Marine Science (0697) | Tropical coral reefs

Welcome to Tropical Coral Reefs: The Marine Rainforests!

Hello future marine ecologists! This chapter dives into one of the most exciting and complex ecosystems on Earth: the tropical coral reef. Often called the "rainforests of the sea," these habitats are bursting with life and biodiversity.
Understanding coral reefs is crucial because they show us how organisms rely on each other (mutualism) and how sensitive life is to changes in the environment. Let's explore these incredible underwater structures!

5.7.1 & 5.7.2: The Coral Animal and Its Structure

What exactly is coral?

When you see a coral reef, you are looking at a massive structure built by tiny animals called coral polyps.
Corals are classified as animals that live together in large colonies.

Structure of a Typical Coral Polyp

A coral polyp looks a bit like a tiny, upside-down jellyfish living in a hard, protective cup.

• Tentacles: These are used to capture small prey (like zooplankton) from the water, usually at night.
• Stinging Cells: Located on the tentacles, these cells help the polyp capture and stun its prey.
• Mouth: Used for feeding and expelling waste.
• Stomach (or gastrovascular cavity): Where the captured food is digested.
• Calcium Carbonate Skeleton: The hard, white cup that the polyp secretes around itself. This is what forms the massive structure of the reef over thousands of years.

5.7.4: The Amazing Partnership – Coral and Zooxanthellae

Mutualism: An essential relationship

Most tropical corals have a symbiotic relationship with microscopic algae called zooxanthellae (pronounced zoo-oh-zan-THEL-ee). This relationship is mutualistic, meaning both organisms benefit significantly.

How the Mutualism Works

Zooxanthellae are photosynthetic microorganisms that live *inside* the tissues of the coral polyp.

Don't worry if the name is long—just remember they are tiny plants living inside the animal!

1. What the Zooxanthellae provide (to the coral):
   • Through photosynthesis, they produce oxygen and crucial organic nutrients (like glucose and amino acids).
   • The coral receives up to 90% of its energy needs directly from these algae. This allows the coral to grow much faster than if it relied only on catching plankton.
2. What the Coral provides (to the zooxanthellae):
   • Protection/Shelter: The zooxanthellae are safely housed within the polyp's tissue.
   • Raw materials: The coral's waste products (like carbon dioxide and nitrogen compounds) are recycled and used by the algae for photosynthesis.

Did you know? The zooxanthellae are also responsible for giving corals their beautiful, vibrant colours! If the coral gets stressed (usually by high temperatures), it expels the algae, leading to coral bleaching (turning white).

Key Takeaway: Without zooxanthellae, tropical corals would starve and reefs could not grow at the incredible rates needed to build large structures.

5.7.5 & 5.7.6: Environmental Factors Affecting Coral Distribution

Corals are fussy creatures! They can only thrive in very specific conditions, which is why reefs are limited to the tropics.

Abiotic (Non-living) Factors for Coral Survival:

• Temperature: Corals need consistently warm water (usually between 23°C and 29°C). If the water is too hot, it causes bleaching. If it's too cold, they cannot survive or grow fast enough.
• Light Intensity: Since the zooxanthellae rely on photosynthesis, corals need high light intensity. This restricts them to shallow, clear water (the sunlight zone).
• Clear Water: Corals need water that is low in sediment and suspended particles. Turbid (cloudy) water blocks the light needed for photosynthesis.
• Hard Substrate: Coral larvae need a hard surface (like rock) to attach to before they can grow into colonies. They cannot settle easily on soft sand or mud.

The Clear Water Mystery (Low Nutrients)

It might seem strange, but the water surrounding a vibrant coral reef is actually very low in nutrients (like nitrate and phosphate).

The water is clear because it is low in nutrients; there aren't enough nutrients to support huge populations of plankton and algae that would make the water cloudy.

The Explanation: The nutrients that are present are quickly taken up and held within the organisms living there (the coral, the zooxanthellae, and other reef inhabitants). The mutualistic relationship is a very efficient nutrient recycling machine!

Biotic (Living) Factors that Affect Coral Distribution:

Other organisms in the ecosystem can significantly affect where corals are found.

• Parrotfish: While they help clean the reef by scraping off algae, they also eat coral polyps and sometimes whole sections of the skeleton.
• Crown-of-Thorns Starfish (COTS): These are major predators that consume large areas of live coral tissue, often leading to significant damage if their populations explode.

Key Takeaway: Corals are restricted to shallow, warm, clear, low-nutrient water with hard surfaces due to their dependence on photosynthetic zooxanthellae and the influence of specific predators.

5.7.7 & 5.7.8: Coral Reef Ecology and Adaptations

Coral Reefs are Hotspots for Life

Coral reefs are known for having extremely high species richness. This means that for a given area, there is a huge number of different species living there. This complexity provides many niches (roles and habitats) for organisms to live in.

Adaptations of Reef Organisms

Life on the reef is tough—there is intense competition and high predation pressure. Organisms have developed incredible adaptations to survive:

Triggerfish

• Adaptation: They have an adapted dorsal fin that can be locked upright.
• Function: This fin allows the fish to hold itself tightly into crevices (small gaps in the rock) for protection from predators while sleeping or hiding. Pulling them out is nearly impossible!

Parrotfish

• Adaptation: They possess a specialised set of grinding teeth (a beak-like structure).
• Function: Used for eating coral (biting off chunks of coral skeleton to access the polyps) and scraping algae off the reef. They are vital for reef health.

Nudibranchs (Sea Slugs)

• Adaptation: They can absorb toxins from the prey they eat (e.g., stinging cells from anemones or hydrozoans).
• Function: They become toxic or distasteful to predators, acting as a built-in defence mechanism. Their bright colours often advertise this danger.

Key Takeaway: Coral reefs support high species richness, and many organisms show specific physical or chemical adaptations (like the triggerfish lock or nudibranch toxins) for survival.

5.7.9: The Darwin-Dana-Daly Theory of Atoll Formation

Not all coral reefs look the same. Some form rings with a lagoon in the middle—these are called atolls. Charles Darwin, along with geologists James Dana and Alexander Daly, proposed a theory explaining how these structures form.

Step-by-Step Formation of an Atoll

The theory states that atolls begin as a reef growing around a sinking volcanic island:

1. Stage 1: Fringing Reef
   

   A fringing reef starts growing around a newly formed, active volcanic island in the shallow, warm, tropical ocean. The reef is close to the shore.

2. Stage 2: Barrier Reef
   

   The volcanic island begins to slowly sink (subside) under its own weight over millions of years. The coral, however, continues to grow upwards and outwards (towards the light and nutrients). A deeper body of water (a lagoon) forms between the main reef structure and the sinking island. This structure is now a barrier reef.

3. Stage 3: Atoll
   

   The volcanic island completely sinks below the surface of the water. All that remains is a ring of coral surrounding a central lagoon. This is a mature atoll. The coral has grown fast enough to maintain its presence near the surface despite the underlying subsidence.

Analogy: Imagine a small toy boat (the island) sinking slowly in a bath. You keep adding rings of playdough (the coral) to the edge of the boat, making sure the top of the playdough ring stays at the water surface. Eventually, the boat sinks completely, and only the ring remains.

Key Takeaway: The Darwin-Dana-Daly theory explains atoll formation as a sequence of reef types (fringing, barrier, atoll) resulting from the continuous upward growth of coral keeping pace with the slow subsidence of a central volcanic island.