Field in Focus

Field in Focus is a video series that brings you into the field with Smithsonian scientists working to save species around the globe.

Coral reefs support one-quarter of all ocean life, help feed millions of people, and protect homes and cities. Yet, they are threatened by the global effects of climate change. As oceans warm and become more acidic, corals may not be able to adapt fast enough to survive. If reefs disappear, the entire world would feel the impact. Smithsonian scientists have spent years studying coral reefs and thinking creatively about how to save them. Follow them into the field to see why their work is bringing new hope to the future of coral conservation and restoration.

Update as of Sept. 7, 2021: The coral larvae in this film are now two years old and thriving under human care! They represent one of the largest living wildlife populations ever created from cryopreserved material. Read more about this success.

Coral 101

They may look like plants, but corals are actually animals. They come in many shapes, sizes and colors, but nearly all corals live in colonies formed by many tiny, genetically-identical individuals, called polyps.

There are soft corals with thick, pliable branches and thin, flexible fan corals that anchor in the mud or sand. Some are found deep in the ocean’s dark, cool waters, while others inhabit the warm, shallow tropics.

Stony corals produce and grow hard, calcium carbonate skeletons (like human skeletons that develop over time). They are the builders and architects of the oceans, forming reef structures as they grow. So, corals are both animals and habitat. For this project, Smithsonian scientists focused their efforts on a species of stony coral, called elkhorn coral (Acropora palmata).

Elkhorn coral is named for its thick branches which resemble an elk’s antlers. It is one of the largest coral species in all the oceans, and its unique shape and structure helps protect shorelines against damaging wave action. Elkhorn corals also create hiding places for fish and other marine animals. So, in some sense, they are the apartments of the oceans.

Coral Reproduction

Like most coral, elkhorn coral reproduces in two ways. The first is by fragmentation, which happens when small pieces of an elkhorn coral break loose. These pieces can jam into crevices, reattach and form new colonies that are clones of the original. Coral can also reproduce through sexual reproduction, which creates new individuals. Those individuals may have new genetic traits that could help coral adapt to changing ocean conditions.

Many corals, including elkhorn, are hermaphrodites – which means they produce both male and female gametes. Most elkhorn coral spawn for just a few nights each summer, following a full moon. About three hours after sunset, the corals simultaneously release their egg-sperm bundles into the water. The bundles float to the surface of the sea, where they break apart and become fertilized eggs.

Life Stages of an Elkhorn Coral

1. Embryo

After an egg is fertilized, its cells repeatedly divide as it develops into a swimming larva.

2. Larvae

Elkhorn coral larvae drift through the water for four to five days before settling onto a hard surface, usually another healthy reef.

3. Settled Polyp

Now attached, the larva becomes a polyp. It starts to secrete a hard skeleton made of calcium carbonate. Soon after, many corals take algae into their tissues. The algae have a symbiotic relationship with coral. They use their photosynthetic machinery to produce sugars that feed coral.

4. Colony

The polyps start to divide, making more genetically identical polyps that form a colony. Colonies can grow into massive structures containing hundreds of thousands of polyps that attach to each other. These dense skeletons can form the foundation of new reefs when they die. Once they reach sexual maturity, the life cycle begins again.

The Challenges Coral Reefs Face

Coral reefs are hubs of biodiversity, rivaling rainforests on land. They provide shelter for marine plants and animals, form a buffer between land and sea, bolster the global economy and are a source for new, potentially life-saving medicines. Despite their importance, coral reefs cover less than 1% of the entire ocean — and they are rapidly disappearing. Reefs are threatened by over-fishing, harvesting, development and pollution. The damaging effects of climate change, such as ocean acidification and warming waters, also make corals more susceptible to bleaching and disease.

Half of the world’s corals have died in the last 100 years alone, and three quarters of the ocean’s remaining coral reefs are expected to experience annual bleaching events by the end of the century. The Caribbean has already lost 98% of its elkhorn coral.

A large elkhorn coral underwater on a sunny day in Curacao

As coral populations are threatened and die, fewer individuals are left to reproduce, causing these populations to become less genetically diverse. Genetic diversity is critical for wildlife health, especially for animals that are impacted by a changing environment. A biodiverse population of coral is better equipped to endure environmental pressures, like warming waters, because it is more likely that some individuals will have genetic variations to help them adapt and survive.

Maintaining genetic diversity among corals can be challenging. The adults are attached to a sea bottom, while the floating larvae are restricted by ocean currents and the space available to settle. This limits the number of genes that can be shared among corals. For example, the eastern population of elkhorn coral in Curaçao would not naturally meet and reproduce with the western population in Florida. One population might have advantageous traits that could help corals survive warming waters, so how can two separate populations come together?

Scientists are creating these long-distance romances by taking coral sperm from one area and introducing it to coral eggs from another area. This process is called assisted gene flow and was first demonstrated with coral on Australia’s Great Barrier Reef. However, corals from the same species that live far apart don’t always spawn at the same time. To overcome this obstacle, Smithsonian scientists developed an advanced reproductive technique, called sperm cryopreservation.

They first collect and freeze coral sperm to preserve it. Then, they can transport the cryopreserved sperm, thaw it and introduce it to fresh eggs when the time is right. For the first time, the researchers tested assisted gene flow on corals using frozen sperm to match the eastern and western populations of elkhorn corals. The coral sperm that they used had been frozen and stored for nearly 10 years, and the young elkhorns that resulted are thriving under human care in Florida. The success of this first trial introduces a brand-new tool for coral restoration and the future survival of the ocean’s reefs.

Photo and Video Gallery

A coral researcher dives underwater at night holding a flashlight to look at an elkhorn coral

Estimating the exact date and time that coral will spawn can be tricky. Scientists in Curaçao dove for more than 40 nights before the elkhorn corals they were monitoring finally released their egg-sperm bundles. The team's patience paid off, and they successfully collected the samples they needed.

An underwater photo of the Great Barrier Reef. Different types and colors of coral form the reef and large fish can be seen swimming in the water behind them.

Some shallow, warm-water corals get their amazing green, brown and red color from algal dinoflagellates that can photosynthesize. The algae and coral have a symbiotic relationship. Algal symbionts live within the coral tissues where they are protected and in return provide food for the corals. Like other plants, the symbionts use sunlight to produce sugars, and the corals consume the sugars. Symbiotic algae provide up to 75% of a coral’s food.

A view from the water of colorful buildings on the shoreline of Curacao under a pink and blue sky during the sunset

Curaçao is part of the Lesser Antilles — a group of islands in the Caribbean Sea — and has a population of about 160,000 people. Pictured here is Willemstad, Curaçao's capital city.

Three large tanks where coral sperm samples are frozen and stored (cryopreserved). This portion of the coral biobank is located at a laboratory in Hawaii.

This small biorepository, donated by the Smithsonian Women’s Committee and located at the laboratory in Hawaii, holds cryopreserved coral samples. It forms the backbone of the team’s conservation operation, allowing them to store samples for short periods of time before shipping them for long-term storage to places like the USDA’s National Animal Germplasm Program.

Green sea turtles are herbivores. They graze on the marine algae growing on coral reefs. These turtles also stop by reefs with "cleaning stations," where reef fish remove pests and other fouling organisms from their shells and delicate skin.

Coral reefs host 25% of all life in the ocean. In the face of changing oceans, healthy and diverse reefs like this one are degrading and will disappear unless strong conservation actions are taken.

An underwater photo of a bleached mushroom coral among other healthy, colored corals

An increase in water temperature of just 2 degrees Celsius over many weeks can cause corals to bleach. What is coral bleaching? Corals get most of their color and food from symbiotic algae (symbionts). Warming waters can cause the algae to stress and the fragile symbiotic relationship to break, resulting in the corals expelling the algae. When a coral loses its symbionts, its tissue becomes translucent and its skeleton becomes visible. The coral appears bone-white or “bleached.”

Marine researcher Claire Lager is equipped with scuba gear as she examines an underwater coral nursery. The coral "tree" is built of PVC pipe and has pieces of coral hanging from its branches.

The Smithsonian team and partners also built and are caring for coral tree nurseries in the Pacific Ocean, specifically in Hawaii and Moorea. Here, scientists can study how climate change impacts coral growth and develop better methods of growing coral for future reef restorations.

Most species of octopus are nocturnal hunters and are often seen during nighttime coral spawning events. During the day, octopuses utilize space within coral reefs for shelter.

An underwater photo of a large elkhorn coral with branches that resemble the antlers of an alk

Scientists estimate that some coral reefs are between 5,000 and 10,000 years old. However, fossil records suggest that coral reefs of some type have existed in our oceans for about 50 million years.

An underwater photo of a large, colorful elkhorn coral with many branches that resemble the antlers of an elk. Other large corals can be seen in the background.

Although a single stony coral polyp is just a few millimeters in diameter, large colonies can collectively weigh several tons. Elkhorns are the giant sequoias of coral reefs.

Smithsonian coral researchers wearing protective gloves and masks look inside a tank with liquid nitrogen

Smithsonian scientists and partners invented the processes and tools for cryopreserving corals. Now, more than 37 species of coral are conserved in a global coral biorepository.

Corals are both animals and habitat. They create shelter for other wildlife, much like trees in a forest. Large stony corals, like elkhorn corals, are critical to a healthy and diverse reef ecosystem. In the Caribbean, these major reef-building corals provide homes for many marine species, and also help protect human homes from damaging storms.

An elkhorn coral underwater in the dark with a light shone on it revealing the eggs it is releasing

Spawning happens quickly once the elkhorn corals’ egg-sperm bundles are released into the water. The entire process is complete in about 45 minutes. Corals are one of the most reproductively restricted animals on the planet. Some species reproduce on just two nights a year for about 40 minutes each night.

An underwater photo of a bleached coral next to a healthy coral. The healthy coral is vibrant and colorful, while the bleached coral is a stark white.

Corals can survive bleaching for a short period of time by using their stinging tentacles to capture food, but they may die if warm temperatures persist for about 10 or more weeks. Corals that do survive a stressful bleaching event are weakened, more vulnerable to disease and less likely to reproduce. As seen in this photo, some species of coral are more vulnerable to bleaching. Susceptibility to bleaching can also vary between individuals of the same species.

Meet the Smithsonian Conservation Biology Institute Team

A photo of coral researcher Mary Hagedorn in Curacao

Mary Hagedorn, Ph.D., Senior Research Scientist

Mary Hagedorn received her Ph.D. in marine biology from Scripps Institution of Oceanography and has worked in aquatic ecosystems across the globe, from the Amazon to Africa. She has taught university classes, lectures frequently, maintains an active laboratory with graduate and postdoc students, and is a successful researcher and grant writer with collaborators in more than 30 institutions around the world. In 2000, she received the prestigious George E. Burch Fellowship in Theoretic Medicine and Affiliated Theoretic Sciences. She was nominated for the Pew Fellowship in Marine Conservation in 2005 and was a 2012 finalist for the Rolex Award for Enterprise. She created the first genome repository for endangered coral species in the Caribbean, Hawaii and the Great Barrier Reef and has distributed this germplasm to frozen banks around the world. Today, she is the director of the Reef Recovery Initiative, a global coral conservation program.

A photo of Smithsonian Conservation Biology Institute coral researcher Clair Lager in Curacao

Claire Lager, Lab Manager/Biological Technician

Claire Lager has a background in coral reef ecology and earned her master’s degree from the University of Hawaii at Manoa. Her passion for coral reefs and desire to improve coral conservation led her to join Mary Hagedorn’s team at the Smithsonian Conservation Biology Institute, where she has worked since 2016.

This project is a collaboration between the Smithsonian Conservation Biology Institute, CARMABI Research Station, The Florida Aquarium Center for Conservation and Mote Marine Laboratory, with support from colleagues at NOAA Fisheries Southeast Regional Office, USDA National Animal Germplasm Program, Hawaii Institute of Marine Biology, University of Amsterdam, University of Florida, SECORE International and the Coral Restoration Foundation.

This project was made possible by the Paul G. Allen Family Foundation, Paul M. Angell Family Foundation and Volgenau-Fitzgerald Family Fund.