Coral Polyps — Tiny Builders
Coral reefs are built by and made up of thousands of tiny animals—coral “polyps”—that are related to anemones and jellyfish. Polyps can live individually (like many mushroom corals do) or in large colonies that comprise an entire reef structure.
A polyp has a sac-like body and an opening, or mouth, encircled by stinging tentacles called nematocysts or cnidae. The polyp uses calcium and carbonate ions from seawater to build itself a hard, cup-shaped skeleton made of calcium carbonate (limestone). This limestone skeleton protects the soft, delicate body of the polyp. Coral polyps are usually nocturnal, meaning that they stay inside their skeletons during the day. At night, polyps extend their tentacles to feed.
Most coral polyps have clear bodies. Their skeletons are white, like human bones. Generally, their brilliant color comes from the zooxanthellae (tiny algae) living inside their tissues. Several million zooxanthellae live and produce pigments in just one square inch of coral. These pigments are visible through the clear body of the polyp and are what gives coral its beautiful color.
Hard Corals
Hard corals grow in colonies and are the architects of coral reefs. Hard corals—including such species as brain coral and elkhorn coral—create skeletons out of calcium carbonate (also known as limestone), a hard substance that eventually becomes rock. Hard corals are hermatypes, or reef-building corals, and need tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee) to survive. Generally, when we talk about coral, we are referring to hard corals.
Learn more about how hard corals reproduce and how corals grow.
The variety of coral shapes and sizes largely depends on the species. Some corals form hard and pointed shapes, while others form soft and rounded shapes. The shape of coral colonies also depends on the location of the coral. For example, in areas with strong waves corals tend to grow into robust mounds or flattened shapes. In more sheltered areas, the same species may grow into more intricate shapes with delicate branching patterns.
Soft Corals
Soft corals, such as sea fingers and sea whips, are soft and bendable and often resemble plants or trees. These corals do not have stony skeletons, but instead grow wood-like cores for support and fleshy rinds for protection. They are referred to as ahermatypes, or non–reef building corals, and they do not always have zooxanthellae.
How Reefs Are Made
Coral reefs are built by coral polyps as they secrete layers of calcium carbonate beneath their bodies. The corals that build reefs are known as “hard” or “reef-building” corals. Soft corals, such as sea fans and sea whips, do not produce reefs; they are flexible organisms that sometimes resemble plants or trees. Soft corals do not have stony skeletons and do not always have zooxanthellae. They can be found in both tropical seas and in cooler, darker parts of the ocean.
The coral polyps that build the reef survive by forming a symbiotic relationship with microscopic algae called zooxanthellae. The polyps offer the algae shelter while the zooxanthellae create energy—through photosynthesis—that the corals use as food. In a sense, the coral polyps are “farming” the algae. The waste products of the polyps also serve as food for the zooxanthellae. Corals are also predators; they extend their tentacles at night and capture tiny organisms (zooplankton, small fish, or other potential food item) that happen to be floating by with stinging cells called nematocysts. The captured prey is then moved into the polyps’ mouths and digested in their stomachs.
Other types of animals and plants also contribute to the structure of coral reefs. Many types of algae, seaweed, sponges, sediment, and even mollusks like giant clams and oysters add to the architecture of coral reefs. When these organisms die, they also serve as foundations for new corals.
Other Resources
Coral reefs are built by and made up of thousands of tiny animals—coral “polyps”—that are related to anemones and jellyfish. Polyps can live individually (like many mushroom corals do) or in large colonies that comprise an entire reef structure.
A polyp has a sac-like body and an opening, or mouth, encircled by stinging tentacles called nematocysts or cnidae. The polyp uses calcium and carbonate ions from seawater to build itself a hard, cup-shaped skeleton made of calcium carbonate (limestone). This limestone skeleton protects the soft, delicate body of the polyp. Coral polyps are usually nocturnal, meaning that they stay inside their skeletons during the day. At night, polyps extend their tentacles to feed.
Most coral polyps have clear bodies. Their skeletons are white, like human bones. Generally, their brilliant color comes from the zooxanthellae (tiny algae) living inside their tissues. Several million zooxanthellae live and produce pigments in just one square inch of coral. These pigments are visible through the clear body of the polyp and are what gives coral its beautiful color.
Hard Corals
Hard corals grow in colonies and are the architects of coral reefs. Hard corals—including such species as brain coral and elkhorn coral—create skeletons out of calcium carbonate (also known as limestone), a hard substance that eventually becomes rock. Hard corals are hermatypes, or reef-building corals, and need tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee) to survive. Generally, when we talk about coral, we are referring to hard corals.
Learn more about how hard corals reproduce and how corals grow.
The variety of coral shapes and sizes largely depends on the species. Some corals form hard and pointed shapes, while others form soft and rounded shapes. The shape of coral colonies also depends on the location of the coral. For example, in areas with strong waves corals tend to grow into robust mounds or flattened shapes. In more sheltered areas, the same species may grow into more intricate shapes with delicate branching patterns.
Soft Corals
Soft corals, such as sea fingers and sea whips, are soft and bendable and often resemble plants or trees. These corals do not have stony skeletons, but instead grow wood-like cores for support and fleshy rinds for protection. They are referred to as ahermatypes, or non–reef building corals, and they do not always have zooxanthellae.
How Reefs Are Made
Coral reefs are built by coral polyps as they secrete layers of calcium carbonate beneath their bodies. The corals that build reefs are known as “hard” or “reef-building” corals. Soft corals, such as sea fans and sea whips, do not produce reefs; they are flexible organisms that sometimes resemble plants or trees. Soft corals do not have stony skeletons and do not always have zooxanthellae. They can be found in both tropical seas and in cooler, darker parts of the ocean.
The coral polyps that build the reef survive by forming a symbiotic relationship with microscopic algae called zooxanthellae. The polyps offer the algae shelter while the zooxanthellae create energy—through photosynthesis—that the corals use as food. In a sense, the coral polyps are “farming” the algae. The waste products of the polyps also serve as food for the zooxanthellae. Corals are also predators; they extend their tentacles at night and capture tiny organisms (zooplankton, small fish, or other potential food item) that happen to be floating by with stinging cells called nematocysts. The captured prey is then moved into the polyps’ mouths and digested in their stomachs.
Other types of animals and plants also contribute to the structure of coral reefs. Many types of algae, seaweed, sponges, sediment, and even mollusks like giant clams and oysters add to the architecture of coral reefs. When these organisms die, they also serve as foundations for new corals.
Other Resources