Ocean polyps are small, simple marine invertebrates. Despite their plant-like appearance, they are animals. Their basic body form is a soft, sac-like structure attached to a surface on the ocean floor. These organisms belong to a larger group of animals called Cnidaria, which also includes jellyfish. Polyps represent one of the two main body forms in this group.
Anatomy and Function of a Polyp
A polyp has a simple body plan, consisting of a columnar, vase-shaped body. This structure is a hollow sac with two layers of cells separated by a gelatinous layer. The body is closed at one end where it attaches to a substrate, while the other end has a single opening that functions as both a mouth and an anus. This opening leads to an internal space called the gastrovascular cavity, where digestion occurs.
Surrounding the mouth is a ring of flexible tentacles used to capture food and defend against predators. The tentacles are armed with thousands of microscopic stinging cells called nematocysts. When a small animal, such as zooplankton, brushes against a tentacle, these nematocysts rapidly discharge a tiny, venom-tipped harpoon that can paralyze or kill the prey. The tentacles then guide the captured food into the mouth.
Colonial Polyps and Reef Building
Many types of polyps, particularly stony corals, live together in large, interconnected colonies. These colonies consist of thousands of genetically identical clones connected by living tissue. This colonial lifestyle is fundamental to the creation of the planet’s largest biological structures: coral reefs. The process begins when a single polyp attaches to a hard surface and divides itself, budding into new polyps.
These reef-building polyps construct a hard, protective skeleton by extracting calcium and carbonate ions from seawater. They secrete calcium carbonate (CaCO₃) to form a cup-like structure called a calyx, into which the soft polyp body can retract for safety. Over many generations, as new polyps build their skeletons on top of old ones, the massive and complex framework of a coral reef is formed. Only the outermost layer of a coral structure is covered in living polyps.
The survival and growth of these reef-building polyps depend on a symbiotic relationship with microscopic algae called zooxanthellae. These algae live inside the polyp’s tissues, which are actually transparent. The zooxanthellae photosynthesize, converting sunlight into energy and providing the polyp with up to 90% of its nutritional needs. This relationship also gives corals their characteristic vibrant colors.
Solitary Polyps and Other Life Cycles
Not all polyps live in colonies or build reefs. Some, like the sea anemone, live a solitary existence. A sea anemone is a single, large polyp that attaches to rocks or the seafloor with its adhesive pedal disc. While structurally similar to a coral polyp with a columnar body and stinging tentacles, they are much larger and live independently. Anemones feed on small fish and shrimp that stray into their tentacles.
The polyp form is also just one stage in the life cycle of other marine animals, like jellyfish. The life cycle of many jellyfish species alternates between a polyp stage and the familiar free-swimming medusa stage. A jellyfish begins its life as a tiny, free-swimming larva that settles and develops into a stationary polyp. This polyp can reproduce asexually by budding before eventually transforming and releasing small medusae, which then grow into adult jellyfish.