During August and September of 2006, I worked in La Parguera, Puerto Rico assisting with coral spawning research as part of an internship program through the University of North Carolina, Wilmington (UNCW) and the University of Puerto Rico, Mayaguez (UPRM). The goals of this research project were to increase the knowledge of collecting coral gametes, of rearing and settling coral larvae, and to perform several experiments on the environmental tolerance limits of coral larvae and ecological interactions of corals with their environment. The spawn was collected from colonies of the glorious, but threatened, Elkhorn coral, star corals and several others. The spawn was collected in the field and from captive colonies of species that were collected for the spawning event and later replaced on the reef. Although our efforts were headquartered at the UPRM marine station on Isla Magueyes, our field work was carried out at many reef sites around Puerto Rico.

We were fortunate that a group from Project SECORE was conducting a workshop on sexual coral reproduction at the same location where we conducted our research. SECORE (SExual COral REproduction) Project is an international initiative of public aquariums and coral research institutions dedicated to conservation through the sustainable management of coral reefs. The SECORE group and my group had very similar goals, and throughout the 10-day spawning blitz we helped each other in various ways. This was a thoroughly enriching experience for me, not only for all that I learned about working with sexual coral reproduction, but also for the repetitive reef diving, which gave me an intimate view of almost all of the Caribbean's stony coral species.

The Corals

Before the corals spawned, many preparations had to be made. All the activities leading up to the big event focused on keeping the coral larvae alive and rearing them to the proper developmental stages. Our research efforts focused on several species: Acropora palmata (Elkhorn coral), Montastrea faveolata (Mountainous Star coral), Montastrea cavernosa (Great Star coral), Diploria strigosa (Brain coral), and Favia fragum (Golfball coral). All but the last of these species are important reef builders. Because corals grow attached to the reef bottom, the only times in their lives when they move around is when they begin life as drifting gametes and then, later in their life, when they are swimming larvae called planulae. Planula larvae are very sensitive to small variations in environmental conditions. The effects of elevated temperature can have strong consequences for the survival of coral larvae (Bassim and Sammarco, 2003) and sedimentation can have large consequences for the settlement success and long term survival of juvenile corals (Hodgson, 1990, Babcock and Davies, 1991). The coral spawning research conducted in Puerto Rico was aimed principally at increasing our understanding of the characteristics of corals' early life history in order to learn more about these important reef-building stony corals.

Figure 1: Elkhorn coral is a major builder of Caribbean coral reefs. It grows to become the largest and most recognizable coral in the world, sometimes attaining the size of a small tree. Unfortunately, the abundance of Elkhorn coral has decreased dramatically in recent years. This summer it became the first coral to be listed as a threatened species under the Endangered Species Act.

Acropora palmata is commonly known as Elkhorn coral because its colonies are composed of thick, broad branches, and colonies grow to produce some of the largest colonies of all corals. Elkhorn coral is critical in building reefs because it can grow very fast, and because it produces a sturdy reef structure. Although entire reef zones formerly consisted almost entirely of this species, in recent years this species has declined by 90-99% of its former population abundance throughout most of its range. The decline has been alarming enough to warrant listing this coral as a threatened species protected by the Endangered Species Act. Acropora palmata occurs almost exclusively in very shallow, very high energy environments. Even with an aquarium system designed specifically for A. palmata, the track record for this species' captive survival is very poor, although few proper efforts have been made. The decline of this important Caribbean reef building species underscores the importance of learning how to propagate the species both sexually and asexually.

Figure 2: Mountainous Star coral grows into massive mounds which can be much larger than the colony in this picture.

As Elkhorn coral's abundance has declined, one of the coral species that has become increasingly important for reef building is Montastraea faveolata, the Mountainous Star coral. Montastraea faveolata, M. annularis and M. franksi are sibling species which predominantly grow into massive forms, and they occur in a wide range of habitats. Although these species grow more quickly than many others, their growth rate is much slower than Elkhorn coral. Unfortunately, although we had intended to do a lot of work with this species, very high temperatures from the previous year caused many colonies to undergo reproductive failure. During the winter of 2005, an abnormally large heat content existed around Puerto Rico and the Virgin Islands. The higher than normal water temperatures lasted many months, and it caused a great deal of bleaching in this species. The bleaching event overlapped with the beginning of its gamete maturation period. Even months after the bleaching, many of the really large, mature colonies of M. faveolata showed yellow blotching which may have been due to impartial bleaching recovery or the onset of disease. In the end, we witnessed only a scant dribble of egg/sperm bundle release in this species, so we pretty much scrapped our plans to use it for our experiments.

Figure 3: It was recently discovered that orange specimens of Montastrea cavernosa, such as this one, get their fluorescence from the presence of nitrogen-fixing bacteria (Lesser et al, 2004).

Montastraea cavernosa is one of my favorite corals in the Caribbean. It is an important reef building coral which can grow to over 2m across. Its growth forms can be massive and encrusting, occasionally plating or pillar-like, but it is most often sub-massive. I have long known that this coral occurred in an attractive red/orange morph, but the range of colors that I observed in Puerto Rico alone was highly diverse. The orange fluorescence of some specimens is known to be caused by the presence of symbiotic nitrogen-fixing cyanobacteria (Lesser et al, 2004). Within this species, there appears to be two growth forms, which some taxonomists suspect may be different species. The two forms may also be reproductively isolated because they may broadcast their gametes a few days apart. The issue is muddled by the existence of intermediate colonies that display characteristics of both forms in different parts of the same colony.

Figure 4: Some specimens of Diploria strigosa contain striking blue colors.

The Atlantic Brain coral Diploria strigosa is a very common and easily recognized species on Caribbean reefs. It is characterized by a massive growth form and usually strongly meandroid corallite valleys. This species' colonies often grow into large spherical mounds, and it frequently hosts many commensal Christmas tree worms (Spirobranchus giganteus). Diploria strigosa is the most abundant of the three species in the genus Diploria. The genus' other species include the sharper grooved D. clivosa, which is generally found in water less than 7m, and the usually larger grooved D. labyrynthiformis, which occurs in the same depth range as D. strigosa but which persists at depth where D. strigosa does not occur (personal observation).

The final species used for our research was the Golf Ball coral, Favia fragum. This species is different from those previously discussed in that it attains a maximum size of only several inches in diameter. It often grows in a spherical shape, hence its name, and it also differs in that it is a brooding coral instead of a broadcast spawner. This species is common and widespread occurring in a range of environments from calm reef zones such as back reefs, mangroves and seagrass beds, but it can also occur in the sheltered microhabitats of high energy reefs. This species is not a critical reef building species, but it still serves as a model species for studying brooding corals because it is very abundant, it matures at a relatively small size and it releases larvae every month.

Before the Spawn

Even before the main team arrived in Puerto Rico, many preparations had been made in North Carolina and in La Parguera. While we were in Wilmington, the other interns and I worked to build spawn collectors, larval housing chambers and larval settling chambers. The materials we used to construct these items were all some form of nylon cloth which we connected by sewing. Although it was the last thing I expected to learn from a coral spawning internship, using a sewing machine was the first thing I learned to do on this job. Meanwhile, in La Parguera about 100 limestone blocks in two shapes were placed at different sites on a couple of reefs to obtain the necessary biotic film that encourages coral larvae to settle (Heyward and Negri, 1999). One of the types of blocks was conditioned with varying degrees of shading and grazing in order to produce small communities of encrusting organisms. Because coral larvae can sense biological cues to settlement (Morse et al, 1996), the experimentally conditioned blocks were used to see the community effects on coral settlement and survivorship.

Once we arrived in La Parguera, we made our facilities conducive to raising coral larvae and to performing our research experiments. One of our first tasks was to organize our seawater culture area and to re-engineer most of the plumbing. We serviced the rapid sand filter and the micron filter, which processed all the feed water from the seawater reservoir. We also reworked most of the plumbing to deliver one circuit of really slow, high pressure water to the dozen or so coral larvae chambers and another circuit of really high water flow for water tables we used to house spawning branches of A. palmata, and colonies of M. cavernosa and D. strigosa. Because water movement is one of my areas of interest and expertise, my project was to configure the maximum water flow tank which was used to house the A. palmata branches. One issue with increasing the water circulation in our water tables was that the incoming water was arriving at 31°C / 87°F, so it was important that the water flow mechanisms I added would not increase the water's temperature. In the end, I opted for a three-pronged approach to moving water in this tank. I added a Penductor to the line feeding incoming water to the tank. Two Vortech pumps on opposite sides of the water table were switched on alternately using a Chauvet light timer. The final effort was a tall 30-gallon polycarbonate cylinder, which was converted for use as a self-starting siphon surge device. The combination of all these water flow devices resulted in strong mass water movement, producing flow speeds in excess of 30cm/s in many parts of the tank. Some of the branches we collected for this tank succumbed to tissue loss within a couple of days, perhaps due to the high water temperature. After this initial setback, most of the branches seemed to tolerate this high flow tank well; they all showed great polyp extension, and we suffered no subsequent losses.

Figure 5: This image shows the Penductor, Vortech pump and surge device that provided mass water movement for this tank, which was dedicated to temporarily housing spawning branches of Acropora palmata.

The Sites

The coral gametes were collected from wild and captive-held coral colonies at several locations around La Parguera and Rincón, Puerto Rico. Isla Magueyes is at the center of the Parguera reefs (satellite view). This is where we built our coral rearing systems, and it is where we coordinated the boating excursions and diving activities. For captive-held corals, we collected about eight large branches of A. palmata and 12 colonies each of D. strigosa and M. faveolata. All the colonies were collected from the forereef of Margarita reef and Media Luna reef, which are about halfway to the shelf's edge or approximately three miles from shore. Small colonies of F. fragum were collected from the calm backreef of St. Cristobal Reef and near the shore on Isla Magueyes.

To collect coral gametes from the field, it is necessary to plan the collection of coral spawn where there is an abundance of the target species, because usually not all colonies of the same coral species spawn on a given night. Fortunately, Puerto Rico has a couple of sites where A. palmata are still the dominant species with 100s-1000s of colonies present. The northern coastline of Puerto Rico is exposed to strong currents and waves. All this water movement whips around the island's northwestern tip, which provides great circulation and favorable growing conditions for Elkhorn coral at Rincón and Baja Gallardo where some healthy stands still exist. My research group went only to Rincón, but the SECORE group divided its members and collected from both sites. The reefs at Rincón were adjacent to the shore so it was practical for us to do our diving and spawn fertilization work from there. Montastrea faveolata is much more abundant than A. palmata overall, so for this species we were able to deploy from Isla Magueyes to another reef in Parguera called Turramote reef, but since the colonies that didn't die from the previous year's bleaching event still failed to produce any significant quantity of gametes, it was really disheartening to go to this reef on successive nights only to see such giant colonies release egg/sperm bundles only from tiny patches of the colony.

The Spawning

Coral spawning behaviors can be classified based upon whether spawn is broadcast into the water column or whether it is brooded internally. Broadcast spawning corals release eggs and sperm into the water column, and they tend to have an annual reproductive cycle. If a broadcast spawning coral is hermaphroditic, it packages egg and sperm into buoyant bundles; otherwise, eggs and sperm are released separately. Egg and sperm float to the surface where they mix, fertilize and are carried away by water currents. Brooding corals either release sperm into the water column or self-fertilize, in which case no sperm is released. The internally fertilized egg is brooded within the parent coral's tissue until it becomes a larvae, at which point it exits the parent's polyp, usually from the mouth or through a tentacle's tip. Although broadcast spawners release a massive quantity of tiny eggs one or two nights per year, some brooders produce an order of magnitude fewer larvae which are released periodically, all month long or for a few nights each month throughout most months of the year. The planulae and resulting primary polyps of brooding corals are usually much larger than those of broadcasting corals. Of all the corals we worked with, F. fragum was the only brooding coral.

The successful collection of broadcasting coral spawn depends on overcoming two obstacles: the timing of the spawning event must be correctly estimated, and the gametes must be physically collected. It is important to note that corals' reproductive effort begins well before it culminates with mass spawning events during later summer months. The species we worked with begin producing gametes as early as November - the month when stony coral spawns appear to be influenced by a combination of water temperature (Mendes et al, 2002) and exposure to sunlight (Penland et al, 2004). For our coral species of interest, the spawning can occur over one to four days, and this typically can occur anywhere between seven to 15 days after the full moon. The spawning occurs after sunset and the precise timing of release depends on the species and the location. At Rincón, the Elkhorn began releasing gametes after 9pm, but the brain corals did not begin releasing them until nearly midnight. In some places the time of spawning is so predictable that you can set your watch by it, although this is not usually the case. The entire spawning effort is a very concerted event which ends abruptly. Although the cues we used were reliable to predict gamete release for the corals in our research area, the timing of coral spawning is a very dynamic mechanism, mediated by a variety of factors which we are only beginning to understand.

Collecting the gametes of broadcasting corals varied depending on the type of coral. For species with separate sexes such as Montastrea cavernosa, it is not practical to collect sperm in the field so mature colonies were spawned in captive aquaria. In Puerto Rico, Diplora strigosa spawns very late so this species was also spawned in aquaria. Once these two species had spawned, we collected the fertilized eggs that accumulated at the surface. Spawning corals in captivity is fine for producing a modest quantity of larvae, but in order to obtain massive amounts of coral gametes, we had to collect gametes in the field using spawning nets or spawn catchers. The spawn catchers are simply large cones with a buoyant cup at the tip and a thin sleeve at the perimeter which encloses semi-rigid tubing to hold the fabric into shape. Because coral egg bundles are buoyant, the spawn catcher's conical shape collects floating egg bundles from a large area or whole colony and guides them into the floating collecting cup at the tip. When it appeared that no more eggs were being collected into the cup, we quickly capped it off and brought the sample to shore where it could be mixed with other spawn.

The collection of coral gametes is not as straightforward as it sounds. When we used spawn catching nets in the surge of Rincón's Elkhorn reefs, our spawning efforts yielded only about 100,000 eggs. Meanwhile, the SECORE crew plankton mesh hand collectors to collect gobs of spawn that accumulated at the surface. Although our current number of eggs could have yielded enough larvae for our experiments, after final attrition by fertilization success and survival rate, we had a very small margin of error to meet our desired quotas. Very generous donations of gametes and larvae from SECORE afforded our research a much more comfortable abundance of larvae. I want to take this opportunity to thank SECORE for their contributions to our research experiments.

Figure 6: This bin's volume is about 3 gallons (10 liters), and it contains several tens of thousands of developing Acropora palmata eggs. The inset shows round and uniform eggs that are healthy; everything else is the result of spawn material which is breaking down.

After the Spawn

Once the spawn has been collected, the stress of collecting subsides and is replaced by the pressure of using the spawn to produce coral. Many different stages occur between the time an egg is fertilized until the larvae becomes primary polyp. Depending on the species, the gametes' development occurs at different rates, and it is imperative to know the timing and requirements of each stage in order to successfully produce corals from coral spawn.

Fertilization occurs within the first couple of hours after spawning, and is one of the most critical steps in the entire culturing process. Not only does fertilization ensure that there are actually zygotes for development into coral larvae, but it also determines the amount of maintenance that will be performed later. Immediately following spawn collection, the egg bundles must be allowed to break up so that the egg and sperm can mix and become fertilized. Some species' gametes do not self-fertilize, so it is desirable to have egg and sperm from many different colonies in order to maximize fertilization success. Throughout the fertilization period, the proportion of spawn is manipulated to achieve an ideal ratio of egg and sperm. If there is too little sperm, then not all of the eggs will be fertilized; if there is too much sperm, then some eggs will be fertilized multiple times (polyploid), and they will die (although some species of Acropora are normally polyploid). If unfertilized or polyploid, it will not develop, and the gametes' rich nutrients will break down and can foul the culture water. After about two hours, the eggs are rinsed repeatedly to remove all of the sperm, as well as any debris in the culture water. Once the post-fertilization rinse is finished, we held the eggs overnight in unstirred, shallow containers so that they could undergo their first few cleavages.

The following morning, those eggs that did not develop properly start breaking down, negatively affecting the culture water quality. After several more rinses the developing eggs were transferred to shallow trays containing a very slow influx of fresh seawater. The drain was filtered by a Nytek screen with a very large surface area and a very small mesh size. At this point the eggs are still very buoyant. They tend to accumulate both at the surface and on the sides of the container, so gentle water movement and occasional manual agitation is required to keep the eggs evenly dispersed. As the fertilized eggs develop, they lose buoyancy by metabolizing lipids, and they also develop cilia and begin swimming. When the larvae disperse a little more evenly throughout the water column, they begin to stick to the filter screen so the filter must be frequently rinsed and backflushed. The duration of this developmental period depends mostly on the species. The typical fertilized A. palmata egg takes about five to seven days to develop to the larval stage, but some Diploria species exhibit break-neck developmental speed. Diploria can develop to fully swimming larvae in 18 hours although larval development ultimately depends on the species, the location and the conditions. In order to be prepared for the next culturing stage, the batches of coral embryos must continually be sampled and analyzed for developmental progress.

Even within a batch of coral eggs, not all of the eggs develop into planulae at the same rate, and not all planulae are ready to settle at the same time. Once the majority of the eggs had developed into planula larvae, they were mostly ready for use in our various experiments. Some batches of larvae had to be inhibited from settling in their containers so that they could be used in settling experiments. Planula settlement may be encouraged by the presence of biofilm as a chemical indicator that a substrate is suitable for settling (Heyward and Negri, 1999). Therefore, in order to discourage premature settlement, we frequently removed all of the larvae from a given container in order to wipe down and remove the biofilm from their culture vessel. Even with these precautions we still had settlement on everything from the container to the PVC to the screen itself. (Editor's note: The microenvironment and the searching behavior of larvae settling probably does depend on numerous cues, but settlement may not require biofilms. There are things you can do to inhibit settlement and things to enhance it, but the requirement of cues is very uncertain at this point.)

Figure 7: This is one of a dozen or so trays used to culture coral larvae. You can see that the developing eggs tended to gather into clumps at the surface and on the sides.

Researchers still have only a very modest knowledge of the captive care of most coral larvae. Planula larvae are tiny creatures, and they interact with other tiny creatures, sometimes not for the best. Toward the end of our culturing effort, a ciliated coral predator invaded our system, and it took a toll on some of our coral recruits. This time-lapse video shows a small plague of ciliates almost completely consuming a large primary polyp of F. fragum within the span of about an hour. We had intended to use the Favia recruits for Diadema grazing experiments, but with the emergence of this unforeseen factor, we decided instead to document and learn what we could from this still unidentified protozoan. Ironically, these ciliated predators superficially resembled planulae in size, color and movement. Because the ciliates consumed coral tissue and green fluorescent proteins, the new pest was particularly easy to see using fluorescence enhancing equipment.


Working with corals in the wild and in captivity is still not an exact science. Unforeseen events can occur, and in order to learn from the experience regardless of the outcome, the expectations of the scientist and aquarist must be flexible enough to roll with the punches. This season, I was rewarded with two such learning events; once during our collection of gametes and the other during the culture of larvae. For the last two decades aquarists have been the pioneers of coral culture. Currently, the professional aquarists that participate in SECORE are at the forefront of this new aspect of coral culture. I hope that in time our knowledge of coral spawning will increase so that further breakthroughs in coral culture will afford home aquarists the opportunity to work increasingly with sexual coral reproduction. Aquarists frequently report the recruitment of coral spat from larvae released by Pocillopora, Tubastrea and some Euphyllia species. Many of the highly coveted, large-polyped species of Scolymia, Cynarina, Dendrophylia and Balanophylia are brooding corals which command high prices because they are only currently available by the collection of scarce, wild specimens. The rearing of larvae released by brooding corals is relatively easy, and although fragmentation has allowed for aquarists to asexually reproduce many coral species, many other corals are still harvested from the wild in great quantities because there are no known methods of reproducing them in captivity. Because of the decline in the abundance of some of these species, the European Union occasionally suspends their importation. I hope that, in time, aquarists become increasingly willing to put in the effort to advance the techniques for reproducing corals in captivity so that we can alleviate harvesting pressure on wild-collected colonies, and so the hobby can continue to increase our understanding of coral biology as a whole.

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Coral Spawning in Puerto Rico by Jake Adams -