Corals possess a vast range of lifespans, with some living for only a few years and others surviving for millennia. This variation is due to their unique biology, as corals are not single organisms but colonies of many small animals called polyps. These creatures collectively build the massive structures known as reefs. A coral’s longevity is a story of both individual polyp lives and the persistence of the larger colony.
The Lifespan of a Single Coral Polyp
The tiny animals that build coral reefs are called polyps. Each polyp, a sac-like creature with a mouth ringed by tentacles, secretes a calcium carbonate skeleton. The lifespan of an individual polyp is short, lasting only a few years; for instance, polyps of the common bumpy coral Porites live for about two to three years.
The arrangement of polyps within a colony can influence their individual lifespans. In bumpy, irregular colonies, polyps grow outward in straight lines, and their paths may intersect. When two polyps meet, one will eventually perish, which limits its lifespan.
The Age of a Coral Colony
The age of many coral formations is achieved through continuous regeneration. A colony sustains itself and grows through asexual reproduction, or budding, where new polyps replace old ones on the shared skeleton. This process allows the colony, as a genetic individual, to persist far beyond the lifespan of any single polyp, with some having a genetic makeup thousands of years old.
Lifespans vary significantly between different types of coral colonies. Faster-growing branching species, like staghorn coral (Acropora cervicornis), grow up to 8 inches per year but may only live for several decades. In contrast, massive, slow-growing forms like boulder corals can live for hundreds of years. The most ancient examples are found in the deep sea, where some black corals are dated to be over 4,000 years old, making them among the longest-living animals on Earth.
Factors Influencing Coral Longevity
A coral colony’s ability to reach its maximum age is dependent on its environment and inherent traits. Corals require stable conditions, including clear, shallow saltwater with ample sunlight and warm temperatures above 70 degrees Fahrenheit. Slower growth rates are linked to greater longevity, which is why deep-sea corals in more stable environments outlive their shallow-water relatives.
Many threats can shorten a colony’s life. Rising sea temperatures can cause coral bleaching, a stress response where corals expel the symbiotic algae in their tissues, leading to starvation. Ocean acidification, resulting from increased carbon dioxide in the atmosphere, hinders their ability to build calcium carbonate skeletons. Pollution, physical damage from storms, and overfishing also pose risks to their survival.
How Scientists Determine a Coral’s Age
Scientists use several methods to determine a coral’s age. The skeletons of many massive corals provide a historical record similar to tree rings. These corals lay down annual growth bands of varying density, which can be counted to determine age and look back hundreds of years into a colony’s life.
For corals that are potentially thousands of years old, scientists use radiometric dating. One method is lead-210 dating, which measures the decay of this isotope within the skeleton. Another approach is uranium-thorium dating, which can date samples up to 500,000 years old. Genetic analysis can also track mutations that accumulate over time, providing an estimate of when the coral’s genome first formed.