Coral polyps are the individual animals that collectively build the structures of coral reefs. These tiny organisms have a more complex diet than many realize, which is fundamental to their survival and the overall health of the reef ecosystems they create. Understanding what and how coral polyps eat reveals the processes supporting Earth’s biodiverse marine environments.
Symbiotic Nourishment
Reef-building coral polyps rely on a symbiotic relationship with microscopic algae called zooxanthellae. These algae live within coral tissues, providing a significant portion of their energy. Through photosynthesis, zooxanthellae convert sunlight into energy-rich compounds transferred to the coral.
This efficient partnership transfers up to 90-95% of organic material from zooxanthellae directly to the coral. In return, the coral provides the algae with a protected environment and nutrients like carbon dioxide, nitrogenous wastes, and phosphorus, byproducts of their metabolism. This internal food production explains why reef-building corals thrive in shallow, clear, sunlit waters.
External Food Sources
While zooxanthellae provide nourishment, coral polyps also capture external food from the water column. This feeding obtains nutrients like nitrogen and phosphorus, often scarce in tropical reef waters. Their external diet includes microscopic organisms and organic matter.
Coral polyps consume zooplankton, tiny animals such as copepods, small crustaceans, and larval stages. They also feed on phytoplankton, microscopic plant-like organisms, and organic particles like detritus and dissolved organic matter (DOM). This dual feeding strategy allows corals to supplement their photosynthetically derived energy with building blocks for growth and development.
Feeding Mechanisms
Coral polyps employ specialized mechanisms to capture external food. Sessile, they cannot move to find food, relying on extending their tentacles. These tentacles are armed with stinging cells called nematocysts, which can paralyze and capture small prey. Prey size depends on the coral polyp’s size.
Once prey is ensnared, the tentacles move it towards the polyp’s mouth. The mouth leads to a gastrovascular cavity, functioning as both stomach and circulatory system. Digestion occurs within this cavity through enzyme secretion. Corals also use hair-like structures called cilia to create water currents, directing food particles, including bacteria and detritus, towards their mouths. Some corals secrete mucus nets to trap finer particles, then consumed.
Diet’s Role in Reef Health
The diverse diet of coral polyps supports the health and sustainability of the coral reef ecosystem. Deriving energy from both internal photosynthetic symbionts and external feeding allows them to adapt to varying environmental conditions. This dual feeding strategy supports their growth and calcification, the process of building their calcium carbonate skeletons, forming the reef structure.
A varied diet also contributes to coral reproduction and resilience. Disruptions to either food source, such as rising ocean temperatures leading to coral bleaching (the expulsion of zooxanthellae) or pollution reducing plankton availability, can severely impact coral health. The feeding habits of coral polyps are thus interconnected with the health and biodiversity of coral reefs.