to shore along islands and small seas, survive best on rocky substrates, most susceptible to humans; ex: Red Sea.
• Barrier reefs: similar to fringing reefs but further from shore, less susceptible to humans, susceptible to wave action (especially if piercing the surface), forming lagoons, and predation; ex: Great Barrier Reef.
• Atolls: ring-shaped reefs, formed above collapsed undersea volcanic mountains, susceptible to wave action and predation, not as susceptible to humans; ex: Indo-West Pacific.

Reef Adaptations

• Color: coloration helps them blend in, indicates territory, safe passage, or toxicity; ex: yellow tang.
• Size: being small enables them to fit into crevices; ex: damsel.
• Feeding: some filter, suspension and deposit feeders; long tube-like mouths or radula (rasping tongue) or claws or large teeth; ex: parrotfish.
• Wave action: Tails, suction, "glue," byssal threads, ex: seahorse.
• Living together: symbiosis (mutual, commensal, predatory, competitive, parasitic); ex: anemonefish.