Or Perhaps Something Else? We explain how polyps are connected to both kingdoms and what they ultimately belong to.
At first glance, coral reefs resemble fantastical underwater gardens with branching structures similar to trees and vibrant colors characteristic of plants. Because of this resemblance, scientists debated the nature of corals for centuries. For example, the ancient Roman naturalist Pliny the Elder considered them a “third form of life”—neither animal nor plant. But what do modern scientists think?
How Corals Are Connected to Plants
The tissues of polyps are home to microscopic algae—zooxanthellae. These algae, like all plants, are capable of photosynthesis and produce nutrients that provide up to 90% of the corals’ energy. In return, the polyps provide the algae with protection and access to sunlight. This mutually beneficial alliance allows corals to thrive in nutrient-poor tropical waters.
It is the zooxanthellae that give corals those same vibrant colors that so impress tourists. Without them, only the white limestone skeleton remains, and the reefs become bleached.
Why Corals Are Classified as Animals
The true nature of corals was only revealed with the invention of the microscope in the 18th century, when scientists discovered their animal cell structure. From that moment began a fascinating scientific journey into the world of these creatures, which continues to this day and brings new discoveries. For example, a separate coral reef 500 meters high—significantly taller than the Eiffel Tower—was recently discovered at the northern edge of Australia’s Great Barrier Reef.
Coral polyps belong to the class of marine invertebrate animals; they are close relatives of jellyfish and sea anemones. Each polyp is a miniature animal ranging in size from a few millimeters to centimeters, with a mouth opening surrounded by tentacles equipped with special stinging cells for hunting plankton.
So what unites these tiny polyps with other animals? First and foremost are common features of structure and life activity. Like all animals, polyps are heterotrophs, meaning they feed on ready-made organic substances—in their case, microscopic plankton. They have a digestive system, albeit a primitive one: a mouth opening leading to a gastric cavity. To capture prey, they use tentacles with stinging cells that paralyze the victim. Furthermore, polyps demonstrate responses to stimuli—for example, they can contract in response to touch. This indicates the presence of a primitive nervous system.
How Coral Reefs Are Formed
Polyps form coral reefs. But how do these underwater gardens, which so impress tourists, come to be? The secret lies in the polyps’ unique ability to build limestone skeletons. Each polyp secretes an external skeleton of calcium carbonate around itself, extracting it from seawater. When polyps die, their skeletons remain, becoming the foundation for new generations. Over thousands of years, these accumulations form reefs that can reach colossal sizes. For example, the Great Barrier Reef off the coast of Australia is so enormous that it is visible from space.
Thus, coral reefs are not just underwater landmarks. They are living testaments to nature’s amazing ability to create complex interconnections between different forms of life.
