If you ever want to stump someone in a game of “Animal, Vegetable, or Mineral,” pick coral–it’s all three. The coral in a true sense is an animal: a sessile marine invertebrate in the same family as the jellyfish (technically, they’re in the same phylum). Many corals also biosecrete a calcium carbonate skeleton and host symbiotic algae, so you can get away with pretending they’re rocks and plants, too.
When corals want to reproduce, they have several options. Broadcast spawning, which is what we’re interested in, happens when gametes (sperm and eggs) are released into the water column, where they mix with the gametes of other broadcasters of the same species. Fertilization occurs externally, and the larvae produced from the union float around at the mercy of the ocean for a few days before developing the ability to swim, to settle out on a substrate, and to acquire their algal symbionts.
Think for a moment about how coordinated a spawning event must be for it to work. Every colony that wants to reproduce needs to release its gametes on the same night. Even more, they need to release at the same time, because the sperm is only viable for about three hours. It’s a very narrow window, but evolution has converged such that many species spawn at the same time on the same night in a predictable way, using cues such as the lunar cycle and water temperature. So, how do you plan your research around a spawning?
Step One: Find an Appropriate Reef.
Actually, this is a difficult process. Acropora palmata is in peril. Though this hermatypic (reef-building) coral used to be quite prominent in the Caribbean, its biomass has declined so much in recent decades that it has been listed as threatened under the Endangered Species Act. Vast fields of barren elkhorn skeleton are spread throughout the Caribbean Sea. We needed to find a reef with a suitable number of corals, a consistent spawning history, and enough diversity to support several different crosses (combinations of egg from one parent and sperm from another). Practically speaking, we needed a reef that was close to shore so we would have enough time to get into the water, gather the spawn, get out, and start the fertilizations within that three-hour window. We also needed it to be within U.S. jurisdiction to ease the approval of permits and the transport of our samples back to Pennsylvania. The Tres Palmas Marine Reserve in Rincon, Puerto Rico, fulfilled all these requirements.
Step Two: Get There.
Sounds easy, right? Just hop on a plane. But it’s not the people that are difficult to transport; it’s the equipment. After spending months planning your experiment and figuring out what you definitely need, what you might need, and what you hope you won’t need but you have to take anyway, it all has to be packed and shipped. We sent about nine boxes to Puerto Rico in advance and brought four on the plane with us, along with our computers and dive gear. We found room for clothes in there somewhere…
Step Three: Build.
Now that you’ve rented a house right off the reef and your gear has arrived, it’s time to construct the aquaria that will hold the developing larvae. Our experiment revolves around rearing larvae from specific crosses at three different temperatures, so we had to build three aquarium systems to accommodate them. These systems each involve heaters and chillers, seawater pumped from the ocean, a sump, replicate tanks, and kriesels (cups) for larvae with mesh and spray racks to provide circulation.
Step Four: Set Your Nets.
So you know where the coral colonies are, and based on previous years you know approximately when they’ll spawn (for A. palmata in Rincon, it’s usually 9 PM on the fifth day after the August full moon), so now you need some way of collecting the gametes when they’re released. Coral spawn is positively buoyant, so it can gather at the surface and mingle with spawn from other colonies. We constructed nets with containers at the top that trap the gametes as they float up. The nets are placed on the reef at 6 PM and removed after spawning. They cause only mild abrasions on the edges of the branches they cover, and we leave enough of the colony uncovered so that we’re not stealing all of the spawn it’s invested so much energy into producing.
Step Five: Spawning!
Now you sit back and wait. After ‘setting,’ where the coral polyps move the gamete bundles (packages of sperm and eggs) into their central cavity, the bundles are ejected into the water column. It’s something you definitely need to see–in person if you get the chance. But if you don’t have the luxury of a tropical ocean in your backyard, the videos below are for you. The first shows Acropora palmata releasing gamete bundles, while the second shows the nets in action.
Step Six: Processing and Crosses.
After only a few minutes, the bundles start to break up, and the eggs and sperm separate. Our crosses need to be performed at this time. What follows is organized chaos: the collection containers are all labeled so we know which colonies produced which bundles; batch cultures are made by combining equal volumes of bundles from all the parents; eggs are washed through a filter to remove sperm and reduce the chance of self-fertilization; separated eggs and sperm are combined in different combinations to produce genetically distinct crosses; the crosses are allowed an hour in a cup to fertilize, along with a little Marvin Gaye to set the mood.
Then the new larvae are washed again to clean them; at the same time, eggs and sperm from each parent are sampled and preserved in various media for later genetic tests and developmental comparisons. Usually we don’t even have time to remove our neoprene wetsuits before we’re pipetting off layers of eggs and labeling vials. Everything happens all at once and at this point it’s late and you’re tired but you just keep going. Samples of the fertilized larvae need to be taken every few hours to track development. Once the larvae have been transported to the kriesels, they require daily washings to prevent fowling. It’s a lot of work, and most of it takes place in the small hours of the morning. That’s why the blog hasn’t been updated for a few days!
Below you’ll find two videos of processing: the first of Iliana filtering eggs and the second of Dannise injecting fertilized larvae into the kriesels.
Step Seven: Settlement and Outplanting.
After a few days, the larvae become capable of swimming, and will seek out substrate on which to settle. We provide it in the form of settlement tiles: preconditioned ceramic tiles that were placed out in the ocean months ago and have since been accumulating a biofilm. Without that natural layer, coral larvae generally won’t settle. This is the stage we’re at currently, with most of our larvae starting to swim in circles. Today we switched them over to kriesels without mesh into which we also dumped some freshly isolated zooxanthellae (the symbiotic algae). Broadcast spawn are born without their own complement of zooxanthellae–by providing different types at different temperatures, we’ll test to see if thermal stress affects the specificity of the symbiosis. Once the larvae settle onto the tiles, we’ll dive back onto the reef and attach the tiles to the natural substrate. Periodic checks by local collaborators will help us monitor how well the settled larvae (now termed ‘spat’) survive.
Step Eight: Finish Up.
With the experiment done and the larvae either returned to the reef or shipped to the SECORE zoos, we’ll need to break down our equipment and store it for next year. Then it’s ‘Adios, Puerto Rico,” and we return to landlocked Pennsylvania to perform our molecular and population genetic tests. We’ll have a lot of data to sort through, so we have our work cut out for us. With any luck, we’ll be able to figure out a better way of conserving our reefs by incorporating genetic diversity into management strategies.
So that’s coral spawning in a nutshell. We still have a week left, and most of the incredibly time-sensitive work has been done, so expect regular blog updates to continue. We’re trying to work out a good way to share the multitude of photographs we’ve all taken, and we’ll be posting some brief interviews shortly. Though spawning is over, our work is only really beginning. We hope you can stick around and continue to learn more about our reef conservation research!