For the coral reef tank: Add two parts lighting, one part water movement, a bakers dozen of snails and a pinch of food. Can the same recipe be used for growing juvenile corals?

In 2005, CITES reported that over 1.5 million individual corals were exported, mostly from the Indo-Pacific region, for the purpose of distribution within the aquarium trade. Many of these corals were harvested from reefs that are already threatened with significant environmental impacts. Although the harvesting of corals for educational and aesthetic purposes is not as detrimental to reefs as pollution or global warming, any effort to reduce the impact, no matter how small, must be examined if we are to keep our reefs alive and healthy.

Growing corals from sexually produced larvae and planulae allows me to obtain corals for the aquarium trade without having to harvest or sacrifice any adult coral colonies from existing reefs. The obvious benefit is that the harvesting pressure on the reef is reduced to nearly zero. This form of coral farming adds to the genetic diversity of corals within the aquarium trade and will ensure a sustainable income for communities that use their reefs as a resource.

In a previous article I discussed the reproductive strategies of corals and how it relates to the challenge of farming corals from larvae on a commercial scale. Corals produce many offspring but only a few will survive to adulthood. This may sound like a waste of energy on part of the adult coral, but the reef is an environment full of competitive pressures. Predators, competition with other reef organisms, other species of corals, and even their own kind contribute to the high mortality of coral offspring. My job as a commercial coral farmer is to not only reduce the pressures, but to create a habitat that promotes the growth of a large percentage of offspring. In this article I will discuss the husbandry issues that revolve around growing corals from larvae and planulae.

Decades of scientific research as well as anecdotal evidence, has lead to a comprehensive understanding of the requirements for captive corals. Dozens of books on the subject of coral husbandry have been authored by professional aquarists. Most of them present a number of recipes on how to maintain corals in the captive environment. While each recipe may be slightly different, all of the authors agree that the aquarist must provide the corals with the essential environmental and nutritional requirements such as sunlight, adequate water motion, some form of algae control, and nutritional food types. Therefore, all I would have to do is find a recipe that I subscribe to the most and presto, I will have corals grown from larvae for sale within the aquarium trade. Not much of a challenge, right?

The problem is that it is not that simple. Most, if not all recipes for the successful maintenance of corals involves the requirements to fulfill the needs of adult colonies. There aren't any recipes that cater to the specific needs of new recruits and juvenile corals. Some aquarists write about sexual reproduction in their captive systems. While this information is helpful, the difference between corals reproducing and settling in aquariums and what I am trying to do is that much of the reproduction in aquariums is opportunistic and not directed on a large scale. In order to compete with other farms I have to ensure that a large percentage of the larvae and planulae settle and grow to marketable sizes. It is easy to understand why there isn't a recipe since this approach to coral farming has not been done before.

In order to understand my challenges, I have to put the corals I am trying to raise into perspective. Newly settled planulae and larvae are small. How small? Look at the lettering on the either side of a penny. Any one of the letters is about equal to the size of the new recruit. On average, metamorphosed larvae and planulae are about 1 mm in diameter.

The Pocillopora damicornis recruits in this photo are 2 days post metamorphosis.

Another consideration is the habitat in which they find themselves in once they transition from the planktonic larvae into a settled coral polyp. Many studies have shown that recruits found on substrates in shaded or cryptic environments survive much better than recruits found on exposed substrates (Birkeland et al. 1981; Babcock and Mundy 1996). The environmental parameters in the cryptic habitat include reduced light levels and water circulation. An example would be the reef substrate under a coral colony or overhang. While many organisms adapt quite readily to these environments, organisms that require high amounts of light and water movement, like many corals, would have a hard time surviving in such an environment.

If this is the conditions that the 1 mm sized juvenile corals finds themselves in, then how is it that they are able to reach adulthood and compete successfully for resources on the reef? More importantly, how does this help in my understanding of what type of requirements need to be provided to foster the growth of the juvenile coral to an adult colony. These are not easy questions to answer. Most of it is speculative and I can only make observations about their preferences and not necessarily state the reasons for their preferences. To get an idea of what I mean, I will use the four important environmental parameters that professional aquarists agree are important for adult coral survival and demonstrate that juvenile corals may not necessarily prosper under the same conditions.

Sunlight

Sunlight is important to all corals that have a symbiotic relationship with zooxanthellae. Zooxanthellae, like all other photosynthetic organisms, have the ability to use sunlight to transform carbon dioxide and water into organic compounds. The products of photosynthesis can then be used by the coral for nutrition and growth. Many adult coral colonies would not survive or prosper if light levels were significantly reduced.

Filamenttous algae growing over a one month old Pocillopora damicornis. The juvenile coral in the photo is 5 mm in diameter.

If this is the case, why do juvenile corals survive better in habitats where light levels are reduced to a fraction of what they would require as adults? There are a number of different possibilities. One possibility is that they survive in the cryptic environment because they remain hidden from predators. Those that settle out in the open are exposed to all type of organisms that find their food on the reef substrate. Predators could find them as an easy target and herbivores may incidentally inflict mortal damage on the coral polyp as they scrap the reef substrate. Hayashibara et al. (2004) proposed that new recruits of Acropora digitifera experienced high rates of mortality (up to 100%) on exposed surfaces due, in part, to algal grazing by fish and invertebrates. Another reason might be that harmful macro-algae are not as prevalent in shaded areas. Sato (1985) was able to show that juvenile Pocillopora damicornis survived better in areas that were not colonized by filamentous algae. Algae can quickly overgrow the tiny coral polyp and restrict light, water movement and food from the coral. In the shaded environment, algae might not be as abundant. Finally, another possible reason is that corals produce pigments called mycosporine-like amino acids (MMAs). These pigments are essential for protection against ultraviolet radiation. However the development of these pigments requires energetic costs on part of the coral. As a new recruit, it is possible that they might not yet be able to 'pay' for the production of these pigments (as limited internal resources may be relegated to growth). In a controlled lab experiment, Suzuki et al. (2004) was able to show that Acropora larvae exposed to strong lighting (500-700 micro mol m-2 s-1) did not settle. Whereas up to 70% of the larvae exposed to darkroom conditions settled and metamorphosed within 9 hours. Further, larvae exposed to strong light did not settle when transferred to the darkroom condition. It was proposed that exposure to strong light may have damaged their ability to settle. For juvenile corals, a shaded environment may be a way to protect themselves from a variety of potential environmental and predatory threats.

Water Movement

While sunlight is essential for the specific purpose of driving photosynthesis for the zooxanthellae, water circulation can have a variety of effects on the health of the coral. Not only will water movement bring food to the coral, but it will also facilitate nutrient and gas exchange between the coral and the surrounding seawater.

Similar to sunlight, good water circulation is important to the health of adult corals. Why do juvenile corals prefer the habitat that offers a reduction in water movement? It could simply be a function of settling in habitats with reduced light levels. What provides the shade also restricts, or blocks water flow. However, it could just as easily be that reduced light levels are a function of the protection they may gain from the exposure to high flow rates. In other words, they may use light levels as a cue to settle in shaded areas for the purpose of avoiding strong water movement.

Exposed reef flats can experience water currents that are in excess of 50 cm sec-1. For adult corals with excessive branching or massive corals with large surface areas, high flow rates ensure that all of the coral polyps around the colony receive adequate circulation. Further, branches and surface area can act as a barrier so that even during a time of strong currents or heavy wave action, polyps on the sheltered side can remain open. For juveniles who have not formed large branches or high surface areas, strong water flow may work against them. Strong water circulation may overpower the tiny polyps and prevent them from completely emerging from their corallites. This would inhibit both photosynthesis as well as the amount of food it can capture.

Two popular types of algae cleaning invertebrates.  Although they mean well, they may inadvertantly harm the tiny juvenile coral. The Pocillopora damicornis in this photo is 2 weeks old.

There are presently no studies that investigated water flow as the single variable for coral settlement preferences. From experience, I have found that new recruits survive even in a zero flow environment. Daily water changes and, later on, food additions were enough to cause the new recruit to grow. However, because water flow is important to corals and may affect their growth rates, good water circulation must be considered as they grow to adulthood. Research on this environmental parameter will prove to be essential for the growth and health of juvenile corals.

Algae Control

I mentioned previously that juvenile corals on the natural reef may prefer the shaded environment as a way to avoid competition with algae. It's true that the reduced light levels may exclude some forms of algae, but not all. For the juvenile coral, even a small tuft of algae could be a life threatening problem, whereas for the adult, it may simply be a nuisance.

In captive systems, like the natural reef, herbivorous invertebrates can prevent the proliferation of harmful algae. Some recipes for successful coral care suggest that adding dozens of hermit crabs and snails to their system is as important as using the proper lighting regime. For the adult coral, these algae eaters are a blessing. For juvenile corals, they may be their worst enemy. The average size of a snail or crab that is placed into reef tanks is about 1 inch or 2.5 cm. The average size of a recently metamorphosed coral is 1 mm. 2.5 cm is equal to 25.0 mm. Therefore, the crab or snail cruising around a new recruit would be 25 times its size. Put another way, the average size of a coral fragment sold from dealers is about 4 inches. That's 10 cm or 100 mm. A crab or snail 25 times that size would be 2500 mm, 250 cm, 100 inches, or 8.33 feet. I think I'd be pretty scared about having an 8 foot snail crawling around my 100 gallon tank, never mind the damage it would do to the 4 inch coral fragment. Although the snail or crab may not feed on the coral, or even care about its existence, it may indirectly cause damage by crawling over it.

Some would argue that a small amount of damage to a coral colony inflicted by a crab or snail is worth the alternative which may be the proliferation of algae. They may be right. A snail or crab that crawls on an adult colony and, in the course of its algae scrubbing, takes out a few polyps is not a big deal. The colony has thousands of other polyps that will function properly. A newly metamorphosed coral has only one primary polyp. Damage to the polyp leads to the death of the coral. If one polyp out of a total of five is destroyed, a juvenile coral will lose 20% of its ability to feed, house zooxanthellae; defend against threats, etc. Sato (1985) made similar conclusions about the potentially lethal affects to P. damicornis by grazing activity.

Note: Many aquarists do not use fish as their main source of algae control. Never-the-less it is important to mention them since they can be just as damaging to the juvenile coral. Fish that scrap algae from the reef are not very careful about where they scrap. A small single polyp coral that is adjacent to a patch of algae could easily be scrapped away during the foraging event.

Food and nutrition

Although corals require sunlight and benefit from their relationship with zooxanthellae; they still require external food sources for growth. Ferrier-Pagès et al. (2003) investigated the effects of three different feeding regimes onStylophora pistillata. Individuals that received the highest amount of zooplankton (6 times per week) exhibited significant increases in protein, chlorophyll a, and chlorophyll c2 per surface area over corals that were not exposed to zooplankton. Further, tissue growth and calcification rates were exponentially higher for corals that were given zooplankton.

(Top photo) Pocillopora damicornis at one month.  Each individual juvenile colony is 6 mm in diameter. (Bottom photo) The same coral at 6 months.  Individual colonies have fused and produced a single colony.  the tiles use dof rhte base measure 5 cm x 5 cm.

It is apparent that food is important to the health of the adult colony and for the juvenile coral as well. However, what type of food is optimal for their survival? Do juveniles feed on bacteria, zooplankton, or dissolved organic matter? In the cryptic environment, how much exposure do they get to zooplankton and other types of plankton? Can their small size handle large prey items? Many aquariums and hatcheries use Artemia fransicana, a representative of zooplankton, to feed adult coral colonies. A. fransicana averages about 500 microns in size. As mentioned before, the average size for a new recruit is 1 mm or 1000 microns, but the actual polyp that would be involved in capturing prey is less than that. The coral polyp, therefore, would have to capture and consume prey that is half of its total size. The concern is that if the recruit spends as much energy to capture its prey as it receives from its prey; growth may be very slow or may not happen at all. This is an important consideration as it not only affects the success of the coral; it may also determine the success of the facility that raises corals from larvae and planulae.

When I consider the habitat that the juvenile corals live in, I find myself wondering if Artemia is a necessary food item. It's expensive and requires extra costs in the form of aeration, water and manpower. Could it be that by providing the ideal physical environment (light levels, water flow, etc.) might be all that is needed. Maybe there are bacteria that the juvenile coral can feed on until it reaches a larger size with more polyps. At that point, the energy to catch larger and more live prey may be outweighed by the nutritional value the coral will gain from the food item. Perhaps they rely, initially, on the zooxanthellae for their nutritional requirements. I am currently gathering data for my study that investigates the affect of different food types on the growth of juvenile corals. Preliminary results suggest that juvenile corals grow faster when fed live foods over the other food types I offered or what they can receive from zooxanthellae alone. Since corals are heterotrophic, these results sound about right. However, keep in mind that I am trying to foster the fastest growth rates. This means that I am trying to provide the best possible nutrition which, I hope, would exceed what the juvenile corals would receive on the reef. Further, as a commercial coral farm, I am trying to do this as inexpensively as possible.

For those who want to grow corals from larvae and had anticipated a checklist of procedures, I apologize. The most important piece of information that can be taken from this article is that the husbandry of juvenile corals requires a different approach than the one used for adult colonies. Much of my success so far is due to the careful observations I make on what type of habitats the juvenile corals thrive in. These observations lead to the formation of specific questions that can be tested scientifically. With the more questions I answer as well as the more experience I acquire, the closer I get to write that recipe. I only have a couple of dozen more questions to tackle. For example, if larvae and juvenile corals do not like strong sunlight, then what is the optimal lighting regime? Further, when is the best time to transition to the lighting that would fulfill the needs for adult colony? What is the optimal water flow speed and when do I increase it? If not crabs or snails, then what can I use for algae control?

Representatives of Pocillopora damicornis at 4 ages.  Clockwise from lower right: 2 weeks; 5 weeks; 2 months; 6 months.  The 6 month old coral is ready for distribution.

My advice to those who have little recruits popping up in their tanks is to continue to do what you have been doing. You have already demonstrated that you provide a good habitat. Document the habitat that you find your juvenile corals in and observe them closely as they grow. Although many of these issues need to be worked through scientific experimentation, a lot can be said for good old fashion observation and experience.

Growing corals from larvae and planulae is a type of coral farming that is still in its infancy but can provide great benefits to the conservation of tropical coral reefs as well as to the aquarium industry. I hope that, with this article, people appreciate how much research still needs to be done into all aspects of their requirements. Growing corals from larvae is the future of the hobby. My goal, through research and experience is to write a recipe for others to use so that this future is realized rather than a future where corals in the aquarium trade are a thing of the past.

Lee Goldman has been growing corals from sexually produced larvae and planulae since 2002. He started Coral Farm Guam, a commercial coral farm in 2004. He will be speaking about his work at the upcoming MACNA XVIII conference in September.

References

1. Babcock, R.C.1985. Growth and mortality in juvenile corals (Goniastrea, Platygyra and Acropora): The first year. Proc. Of the Fifth Inter. Coral Reef Congress Vol 4, Tahiti
2. Birkeland, C., D. Rowley, R.H. Randall. 1981. Coral recruitment patterns at Guam. Proc. 4^th Int. Coral Reef Sym. Manilla 2: 339-344
3. Ferrier-Pagès, C., J. Witting, E. Tambutté and K. P. Sebens^. 2003. Effect of natural zooplankton feeding on the tissue and skeletal growth of the scleractinian coral Stylophora pistillata. Coral Reefs 22: 229 - 240
4. Hayashibara, T., K. Fukuoka, M. Sano, M. Tamaki, H. Shimizu, M. Minagawa (2004). Reef restoration by coral larvae seeding: An experimental study on survivorship of artificially settled Acropora polyps in the first year. 10 Int. Coral Reef Sym. Japan
5. Sato, M. (1985). Mortality and growth of juvenile coral Pocillopora damicornis (Linnaeus). Coral Reefs 4: 27-33
6. Suzuki, G., T. Hayashibara, K. Iwao, M. Tamaki, H. Shimizu, I. Hayashi (2004). Inhibition of the settlement and metamorphosis in coral planulae by strong light. 10 Int. Coral Reef Sym. Japan