Corals and Coral Reefs

What Are Corals?

Corals are fascinating marine animals made up of tiny creatures called polyps. Each polyp builds a hard skeleton from calcium carbonate and looks a bit like a small sea anemone, with a mouth surrounded by tentacles.

Corals form a special partnership with microscopic algae called zooxanthellae. These algae live inside the polyps, providing them with nutrients through photosynthesis—this relationship helps corals grow and thrive.

Corals flourish in shallow, sunlit tropical waters (less than 30m deep), where temperatures range from 22°C to 28°C and salinity stays below 40 parts per thousand. They’re usually found between 30° North and 30° South of the equator.

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Coral Shapes & Colours

Corals can be:

• Solitary, like mushroom corals

• Colonial, like brain, staghorn, and table corals

They come in stunning shapes and colors. The vibrant hues—green, yellow, or brown—are thanks to the zooxanthellae living inside them.

Just like all living creatures, corals feed, grow, and reproduce.

How Do Corals Eat?

Corals feed on tiny drifting animals called zooplankton. Their tentacles are armed with stinging cells known as nematocysts, which they use to paralyze and capture prey.

Most corals feed at night, when zooplankton are more abundant in the water.

How Do Corals Reproduce?

Corals mostly reproduce by spawning, releasing eggs and sperm into the water. This often happens a few days after the full moon—typically in October/November or March/April, depending on the location.

There are two main reproductive strategies:

• Broadcast spawners release eggs and sperm into open water for external fertilization.

• Brooders fertilize internally and release developed larvae.

These larvae, called planula, float for days to months before settling on a surface. Once settled, they grow into new coral colonies by forming skeletons and multiplying through a process called budding.

Growth rates vary:

• Fast growers like Acropora can grow 10–20 cm per year

• Slow growers like brain corals only manage 0.5–1 cm per year

However, only a small number of larvae survive, as many are eaten by fish and other sea creatures.

How Are Coral Reefs Formed?

Over time, old corals die, and new ones grow on top of their skeletons. This ongoing process, happening over thousands of years, leads to the formation of coral reefs.

Today, coral reefs cover more than 600,000 square kilometers of the ocean floor. These ecosystems are among the most biodiverse on Earth, with intricate relationships between animals, plants, and the environment.

Coral Reefs 

Types of Coral Reefs

There are three main types of coral reefs:

1. Fringing Reefs: These form close to shorelines around islands and are marked by a shallow reef flat on the landward side. In some areas, the reef flat rises above sea level to form a reef crest.  The fringing coral reef plays a pivotal role in shielding the Mauritian coastline from waves originating in the open ocean, contributing significantly to tropical ocean ecology. 

2. Barrier Reefs: These develop along the edge of the continental shelf, between the shore and the open ocean. They are separated from the land by a wide, deep lagoon.

3. Patch Reefs: These occur on the irregular seabed of the continental shelf. Corals settle on raised areas of the seafloor, forming isolated patches.

Mauritius 

Mauritius is surrounded by about 150 km of fringing reefs, except for breaks on parts of the southern and western coasts. A short stretch of barrier reef is also found off the southeastern coast near Grand Port. 

Five types of reefs: fringing, reefs, atolls, reef flats and barrier reefs; 43 genera of hard corals; 159 species from 16 families.  

Rodrigues & the Outer Islands

In Rodrigues and St. Brandon, extensive reef systems cover about 200 km² and 190 km², respectively. 

One endemic coral: acropora rodriguensis.    

The Agaléga Islands are entirely encircled by fringing reefs, while coral reefs are also present in the Chagos Archipelago.

Acropora rodriguensis

Why Coral Reefs Matter

Coral reefs play a vital role in marine ecosystems and human livelihoods for many reasons:

• Highly productive ecosystems – Among the most biologically productive environments on Earth.

• Fisheries support – Coral reefs contribute to about 10% of the world’s marine fish production, directly or indirectly.

• Habitat for marine life – Provide crucial feeding, breeding, and shelter areas for numerous marine species.

• Coastal protection – Act as natural barriers, reducing the impact of ocean waves and helping prevent coastal erosion.

• Sand and beach formation – Help in creating beaches, sandbanks, and lagoons; also play a key role in nutrient recycling.

Coral Reef Fishes

Coral reefs are home to some of the most colorful and beautifully patterned fishes in the world. These fish come in an astonishing variety of sizes, shapes, and colors. Even within the same species, there can be striking differences between males and females, and between juveniles and adults.

Many reef fish—like butterflyfish, parrotfish, and wrasses—are corallivores, meaning they feed on corals.

• Butterflyfish use their long snouts to delicately pluck polyps from coral colonies.

• Parrotfish and wrasses break off chunks of coral, which are ground into sand by their powerful teeth. The coral matter is digested, while the indigestible skeleton is expelled.

These feeding behaviors contribute significantly to sediment redistribution and the formation of coral sand.

To avoid predators, many reef fish form large schools, making it difficult for predators to single out individuals. Common schooling species seen in Mauritian lagoons include:

• * Lonchan (Acanthurus triostegus)

• * Chirurgien (Ctenochaetus striatus)

• * Rouget (Mulloides vanicolensis)

• * Masuran (Plotosus lineatus)

Some reef fish may appear attractive but can deliver painful stings. An example is the scorpionfish or laffe volant.

Impacts on Coral Reefs

Coral reefs worldwide are rapidly degrading.
An estimated 10% of global reefs have already been damaged beyond recovery, and 30% more are at risk of collapse in the next 10–20 years.

Major threats include:

• Human-induced pressures:

  • > Nutrification, overfishing, and sedimentation

  • > Anchor damage

  • > Coastal development (hotels, beaches)

  • > Dredging and ski lane construction

  • > Use of harmful fishing practices

• Climate-related disturbances:

  • > Rising sea temperatures

  • > Coral bleaching and diseases

  • > Red tides and crown-of-thorns starfish outbreaks

  • > Natural events like storms, cyclones, and floods

These threats are pushing reefs beyond their ability to recover naturally.

Measures to Protect Coral Reefs

To conserve marine life and reef ecosystems, several protective regulations have been implemented:

# Ban on coral and shell trade
# Seasonal restrictions on net fishing
# Complete ban on all types of underwater fishing
# Strict control on dynamite import and use in fishing
# Prohibition on coral mining for lime production
# Ban on sand mining from lagoons