Coral spawning: a race against time

At 8:45 pm on the night of the new moon, rice corals all across Hawaiʻi begin to release millions of cream-colored bundles into the water. Each delicate package is the size of a pinhead and contains eggs and sperm that were created months before. Their parents have been patiently waiting for a synchrony of cues. When they arrive, if you are lucky enough to be underwater, you can watch as a myriad of gametes emerge from the coral’s polyps and make their journey to the surface like paper lanterns during autumn festivals. Once at the surface, the gentle jostling of the water will cause them to burst; tens of eggs will float, buoyed by fat reserves, while clouds of sperm sink below. The lucky eggs will fertilize and divide and form free swimming larvae. Most will be eaten by predators, a few will find a perfect place on the reef to settle and metamorphose into the first of many polyps, and even fewer will survive the coming weeks. Those polyps will continue to divide and the colony will grow, laying down hard calcium carbonate skeleton that, over time, will contribute to the landscape of the entire coral reef until they are ready to release bundles of their own. And, if left undisturbed, they will continue to do this every summer, like clockwork, indefinitely.  

Some of the disturbances that threaten reef-building corals today are vastly different than those 10,000 years ago and are in great part due to humans. Significant damage can be caused by an influx of pollution from urbanization, runoff from agriculture, sunblock on the skin of beachgoers, overfishing, and development in our coastal communities. However, the most devastating cannot be limited to specific geographical location: a sharp spike in seawater temperature caused by global climate change. Much of our research here in Dr. Ruth Gates’ Coral Lab at the Hawaiʻi Institute of Marine Biology (HIMB), where I’m a PhD candidate, centers on how we can increase coral survival under future climate change conditions, and coral spawning is a busy and exciting time for everyone. In addition to the opportunity to study how future conditions will impact the next generation of corals, spawning material allows us to test large scale rearing efforts and provides a snapshot as to how corals are doing Kaneohe Bay today.

Three field teams made of us Gates Lab graduate students, post docs, lab and project managers, interns and volunteers getting ready to head out for coral spawning. Click and drag to see everybody. (Photo: S. Matsuda)

One of the most common species of coral in Kāneʻohe Bay is the rice coral, Montipora capitata. You can recognize it easily; colonies range from burnt orange to dark chocolate brown and grow in large branching or plating structures. Rice corals are commonly found in the shallow reefs, and are simultaneous hermaphrodites, meaning that individuals produce eggs and sperm concurrently. They are also broadcast spawners whose eggs and sperm fertilize in the water column, which differs from brooders who release free-swimming larvae. Water temperatures, tides, atmospheric pressure, and the cycles of the moon simultaneously signal all the rice corals to release their gametes into the water column. On the night of and a few days following the new moon, for three consecutive months over the summer, at around 8:45 pm, the first bundles start making their way to the surface.

The author peers into a bucket containing a colony of Montipora capitata, or rice coral. (Photo: S. Matsuda)

Millions of rice coral egg and sperm bundles rising (left) and floating on the surface (right). (Photo: S. Matsuda)

On the night of the new moon in June of this year, our wild-type team, graduate students Carlo, Ariana and I, start gathering supplies to head out into the field: life jackets and a first aid kit, headlamps and spotlights, as many 2.5 gallon buckets as will fit on the floor of a small whaler, and plastic bins with mesh-covered windows zip-tied to PCV-handles. Our tools are not glamorous, but get the job done. At 8pm, we cast off and follow a heart trail of marker points (or “favorites”) on our Google map that we planted earlier to help us navigate the Bay in the dark.

We use Google “favorites” to help us navigate to our destination in the dark. (Image: A. Huffmyer)

As we near our destination, we use a spotlight to find the edge of the reef. The buckets are filled with an inch of sea water, and at 8:45 we lean over the edge of the boat with our headlamps trying to locate the first bundles. The light cuts through the water and is quickly rendered useless as dense clouds of plankton gather in its beam. 

Wild-type team scans the surface for bundles. (Photo: Carlo Caruso)

PhD candidate, Ariana Huffmyer, carefully scoops bundles off the side of the boat. Click and drag to look around you. (Photo: S. Matsuda) 

On a good night, millions of bundles will be released into the Bay in a little over 30 minutes. Tonight, after a few false alarms, a steady stream of bundles begins to arrive at the surface. We must work quickly and delicately – once the bundles reach the surface, they will soon burst, and we need time to aliquot them into the fertilization buckets in the right concentrations before they do. There is also another concern: inside the bundles is a toxin that is released if the bundles are crushed, which can kill an entire bucket of fertilized embryos. With this in mind, and as we finish collecting and sorting, the bundles begin to naturally burst and we can see tiny eggs floating at the surface above a cloud of sperm, and we follow the heart trail back to HIMB.

PhD candidate, Ariana Huffmyer, allocating the right densities of bundles to buckets of sea water. (Photo: S. Matsuda)

Once we pull up to the dock, the buckets are loaded onto the back of an old pickup truck that crawls over the gravel road to the outdoor lab. As we near, we are met with a sea of darting red lights from the headlamps of the team working to collect bundles from individual colonies that either bleached or were resistant during the previous bleaching events; the red light from their torches doesn’t interfere with the lunar lighting cues. We head into the sea of lights with the buckets to the platform around our 300 gallon tanks. Excess sperm is removed and new sea water added, and then left to fertilize overnight. Over the next days, cell division will lead to swimming larvae, some which will settle and metamorphose into a single polyp. This polyp will continue secrete calcium carbonate skeleton and bud new polyps, and as it grows will begin to contribute to the structure of the reef.

As rice coral bundles (left) begin to burst (middle), tiny eggs float at the surface while clouds of sperm (right) sink below. (Photo: S. Matsuda)

Microscopic view of rice coral embryos (left), swimming larvae (middle), and settle spat (right). Not to scale. (Photo: S. Matsuda)

Like a forest whose trees are cut, the absence of corals changes the entire ecosystem – animals move out, opportunistic organisms take over, and, depending on the severity of the loss, recovery may slowly begin, although not fast enough for the organisms and people who rely on the reef. Life on a coral reef is a busy but delicate balance, and warming seas can have consequences other than only bleaching. The material we get from this summer’s coral spawning events allows us to study these complex organisms and build strategies for protecting, conserving, and managing coral reefs for future generations.

About the Author

Shayle Matsuda is a science storyteller and PhD student at the Hawai'i Institute of Marine Biology. He uses DNA sequencing and 3D-imaging to study the dynamic symbioses between coral and their algal partners, and is particularly interested in the intersection of technology and biology for creative problem solving and education. He is passionate about science communication (WIRED.com, Story Collider, Nerd Nite), was the creator and host of the science happy hour series, Science, Neat, in San Francisco. He is on the ComSciCon Leadership Team (the National Communicating Science Workshop for STEM graduate students) and a member of the Diversity and Inclusion team at oSTEM. Out of the lab, you can find him making watercolor Sketchnotes (@wrong_whale), tinkering, and enjoying being out in nature as much as possible in Hawaii.