There are underwater animals that can look a lot like plants. They are called corals.

Each one of the animals is called a polyp.

The polyps hook on to the ocean bottom so they don't float around. To attach themselves they make a hard shell-like substance.

This rock-hard substance is made out of limestone, also known as calcium carbonate. The anchor that the plants make may be attached to a rock or other polyps that have died.

When a bunch of polyps are all gathered together they form a structure under the sea called a coral reef.

“In the case of stony or hard corals, these polyp (groups) grow, die, and endlessly repeat the cycle over time, slowly laying the limestone foundation for coral reefs and giving shape to the familiar corals that reside there,” according to the National Oceanic Service’s website. “Because of this cycle of growth, death, and regeneration among individual polyps, many coral colonies can live for a very long time.”

The National Oceanic Service calls reefs the “largest structures of biological origin on Earth,” biological meaning things that are alive.

Coral have an interesting relationship with small plants known as algae. The algae live inside the coral. They also use the coral’s waste to make food for themselves.

As the algae eat, they make oxygen and feed the coral.

This relationship is known as symbiosis, which means two different plants or animals that live close together, each helping the other live.

For coral and algae, scientists believe they have been helping each other for about 25 million years — that’s a long relationship!

Tune in next week to learn about a fish that feeds on coral and in the process creates something very unusual and beautiful.

— Brett French, french@billingsgazette.com

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