Corals are marine animals, related to jellyfish and sea anemones, often mistaken for plants due to their rooted appearance. While adult colonies are fixed, they exhibit dynamic behaviors and forms of “movement” essential for their survival and the growth of reef ecosystems. These movements range from microscopic cellular actions to large-scale changes in colony structure.
The Journey of Larval Coral
The most active movement occurs during the larval stage. Coral larvae, known as planulae, are free-swimming and microscopic, propelled by rhythmic beating of tiny hair-like structures called cilia. This allows them to explore their environment and respond to cues.
Planulae journey through the water column, searching for a suitable place to settle. Their dispersal can range from tens to hundreds of kilometers, facilitating genetic exchange and new colony establishment across reefs. Upon finding a favorable spot, the larva undergoes metamorphosis, converting into a sessile polyp that attaches permanently to the substrate.
Micro-Movements of Coral Polyps
Even after settling, individual coral polyps, the tiny animals that make up a coral colony, display micro-movements. Polyps contract into their hard skeletons for protection or expand their bodies and tentacles to capture microscopic food particles like zooplankton. This expansion and contraction is a behavior for feeding and defense.
Their tentacles, armed with stinging cells called nematocysts, ensnare prey or deter encroaching organisms. These hunting tools allow corals to supplement nutrients from symbiotic algae. Polyps also subtly orient themselves to optimize light exposure for their symbiotic algae or to better position tentacles to catch food from currents.
Colony Growth and Environmental Responses
Coral colonies exhibit dynamic changes through growth and adaptation. They expand by adding new polyps through asexual budding and continuously secreting a calcium carbonate skeleton. This process effectively “moves” the colony by increasing its size and changing its shape over time. The growth involves polyps lifting themselves and depositing new skeletal material, gradually building the reef structure.
Corals also demonstrate adaptive growth forms in response to their environment. Branching corals grow upwards, while plating corals spread horizontally to maximize light capture. These varied shapes, such as massive, encrusting, or foliose forms, are a response to factors like water flow, light intensity, and competition. Furthermore, dislodged fragments can re-attach and grow, representing a passive but significant form of relocation and propagation.