Early-stage heterotrophy and feeding effects on growth and survival of sexually produced elkhorn coral (Acropora palmata) spats
A two-week ex situ experiment on sexually produced elkhorn coral (Acropora palmata) spats from the Dominican Republic demonstrates that while short-term survival remains high regardless of diet, heterotrophic feeding—particularly with freshly hatched Artemia—significantly enhances early post-settlement growth, suggesting that active zooplanktivory can be leveraged to improve coral restoration outcomes.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Coral reefs are the bustling cities of the ocean, teeming with life and protecting coastlines from storms. Yet, these vibrant ecosystems are in trouble, shrinking rapidly due to disease, warming waters, and storms. To save them, scientists are trying to help nature along by breeding new corals in laboratories and planting them back on the reef. This process, known as assisted sexual reproduction, involves collecting eggs and sperm from adult corals during their brief annual spawning events, mixing them in tanks, and raising the resulting babies, called spats, until they are strong enough to survive on the ocean floor. However, these tiny new corals face a critical bottleneck. In the wild, most die before they grow large enough to withstand predators or competition. For decades, scientists have debated whether these tiny spats can feed on their own or if they must rely entirely on the energy stored inside them and the sunlight captured by the microscopic algae living within their tissues. If they cannot eat, they might starve before they ever get a chance to grow.
In a recent study, researchers set out to answer this question for the elkhorn coral, a critically endangered species that once formed massive, tree-like structures across the Caribbean. They wanted to know if feeding these tiny, newly settled spats would help them grow faster and survive better. The team, working in a laboratory in the Dominican Republic, collected gametes from wild colonies and raised thousands of spats in controlled tanks. Once the spats had settled onto ceramic disks and transformed from swimming larvae into stationary coral babies, the researchers began a two-week experiment. They divided the spats into different groups. Some were fed a diet of homogenized brine shrimp, a tiny crustacean often used in aquariums. Others received a blend of commercial plankton, while a control group received no food at all. The goal was to see if adding food to the water would make a measurable difference in how quickly these fragile creatures grew and how many of them survived the two-week period.
The results offered a clear and encouraging answer. The spats that were fed grew noticeably faster than those that were left to rely on their own reserves. Specifically, the group fed with freshly hatched brine shrimp showed the most growth, increasing their surface area by about eight percent more than the unfed group over the course of the two weeks. This might sound like a small amount, but for a creature that is barely visible to the naked eye, gaining that extra size is significant. In the wild, being slightly larger can mean the difference between being buried by sand or algae and successfully claiming a spot on the reef. The study also found that the type of food mattered less than the act of feeding itself; whether the spats ate fresh shrimp, older shrimp, or a commercial blend, they all grew better than the hungry ones. This suggests that the ability to eat is a powerful tool for helping these corals reach a size where they can survive on their own.
Perhaps even more surprising was the discovery of when these corals start to eat. For a long time, scientists believed that elkhorn coral spats were too small and undeveloped to catch prey for at least three weeks after they settled. They thought the tiny polyps, which are the individual animals that make up the coral, simply waited until they were bigger. However, the researchers observed the spats under a microscope and saw them capturing food as early as nine days after they settled. The spats were seen using their tiny, stinging tentacles to grab live brine shrimp and also using hair-like currents to sweep dead shrimp particles toward their mouths. In some cases, they even extended internal threads to feed on prey outside their bodies. This observation challenges the old idea that these corals are helpless in their earliest days and proves they are capable of hunting much sooner than previously thought.
While the feeding helped the corals grow, it did not seem to change their chances of surviving the two-week experiment. Almost all the spats, whether fed or unfed, survived with a success rate of over ninety-seven percent. This high survival rate suggests that for a short period, these young corals have enough energy stored inside them to stay alive without food. However, the researchers noted that in the wild, conditions are often harsher, and the ability to eat could be the key to surviving longer periods of stress or competition. The study also highlighted that the size of the coral at the start mattered; smaller spats grew faster than larger ones, and feeding seemed to help the larger ones maintain their size rather than shrink, which happened to some of the unfed, larger spats. This shrinking effect might have been caused by the warm water in the lab, which can stress larger animals with higher energy needs.
The findings provide a practical path forward for coral restoration efforts. By adding simple feeding steps to the early care of these corals, restoration teams can help them grow larger and stronger before they are planted back on the reef. Larger corals are generally better equipped to survive the dangers of the ocean floor, such as being smothered by algae or eaten by predators. The study suggests that this feeding strategy could be a low-cost, high-impact way to improve the success of coral restoration projects. It turns out that the key to saving these ancient builders might be as simple as giving their babies a meal.
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