Corals are marine invertebrates that form the structural foundation of some of Earth’s most biodiverse ecosystems: coral reefs. These underwater formations are built by tiny animals called polyps, related to jellyfish and sea anemones. Each polyp secretes a hard skeleton of calcium carbonate, and over generations, these skeletons accumulate to create reefs.
Coral reefs support a quarter of all marine species, providing habitat, food, and breeding grounds, despite covering less than one percent of the ocean floor. They protect coastlines from erosion and storms, and offer economic benefits through tourism and fisheries. Understanding coral growth rates is important for conservation efforts to preserve these valuable marine habitats.
Varieties of Coral and Their Growth Rates
Coral species are categorized into hard and soft corals, exhibiting diverse forms and growth rates. Hard corals are the primary reef builders, building rigid, limestone skeletons. Soft corals, like sea fans and sea whips, do not form hard skeletons; their growth involves tissue expansion and they do not contribute to reef structure.
Among hard corals, growth rates vary depending on their morphology. Fast-growing species, often branching or plate-like, allow rapid expansion and colonization. For instance, branching corals like Acropora species can grow between 5 to 10 centimeters per year. These rapid growers are important for building reef complexity.
In contrast, massive or boulder corals, such as Porites species or brain corals, grow much more slowly. Their dense, compact structures offer greater resilience to physical disturbances but grow slower. These corals typically expand by about 0.3 to 2 centimeters per year. While their individual growth is slow, their longevity means they can form large, long-lasting structures that provide stable foundations for reefs over centuries.
Key Factors Shaping Coral Growth
Coral growth rates are influenced by a combination of environmental and biological factors. Light availability plays a key role, especially for reef-building corals hosting microscopic algae called zooxanthellae. These algae perform photosynthesis, providing the coral with energy, fueling growth. Corals grow faster in clear, shallow waters with effective sunlight penetration.
Water temperature also impacts coral growth. Corals have an optimal temperature range, typically 20°C to 28°C (68°F to 82°F), where calcification is most efficient. Temperatures outside this range, especially prolonged elevated temperatures, cause coral stress. This leads to zooxanthellae expulsion, known as coral bleaching, which hinders growth and can cause mortality.
Water quality is another factor. High turbidity from suspended sediments reduces light penetration, impeding photosynthesis and growth. Pollution from land-based runoff, including excess nutrients, can stimulate algae growth that outcompetes corals for space and light, suppressing coral growth. Ocean acidity, from increased atmospheric carbon dioxide, reduces carbonate ions needed for corals to build skeletons, slowing calcification.
Techniques for Measuring Coral Growth
Scientists employ various methods to measure coral growth rates, offering insights into reef health. Direct measurement uses calipers or rulers to measure colony size increase over time. Effective for tracking fragments or smaller colonies, it is labor-intensive and requires repeated site visits.
Photographic monitoring offers a less invasive way to assess growth. Standardized photographs taken at regular intervals allow image analysis software to measure changes in coral area or volume. This technique tracks numerous colonies simultaneously and minimizes disturbance. Digital images provide a permanent record for comparison.
The buoyant weight technique is another common method for measuring skeletal accretion. This involves detaching a coral sample, weighing it in air and water, then repeating after growth. The difference in buoyant weight correlates to calcium carbonate added, precisely measuring calcification. This method requires specialized equipment and careful handling.
Skeletal staining marks the growing edge of the coral skeleton. Corals are briefly exposed to a dye, which is incorporated into the newly formed skeleton. After growth, the coral is collected and sectioned, revealing a distinct stained band marking growth over a known period. This technique provides a clear visual record and is useful for analyzing historical growth patterns from massive coral core samples.
Human Efforts in Coral Restoration
Understanding coral growth rates is key to coral restoration efforts to help degraded reefs recover. Coral gardening is a widely used technique where small coral fragments, from healthy donor colonies or naturally broken, are cultivated in underwater nurseries. These nurseries provide a protected environment for rapid growth, benefiting from increased water flow and reduced competition or predation.
Once coral fragments reach a suitable size, they are “outplanted” onto degraded reef areas. This involves attaching new colonies to the reef substrate, where they grow and contribute to reef structure. Species with faster growth rates, such as Acropora species, are chosen for their ability to quickly establish new colonies.
Microfragmentation is a technique that exploits corals’ rapid growth in smaller pieces. Cutting corals into microfragments allows them to grow significantly faster than larger fragments or whole colonies. This accelerated growth produces new coral material faster, which can be fused to create larger, genetically diverse colonies for outplanting. These methods accelerate natural coral growth and reef recovery, offering a proactive conservation approach.