Coral reefs have survived on Earth for more than 200 million years, and more than 500 million people depend on healthy corals for sustenance as well as for a healthy eco-tourism industry. Coral reefs also protect our coastal areas from erosion and storm surges, and act as underwater “rainforests”, providing habitats for a plethora of species.
In recent years the world has experienced a steady but concerning decline in coral reefs and the health of corals, with scientists estimating that half of the world’s corals have been lost in the past three decades. This is due to rising sea temperatures due to climate change, trawling fishing vessels, ocean acidification, storm events increasing in intensity and frequency, and increased pollutants in the ocean. These impacts are predominantly as a result of human actions. Many creatures are able to adapt and evolve to survive in a changing environment. However, this process takes hundreds of years, and major human-induced environmental changes are occurring fast, posing a threat to marine organisms, including coral reefs.
What Happens When Corals Are Stressed?
When corals are stressed by an unideal environment for prolonged periods of time, they undergo bleaching events. Coral bleaching is when stressed corals expel the colorful symbiotic algae living within their tissues, exposing the limestone skeleton of the corals, causing the corals to turn white. Corals can remain bleached for short periods of time and still recover if conditions improve. However, if the bleaching event continues, the corals eventually die of starvation. Over the past two decades, bleaching resulted in significant coral reef cover loss (including parts of the Great Barrier Reef), as well as the loss of ecosystem services provided by these reefs.
In 2015, scientists published an article in the Proceedings of the National Academy of Sciences (PNAS) that proposed a novel coral management strategy that has the potential to save corals: human-assisted evolution.
What Is Assisted Evolution?
The term “assisted evolution” refers to a range of approaches that involve active human intervention to speed up the rate of naturally occurring evolutionary processes. The purpose of assisted evolution is to enhance certain features and attributes in organisms, such as temperature or disease tolerance, enhanced growth and reproduction. Assisted evolution is not a new practice – humans have been utilizing evolutionary processes for thousands of years in order to create beneficial adaptations and alterations to existing organisms. One such example is selective breeding in dogs – where favorable traits were selectively enhanced and bred into individuals over time, to create new dog breeds.
Recently, naturally occurring behaviors such as acclimatization (when organisms adapt to environmental changes over their lifetimes), trans-generational acclimatization (adaptations to environmental changes are handed down to the next generation) and genetic modification have been used by humans in industries such as agriculture. This has allowed humans to produce crops that are more resilient to droughts and diseases as well as produce higher yields. This technology is allowing humans to alter organisms faster than they normally would through the natural process of evolution.
Assisting Coral Evolution
The authors of the study have proposed four main types of assisted evolution to be investigated for coral reef management.
1. Exposing natural coral stocks to stresses in an attempt to induce acclimatization and trans-generational acclimatization.
When corals are forced to undergo stresses similar to what might be naturally experienced under changing environmental conditions, the corals might potentially eject the current symbiotic algae living in their tissues and replace them with algae that is more beneficial for the new environmental conditions. This strategy will be faster than natural genetic adaptation, and as the symbiotic algae can be transferred from parent to offspring in coral, it may last for several generations.
2. Actively modifying the symbiotic algae.
There are several types of coral symbionts, with some being specifically adapted to certain environments, such as surviving in warmer waters. Actively modifying the symbionts means taking strains of symbionts which naturally tolerate certain environmental conditions, and introducing them to coral reefs that require tolerance to that specific condition. This could mean corals can survive warming waters if they are introduced to symbionts that can tolerate warmer waters.
3. Genetically altering coral symbionts.
This differs from Strategy 2 above, as instead of using naturally occurring symbionts, research would focus on genetically modifying (changing the genetic structure) of the algae to be better suited for a variety of environmental conditions. This would involve growing these symbionts in a lab, and then introducing them to wild coral colonies, hopefully allowing the coral reefs to be better adapted to a variety of environmental changes.
4. Selectively breeding coral strains that are better suited.
This strategy is similar to selective breeding of dogs. It would mean taking the corals that are best at tolerating environmental changes, and selectively breeding those individuals for generations, until a line of corals has been produced that can survive these environmental changes. Selective breeding is the most promising technique, and research done by the Hawai’I Institute of Marine Biology has shown that selective breeding of corals has spawned coral offspring that could cope with 5oF warmer temperatures than offspring from parents that were not specifically selected.
Assisted evolution is still a novel, and controversial, idea in coral reef management. While it provides a glimmer of hope for the future of coral reefs, scientists are still developing their understanding of its possibilities and consequences. Although the strategies proposed above have the potential to ensure the survival of coral reefs, the authors did not discuss the issues that might arise when humans attempt to alter the course of evolution in corals. The strategies suggested are not to kick-start immediate assisted evolution of corals, but rather to encourage other researchers to start considering the possibilities of coral restoration through human-assisted evolution. The complexity of assisted evolution of corals is made more difficult due to the urgency of the problems faced by corals. Although research has suggested that natural processes can keep up with some environmental changes, such as small increases in oceanic temperatures, outcomes remain unclear for all coral species.