Feeding Corals in a Reef Aquarium
How corals get nutrition from light and from feeding: zooxanthellae photosynthesis plus heterotrophic feeding, what to feed by coral type, broadcast vs target feeding, and overfeeding risks.
Overview
Corals feed in two ways at once: many get most of their energy from light, through symbiotic algae living in their tissue, and most also capture food from the water. Understanding this balance helps reefkeepers decide what, how much and how often to feed, which varies a great deal between coral types.
Light: feeding through zooxanthellae
Most reef corals host symbiotic dinoflagellates called zooxanthellae (Symbiodinium) in their tissue. These algae photosynthesise and pass sugars, glycerol, amino acids and other compounds to the coral, supplying a large share of its energy, by some accounts up to around 85 percent in well-lit photosynthetic corals. For these corals, appropriate lighting is itself a form of feeding, and many can survive on light alone, though feeding improves growth and colour.
Heterotrophic feeding
Corals also feed directly, capturing zooplankton, phytoplankton, bacteria, detritus and dissolved organic matter with their tentacles and stinging cells. Most corals do this at night, when they expand their tissue and extend their feeding polyps. This captured food supplies nutrients, especially the amino acids and fatty acids used for tissue and pigment, that photosynthesis alone may not fully cover.
Feeding by coral type
- LPS (large-polyp stony) corals: have large polyps and tentacles and take meaty foods such as mysis shrimp and sinking pellets, often target-fed.
- SPS (small-polyp stony) corals: have tiny polyps and use very fine foods such as phytoplankton, rotifers and amino acids, usually broadcast into the water.
- Soft corals: many absorb dissolved nutrients and take fine particulate 'marine snow' and detritus from the water column.
- Non-photosynthetic (NPS) corals: lack zooxanthellae and depend entirely on captured food.
Broadcast versus target feeding
Broadcast feeding adds food to the water column so corals catch it in the flow; it suits SPS and filter-feeding soft corals. Target feeding delivers food directly onto a coral's polyps, usually with the flow turned down so the food is not swept away, and suits large-polyp corals and especially non-photosynthetic species such as sun corals (Tubastraea), which must have each polyp fed. A feeding response, the extension of polyps and tentacles, shows the coral is ready to eat.
How much and overfeeding
Feeding should be matched to what the corals actually take. Any food not eaten breaks down and raises nutrients, which can fuel nuisance algae, cyanobacteria and pest organisms, and degrade water quality. The usual approach is to feed modest amounts a few times a week for photosynthetic corals and more often for non-photosynthetic ones, while watching nutrient levels and adjusting.