Coral reefs are the treasures of the oceans, home to a wide variety of corals that form complex structures and support incredible biodiversity. But did you know that there are different types of corals? Each type plays a unique role in the marine ecosystem. In this article, we’ll explore the two main categories of corals: hard corals, which build reefs, and soft corals, which are more flexible but equally important.
1. Hard corals (hermatypic)
Hard corals are the primary builders of coral reefs. They have an external skeleton made of calcium carbonate, which gives them the rigidity and durability essential for reef formation. These corals are in symbiosis with microscopic algae called zooxanthellae, which live in their tissues. The zooxanthellae, through photosynthesis, provide the corals with nutrients in exchange for shelter and carbon dioxide, an example of mutualistic symbiosis.
Hermatypic corals are found mainly in shallow tropical waters, where sunlight is sufficient for photosynthesis. The best-known genera include Acropora , Porites and Montipora . These corals form dense colonies and actively participate in reef building. Their forms include:
Branching corals : These corals, like those of the genus Acropora , have branch-like ramifications. They grow quickly, but are fragile in the face of storms.
Massive corals : Porites is an example of massive corals. They grow slowly but are extremely resistant to physical and climatic disturbances. Their round and compact shape allows them to better withstand waves.
Foliate corals : These appear as layers that overlap, like leaves. The genus Montipora often produces these foliate forms. These corals optimize the surface area exposed to light to maximize photosynthesis.
2. Soft corals (ahermatypic)
Unlike hard corals, soft corals do not build calcareous reefs. Their structure is more flexible and contains only a small proportion of calcium carbonate. Because of this characteristic, they do not contribute directly to the physical formation of reefs. They are found in both shallow and deep waters, and not all require sunlight to survive. Some can grow in low-light areas where photosynthesis is limited.
Soft corals, such as Alcyonacea and gorgonians, are often more colorful and have a more "floating" appearance. They have a variety of shapes, such as:
Gorgonians : These soft corals have fan-shaped structures that rise perpendicular to ocean currents, allowing them to efficiently capture nutrients carried by the water.
Leather corals : They are more rigid than other soft corals, but remain flexible. Their texture can be reminiscent of leather, hence their name. These corals can form large colonies and are often colored brown or green.
3. Other types of corals
In addition to hard and soft corals, there are other lesser-known but ecologically important forms of coral. For example, solitary corals (or non-colonial corals) live as individuals rather than in colonies. Some of these, such as Caryophyllia , thrive in colder waters and at greater depths.
Fire corals (genus Millepora ), although they resemble hard corals, actually belong to the class Hydrozoa. They are known for their ability to inflict painful burns upon contact, due to their stinging cells.
The diversity of coral types, both hard and soft, is a testament to the complexity and richness of coral reefs. Each plays a crucial role in the balance of this fragile ecosystem. While hard corals build the very structure of reefs, soft corals provide additional biodiversity that contributes to the richness of reefs. Together, these corals form an essential habitat for a wide variety of marine species, connecting the entire ocean ecosystem. However, this diversity is threatened by global warming and other anthropogenic pressures, highlighting the urgency of protecting these ecosystems that are essential to marine life.
Sources:
Veron, JEN (2000). Corals of the World . Australian Institute of Marine Science.
Sheppard, CRC, Davy, SK, & Pilling, GM (2009). The Biology of Coral Reefs . Oxford University Press.
