Corals are captivating marine invertebrates that form some of the most diverse ecosystems on Earth. Each coral is a colony composed of thousands of tiny, genetically identical animals called polyps. These polyps are related to sea anemones and jellyfish, and each individual secretes a hard, cup-shaped exoskeleton at its base. Corals are broadly grouped into two main functional categories based on their biological structure and their role in the creation of large underwater structures.
Hermatypic Corals: The Reef Builders
Hermatypic corals are the architects of the massive tropical reefs known worldwide. These stony, or hard, corals are defined by their ability to deposit significant amounts of calcium carbonate, which accumulates over generations to form the durable, three-dimensional framework of a reef. (60 words)
Their ability to build these large structures stems from a unique biological partnership. These corals host microscopic, single-celled algae called zooxanthellae within their tissues. This symbiotic relationship is the engine for reef building, as the algae perform photosynthesis and share their energy-rich compounds with the coral host. (70 words)
The enhanced nutrition from the algae significantly increases the rate at which the coral polyps can secrete their hard, calcareous skeleton. Because the zooxanthellae require sunlight, hermatypic corals are strictly confined to shallow, clear, and sunlit waters, known as the photic zone. They also require warm, tropical temperatures to thrive. (70 words)
Ahermatypic Corals: Non-Reef Formers
Ahermatypic corals are those that do not contribute meaningfully to the framework of a large, shallow, tropical reef structure. This group includes a variety of species, encompassing both hard and soft corals. They are distinguished by their lack of reliance on symbiotic algae for survival. (58 words)
Since they do not depend on light-harvesting zooxanthellae, ahermatypic corals must obtain their energy solely by capturing plankton and other small organisms from the water column. They use their tentacles and stinging cells to feed on passing prey, operating as pure carnivores. This nutritional independence allows them to inhabit a much wider range of marine environments. (73 words)
This classification includes many soft corals, which have a flexible structure rather than a rigid calcium carbonate skeleton. Hard ahermatypic corals do secrete a skeleton, but they do not deposit it fast enough, or in the right formation, to create a substantial reef structure. (59 words)
Key Differences in Habitat and Growth
The fundamental biological difference in energy acquisition results in stark contrasts in where these two coral types can live and how they grow. Hermatypic corals are restricted to the photic zone, generally the upper 60 meters of water, because their primary energy source requires sunlight. In contrast, ahermatypic corals have a global distribution, found from the ocean surface down to depths exceeding 6,000 meters, unconstrained by light. (85 words)
Temperature requirements also separate the groups. Hermatypic species need consistently warm, tropical water to support the symbiotic relationship. Ahermatypic species can survive in the frigid temperatures of the deep ocean or high-latitude waters, where they may form deep-water coral banks. They are less sensitive to temperature fluctuations because their metabolism is not tied to the photosynthetic process. (88 words)
The symbiotic relationship also dictates a significant difference in growth rate. Hermatypic corals, fueled by the abundant energy from their algae, can grow relatively quickly, with some branching species expanding up to 10 to 20 centimeters per year. Without this energetic boost, ahermatypic corals grow much more slowly, often only a few millimeters annually. This slow growth explains why they do not form the massive, fast-growing structures characteristic of tropical reefs. (85 words)