Coral reefs are vast underwater structures built by tiny animals called coral polyps, invertebrates closely related to sea anemones and jellyfish. A single polyp is a soft-bodied creature, often only a few millimeters in diameter. As the fundamental units of a coral colony, their collective activities create some of the most diverse ecosystems on the planet. Hundreds or thousands of polyps form a colony, and multiple colonies join together to create a reef.
Anatomy of a Coral Polyp
Each coral polyp has a simple, sac-like body with two main cell layers: an outer epidermis and an inner gastrodermis, separated by a jelly-like substance called mesoglea. At the top of this body is a single opening, the mouth, surrounded by a ring of tentacles. This opening leads to a central digestive cavity known as the gastrovascular cavity.
The tentacles are armed with specialized stinging cells called nematocysts, which contain tiny, coiled barbs that can be discharged to capture prey or for defense. At its base, a reef-building polyp secretes a hard, protective skeleton of calcium carbonate. This cup-like structure, known as a calyx or corallite, houses the soft polyp.
How Coral Polyps Eat and Survive
Coral polyps obtain nutrients through two distinct methods that allow them to thrive in nutrient-poor tropical waters. Their primary strategy involves a symbiotic relationship with microscopic algae called zooxanthellae, which live inside the polyp’s tissues. These algae perform photosynthesis, using sunlight to produce energy-rich compounds. This process provides the coral with up to 90% of its required energy and also gives the coral its color.
In return for a protected environment, the algae supply the coral with oxygen and the organic products of photosynthesis. Coral polyps are also carnivores. At night, many polyps extend their tentacles to capture small organisms floating in the water, such as plankton. When prey comes into contact with the tentacles, the nematocysts fire, immobilizing the organism, and the tentacles then guide the captured food into the mouth for digestion.
Reef Formation and Reproduction
The structures of coral reefs are built over thousands of years through reproduction and skeletal growth. Corals reproduce in two main ways: asexually and sexually. Asexual reproduction is responsible for the growth of an individual colony. This occurs through budding, where a polyp divides and creates a genetically identical clone of itself, which then grows its own calyx.
Sexual reproduction creates new, genetically distinct coral colonies. Most coral species are broadcast spawners, releasing massive quantities of eggs and sperm into the water column. This event is often synchronized with lunar cycles and water temperature to increase the chances of fertilization.
Once an egg is fertilized, it develops into a free-swimming larva called a planula. The planula may drift with ocean currents for days or weeks before settling on a suitable hard surface. After settling, the larva metamorphoses into a single polyp and begins to secrete its calcium carbonate skeleton. Through continuous budding, this single polyp establishes a new colony, contributing to the gradual expansion and formation of the reef.