The Crown-of-Thorns Seastar: A Master's Thesis in Marine Science
Published on 12/20/24
Written by Jessica Colla
If you’re thinking about pursuing a master’s degree in marine science or a related field, you may be wondering: what does the research actually look like? Well, one of the most fascinating and impactful topics I had the opportunity to explore in my master’s thesis was the Crown of Thorns Sea Star (COTS), a creature that plays a controversial and pivotal role in the health of coral reef ecosystems. But why focus on this spiny sea star? Let’s dive in and explore the COTS, its environmental impact, and why it was a perfect research subject for my thesis.
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getting started with a master’s degree:
After living and working in Maui as a scuba instructor and whale watching naturalist for a few years, I decided to pursue a master’s degree in Marine Science & Management. For about a year, I applied to several universities both in the United States and internationally. I was thrilled to receive an acceptance letter from the University of Sydney, which marked the beginning of a transformative journey. I moved to Australia to complete a year of coursework, followed by a research pathway for 2 more years. My focus was on Crown-of-Thorns Sea Stars (COTS) and how their development and lifecycle is impacted by increased nutrient loading from tropical storms—a direct result of anthropogenic climate change. I had the opportunity to work under the guidance of Dr. Maria Byrne, where my research examined how extreme runoff events allowed more COTS sea star larvae to reach adulthood, contributing to catastrophic outbreaks on the Great Barrier Reef.
The experience of collecting, processing, analyzing, and summarizing data for my master’s thesis was challenging but deeply rewarding. This project also reinforced the value of my undergraduate thesis, which had prepared me for the rigor of graduate-level research. Given the competitive nature of the field, especially in today’s climate of “educational inflation,” I firmly believe this level of education was both necessary and informative for my career. I highly recommend this path for anyone hoping to pursue a professional career in marine science, particularly for those who aspire to work in research, policy, or management roles beyond fieldwork and entry-level positions.
Measuring Crown of Thorns Seastar (COTS) larvae allometry (body shape & size) under the microscope at University of Sydney, Australia.
The Crown of Thorns Sea Star: A Closer Look
The Crown of Thorns Sea Star (COTS), also known as Acanthaster planci, is a large predatory sea star that thrives in tropical coral reef ecosystems of the Central & South Pacific & Indian Oceans. These sea stars are often recognized by their striking appearance—growing to nearly 1 meter in diameter, with up to 22 arms and completely covered in 1-inch long, thorn-like, venomous spines to ward of predators! COTS are physically intimidating compared to most docile-looking seastar species and are often depicted as a villain in the underwater world. However, their role in the ecosystem is much more complex.
COTS are specialist predators known as corallivores—or coral eaters—meaning they feed almost exclusively on coral tissue, using their specialized feeding method. When a COTS feeds, it extrudes its stomach from its body, releasing digestive enzymes to break down coral tissues before slurping it back in—almost like drinking a milkshake. While this may sound unpleasant, in healthy balanced ecosystems, this behavior is actually beneficial. COTS help to prevent coral overcrowding by consuming compromised individuals, making space for new coral recruits (baby corals) to grow on the seafloor and increasing overall coral biodiversity on the reef. This is similar to how sharks cull sick or weak prey, maintaining balance in the ecosystem.
A Synchronized Spawning Strategy
Crown of thorns sea stars are highly effective r-strategists - characterized by a high reproductive rate and low juvenile survivorship. They are synchronized spawners, meaning that all sea stars in a given area will gather at a specific time (typically driven by seasonal & lunar cycles) where they release massive clouds of eggs and sperm into the water column, where fertilization occurs. Synchronized spawning maximizes the chances of successful reproduction by increasing the sheer number of potential offspring.
Female COTS are incredibly fertile, with the ability to release up to one million eggs in a single synchronized spawn! However, this abundance comes with a high risk—most of the larval recruits will either be eaten by predators or fail to find enough food and will die off in the planktonic stage. Despite these odds, the sheer number of eggs produced means that some larvae will survive, find enough food, and settle to the sea floor as juvenile sea stars. This reproductive strategy, characterized by producing large quantities of offspring, greatly enhances the likelihood that at least a few of them will survive to maturity and contribute to the next generation.
The Environmental Impact of COTS
However, this balance is being disturbed. COTS populations are exploding in certain parts of the Indo-Pacific, the Great Barrier Reef of Australia, and here in Hawai’i. These population booms—often referred to as “COTS outbreaks”—can wreak havoc on coral reefs, consuming vast swathes of coral, which threatens biodiversity and ecosystem health. Despite being native (indigenous) to these outbreak regions, their impacts resemble the overwhelm of an invasive species.
But why is this happening?
A key part of my research focused on understanding WHY COTS populations have been increasing in certain areas. Normally, these sea stars follow a natural “boom and bust” population cycle, where they experience steady population growth followed by a rapid decline due to resource shortages (in this case, coral). However, scientists have observed that the “boom” phase is becoming more frequent and intense, with COTS outbreaks becoming more destructive to coral reefs and, subsequently, the organisms that call the reef home—which is over 25% of life in the ocean!
This unnatural population surge can be linked to several factors, including:
Overfishing, Over-Harvesting & Poaching — the removal key predators like the King Tritons Trumpet Sea Snail, Red Decorator Crab & Napoleon Wrasse
Climate Change — sea temperature rise & increased storm frequency/intensity weakens coral reefs, making them more vulnerable to COTS predation
Excessive Nutrient Loading & Pollution — excess nitrogen & phosphorus from coastal development, agriculture & pollution boosts COTS larvae survival rates
Negative Feedback Loop — as COTS populations rise, corals they prey on struggle to recover, leading to the degradation of entire reef ecosystems
Coral reefs are not just beautiful underwater ecosystems that feed & shelter marine life—they are vital to humans too! They support a diverse array of species that we rely on for consumption. Coral reefs are the centerpiece of ocean-based tourism, driving coastal economies. Reef structures protect coastal areas from storm surge, and even play a crucial role in medicine! To learn more about the importance of coral reefs, check out this blog post here! Therefore, when COTS outbreaks occur, the impacts ripple outward, affecting both marine species and human communities that rely on healthy coral reefs.
The two theories
Scientists are exploring several theories to address the crown of thorns sea star (COTS) problem, which can generally be grouped into two main categories: 1) "top-down" and 2) "bottom-up."
The Top Down Theory:
One of the key theories in addressing the proliferation of crown of thorns sea stars (COTS) is the "top-down" approach, which focuses on the success of adult COTS and examines why they thrive on our reefs. This line of inquiry led scientists to explore the predators of COTS. Despite being armed with hundreds of venomous spines, COTS have a few natural predators. One of the primary predators identified is the king triton's trumpet, a specialized sea snail that feeds on COTS. However, the king triton's trumpet has a beautiful and highly prized shell, making it a target for the seashell trade. Overfishing, illegal poaching, and the removal of these snails from the reefs have left COTS with fewer threats. Without natural predators, COTS roam more freely, consuming large amounts of coral in areas they might otherwise avoid. This lack of fear allows them to access prime locations for feeding and reproduction, exacerbating their spread. The decline of king triton's trumpets in vulnerable regions, such as the Great Barrier Reef, highlights this issue. A colleague of mine from the University of Sydney, who studied this dynamic, found that after weeks of diving, he could not locate a single king triton's trumpet to study. This discovery not only affected his research but also underscored how scarce these sea snails have become in areas critical to maintaining a healthy reef ecosystem.
The Bottom Up Theory:
Now that we've explored the top-down theory, let's turn to the “bottom-up” approach, which focuses on the early stages of the crown of thorns sea star's lifecycle. Researchers studying bottom-up theories are investigating why COTS planktonic larvae survive and become recruits—baby sea stars that settle on the reef and grow into adults. Unlike the top-down theory, which emphasizes predator pressure, the bottom-up theory looks at environmental conditions that might favor larval survival. These conditions include factors like coastal runoff, global warming, eutrophication (an increase in nutrients), and dead zones (areas with low nutrients).
Eutrophication (excess nutrients), driven increase heavy rainfall and coastal runoff from tropical storms, and uninhibited by coastal development and near-coastal agriculture affected COTS larval success in the Great Barrier Reef (GBR). This theory suggests that higher nutrient concentrations like nitrogen and phosphorus washed into the GBR during the rainy season increased the survival rates of COTS larvae, allowing more of them to settle and grow into adult sea stars. COTS are naturally adapted to oligotrophic (low-nutrient) environments typical of coral reefs (picture the clear blue waters of the tropics). However, they are also highly responsive to nutrient spikes, which trigger changes in their growth patterns to take advantage of the surplus food source and ultimately increase likelihood of survival into adulthood.
My Research in Bottom-Up Theory:
During my research, we tested different nutrient concentrations (from oligotrophic to eutrophic conditions) and varied the timing of nutrient exposure during the larval stage - essentially replicating the GBR’s rainy season during typical COTS larval development. We found that larvae introduced to higher nutrients earlier in their development grew larger and more successfully, with bigger stomachs to accommodate the increased food. In contrast, larvae exposed to high nutrients later in development were smaller and less successful. In simple terms, the extra food in the water column after a heavy rain was creating well-fed “super larvae!” These results support the "enhanced nutrient hypothesis," which suggests that nutrient-rich environments can boost larval survival and settlement, ultimately leading to increased COTS populations and outbreaks.
In reality, both the top-down and bottom-up theories contribute to the factors driving the proliferation and success of crown-of-thorns sea stars.
What’s Being Done to Manage COTS Populations?
Scientists and marine managers have developed a range of strategies to tackle the crown-of-thorns sea star outbreaks threatening coral reefs. These efforts include: physical removal, biological controls, and technological innovations. Each approach has its benefits and challenges, and they are often used in combination to maximize effectiveness.
1. Physical Removal of COTS from Reefs
Divers physically remove COTS from reefs by hand, a labor-intensive but effective method in localized areas (especially tourism hotspots) experiencing outbreaks. Divers use tools like tongs or three-prong spears to collect the sea stars, which are then safely disposed of onshore where they cannot spontaneously spawn or regenerate, harming reefs further. While this approach is effective in small-scale outbreaks, it is highly resource-intensive and time-consuming. The Australian government frequently outsourced this initiative to “platforms of opportunity”—tourism operators who already work in the impacted areas. Large-scale COTS population explosions make physical removal alone insufficient as a long-term solution.
COTS eradication efforts during outbreak event in Fiji
COTS placed in bucket & removed from reef environment
2. Biological Controls: Reintroducing Natural Predators
Biological controls focus on restoring ecological balance by reintroducing or protecting natural COTS predators like the King Triton’s trumpet sea snail. This large marine snail preys on COTS but has been over-harvested for its beautiful shell, severely reducing its population in many areas. Reintroducing the King Triton’s trumpet is a promising approach, but it faces several challenges. These include the difficulty of breeding and reintroducing the snails in sufficient numbers and ensuring their survival in degraded reef ecosystems and the risk of poaching for the shell trade. Scientists are also exploring other potential predators and how predator-prey dynamics could be leveraged to control COTS populations naturally, such as the Titan Triggerfish and the Napoleon Wrasse.
3. Technological Innovations: Underwater Drones and Robots
New technologies, such as underwater drones and autonomous robots, are revolutionizing COTS population management. These machines are designed to identify and target COTS in hard-to-reach areas, making them a valuable tool for large-scale reef management. Some drones are equipped with systems to inject a lethal solution directly into the sea stars, killing them quickly and efficiently. Others are used for monitoring and mapping outbreaks, providing real-time data to guide management efforts. Although these technologies hold great promise, their deployment can be expensive, and they require ongoing maintenance and technical expertise.
4. Coastal Development and Riparian Buffer Management
Coastal development and riparian buffer management play a crucial role in mitigating the environmental factors that contribute to COTS outbreaks. Increased nutrient loading from runoff, often caused by agricultural practices, urban development, and deforestation, exacerbates the problem by promoting the growth of plankton that feeds COTS larvae. By implementing riparian buffers—vegetated areas along shorelines that filter runoff—along with stricter regulations on coastal development, the amount of nutrients entering the marine ecosystem can be reduced. These buffers help stabilize coastal ecosystems, prevent erosion, and minimize the conditions that promote COTS larvae survival. However, the challenge lies in balancing coastal development with conservation efforts, as these areas often compete with the need for housing, infrastructure, and agricultural expansion.
to manage or not to manage?
The fight against COTS outbreaks doesn’t end with these short-term solutions. The increasing frequency of these outbreaks highlights the urgent need for more research to understand the broader biological, ecological and human impacts. Addressing the root causes of these population explosions, like climate change and human activities, is key to creating long-term solutions that protect coral reefs and the species that rely on them.
It is important to recognize that COTS are NOT considered invasive but rather a pest species, as their feeding habits can harm coral reefs, which are vital to human well-being, food security, and the economy. Because of this, population management may be necessary, but it requires careful, research-driven decision-making, often supported by university research & government-funded studies to ensure the right approach is taken.
Crown of thorns sea stars are often vilified as “reef destroyers,” even though this perception is not entirely accurate. Usually the first to blame for coral damage, their intimidating appearance can overshadow the fact that they are NOT the only animals that consume corals on our reefs. Take, for example, the adorable Pincushion Sea Star, which many people find charming. Despite its less intimidating appearance, it is also a specialized predator that feeds on coral tissue using the exact same feeding technique as COTS. The same is true for parrotfish, which have teeth fused into beaks that allow them to rasp coral tissue off the limestone. Parrotfish are also coral specialists, yet they are rarely vilified to the same extent as COTS.
COTS serve as remarkable indicators of human impact on the ocean, with their population growth and behavioral changes directly reflecting the effects of our activities on marine ecosystems.The disproportionate blame placed on the COTS sea star exemplifies how certain animals become scapegoats, which can lead to their mismanagement. Understanding the biology of crown of thorns sea stars (COTS) and their population cycles is crucial before implementing population management strategies. Using population data to assess current numbers on the reef helps determine if intervention is needed. However, actions like culling can be harmful if applied at the wrong time, potentially disrupting a healthy ecosystem.
A Master’s Thesis Experience
In my own experience, researching the Crown of Thorns Sea Star was an incredibly rewarding and challenging endeavor. My research involved not just laboratory work, but also field studies to gather data from reef ecosystems, giving me a firsthand understanding of how these sea stars interact with coral and their environment. It was a deep dive into marine science, requiring both critical thinking and creativity to find innovative ways to monitor and address COTS outbreaks. I have observed COTS on coral reefs in both sustainable and unsustainable population numbers since then, and been part of several management initiatives in 3 different countries.
For anyone considering a master’s degree in marine science or a related field, I highly encourage you to pursue a topic that fascinates you and makes a real-world impact. Research in this field can be challenging, but it is also incredibly rewarding as you contribute to solving the pressing environmental issues of our time.
Conclusion: Hope for Coral Reefs and the Future of Marine Science
While the Crown of Thorns Sea Star can pose a significant threat to coral reefs, the ongoing research and efforts to manage COTS populations are offering hope for the future. New technologies, sustainable practices, and continued scientific inquiry are essential to mitigating the impacts of COTS outbreaks. As we learn more about these sea stars and the complexities of coral ecosystems, we can create a more resilient approach to preserving our oceans.
If you’re passionate about marine science and eager to make a difference, pursuing a master’s degree in this field is an exciting opportunity to contribute to meaningful environmental change. The challenges may be great, but the potential for discovery and positive impact is even greater.
If you're interested in pursuing a master's or are already in the process of considering a research topic, I’d love to hear your thoughts! Leave a comment below with your field of study and your proposed research idea.
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