In recent years, jellyfish have been making waves in our oceans, and not in a good way. Imagine divers in New Zealand trying to fend off millions of tiny, stinging jellyfish, each no bigger than a grain of pepper. Or consider the massive swarm of moon jellyfish in Sweden that clogged up a nuclear reactor’s pipes, causing a shutdown. In the Sea of Japan, enormous Nomura’s jellyfish, weighing up to 200 kilograms and with bells as wide as two meters, are causing chaos for fishermen by breaking nets and eating local fish. These jellyfish are not just a nuisance; they are consuming fish eggs and larvae, disrupting marine farming, and competing with adult fish for resources. If this trend continues, we might face oceans dominated by jellyfish.
So, what can keep these jellyfish in check? Meet the sea turtle, one of the jellyfish’s oldest predators. Many marine animals eat jellyfish, but sea turtles, especially leatherbacks, are particularly effective. Leatherbacks are the largest sea turtles and have a diet that consists almost entirely of jellyfish. Over their 50-year lifespan, they consume over 1,000 metric tons of jellyfish. This is impressive because jellyfish are mostly water and low in calories, meaning a 500-kilogram leatherback needs to eat about 400 kilograms of jellyfish daily to stay healthy.
While some sea turtles target the protein-rich parts of jellyfish, leatherbacks eat them whole, consuming large numbers at once. Jellyfish have tentacles with cells called cnidocytes that contain venomous harpoons, known as nematocysts, which they use to sting and paralyze prey. However, these defenses are useless against sea turtles. The turtles have thick, keratin-covered scales that protect them from stings. Any jellyfish that gets caught is trapped by the keratinized spikes in the leatherback’s esophagus.
Despite being easy prey for sea turtles, jellyfish have a powerful defense: rapid reproduction. They can reproduce both sexually and asexually, allowing them to multiply quickly. In tropical areas, jellyfish reproduce year-round, while in temperate regions, they reproduce in massive blooms. Unfortunately, human activities are making these blooms more frequent. Fertilizer runoff from farms introduces chemicals that harm other fish but encourage jellyfish blooms. Climate change raises water temperatures, speeding up jellyfish reproduction and extending their breeding season. Additionally, marine construction and pollution provide more surfaces for jellyfish polyps to attach and grow.
To address these challenges, we need a variety of policy-based solutions. Protecting natural predators like sea turtles is crucial, as many are currently threatened. Small-scale fisheries often use gillnets that accidentally capture and kill hundreds of sea turtles each year. In the Eastern Pacific, this could lead to the extinction of leatherbacks in the next 60 years. Fortunately, researchers have developed simple tools to reduce these risks. For example, attaching green LED lights to gillnets helps sea turtles, dolphins, and seabirds avoid the nets. Solutions like these allow fishers to support their communities while protecting the ocean’s natural balance.
Research the impact of jellyfish on marine ecosystems and human activities. Prepare a presentation to share your findings with the class, focusing on specific examples such as the incidents in New Zealand, Sweden, and the Sea of Japan. Highlight the ecological and economic consequences of jellyfish overpopulation.
Write a short story from the perspective of a leatherback sea turtle. Describe a day in its life, focusing on its role in controlling jellyfish populations. Include details about its diet, the challenges it faces, and its interactions with the environment.
Participate in a debate about the role of human activities in promoting jellyfish blooms. One side will argue that human actions are the primary cause, while the other will focus on natural factors. Use evidence from the article and additional research to support your arguments.
Create a campaign to raise awareness about the importance of protecting sea turtles as natural predators of jellyfish. Develop posters, social media posts, or a short video to educate your peers about the threats sea turtles face and how they can help in conservation efforts.
Conduct a classroom experiment to simulate jellyfish reproduction. Use simple materials to demonstrate both sexual and asexual reproduction methods. Discuss how environmental factors like temperature and pollution can affect jellyfish populations, drawing connections to real-world scenarios.
Over the past two decades, jellyfish have begun to overwhelm our oceans. In New Zealand, divers swat off millions of stinging jellyfish, each no larger than a grain of pepper. In Sweden, a massive cluster of moon jellyfish shut down one of the world’s largest nuclear reactors by clogging the plant’s essential pipes. In the Sea of Japan, thousands of 200-kilogram Nomura’s jellyfish, with bells two meters in diameter, swarm fishermen, snapping nets and consuming local fish. Around the world, these animals voraciously consume fish eggs and larvae, routinely undermine marine farming efforts, and outcompete adult fish by consuming the resources they need to survive. If things stay on their current trajectory, we could be headed for a future where the entire ocean is thick with jellyfish.
So, is there anything that can keep these gelatinous creatures under control? Enter the humble sea turtle. There are a wide range of marine animals that feed on jellyfish, but sea turtles are among their most ancient predators. While every known species of sea turtle preys on jellyfish at some point in their lives, none consume quite as many as the leatherback. Leatherbacks are the largest species of sea turtle, and they eat jellyfish almost exclusively, devouring well over 1,000 metric tons of them over their roughly 50-year lifespans. This is particularly remarkable because jellyfish are 95% water and very low in calories, so to maintain a healthy weight, the average 500-kilogram leatherback needs to eat roughly 400 kilograms of jellyfish every day.
While some sea turtle species have been documented selectively eating their prey’s protein-rich gonads, leatherbacks eat jellyfish whole—mowing down huge swaths of unsuspecting jellies. Normally, jellyfish aren’t quite this defenseless. Most species have tentacles covered in cells called cnidocytes, which contain venomous harpoons coiled and ready to launch. These barbed structures, known as nematocysts, are released on contact. Jellyfish frequently use this sting to paralyze and kill their food, and it can also irritate the skin of would-be intruders. But it’s completely ineffective against sea turtles. Most of these reptiles have thick scales covered in keratin—the same material that makes up nails and claws. This leathery armor protects their skin while they hunt, and any captured prey that tries to escape is impaled on the keratinized spikes lining the leatherback’s esophagus.
For most sea turtles, these adaptations make individual jellyfish easy prey. However, a jellyfish population’s true defense mechanism is in how quickly they can reproduce. Almost all jellyfish species have evolved to reproduce both sexually and asexually, allowing them to increase their numbers with or without a partner. In tropical environments, jellyfish reproduce constantly throughout the year. But in more temperate climates, species often reproduce all at once in a massive bloom, where jellies multiply at alarming rates. Unfortunately, human activities are making these blooms much more frequent. Fertilizer runoff from farms introduces chemicals that simultaneously harm other fish and prompt blooms. High water temperatures caused by climate change speed up jellyfish reproduction and extend the reproductive season. Meanwhile, both marine construction and pollution dramatically increase the surface area for jellyfish polyps to attach, grow, and mature.
All these issues require a wide range of policy-based solutions. One major way to prevent jellyfish populations from getting out of hand is to protect their natural predators—many of whom are currently under threat. Small-scale fisheries that are essential to communities often use gillnets, which unintentionally capture and kill hundreds of sea turtles each year. In the Eastern Pacific, these practices could drive leatherbacks to extinction in the next 60 years. Fortunately, some researchers have already developed inexpensive tools to minimize these risks. Attaching green LED lights to gillnets has proven useful for allowing sea turtles, dolphins, and even seabirds to avoid fishing gear. Solutions like this should allow small-scale fishers to support their communities while minimizing their impact on our ocean defenders.
Jellyfish – A free-swimming marine coelenterate with a gelatinous umbrella-shaped bell and trailing tentacles. – The jellyfish population in the ocean has increased due to changes in water temperature and overfishing of their natural predators.
Sea Turtle – A large marine reptile with a bony or leathery shell, found in warm and temperate seas. – Conservation efforts are crucial to protect sea turtle nesting sites from human interference and pollution.
Reproduction – The biological process by which new individual organisms are produced from their parents. – The reproduction rate of certain fish species can be affected by changes in water temperature and habitat destruction.
Predators – Organisms that hunt and feed on other organisms for survival. – Sharks are apex predators in the marine ecosystem, playing a vital role in maintaining the balance of marine life.
Marine – Relating to the sea; existing in or produced by the sea. – Marine ecosystems are diverse habitats that include coral reefs, kelp forests, and open ocean environments.
Pollution – The introduction of harmful substances or products into the environment, causing adverse effects. – Marine pollution, such as oil spills and plastic waste, poses a significant threat to oceanic life and biodiversity.
Climate – The long-term pattern of weather conditions in a particular area. – Climate change is causing ocean temperatures to rise, which affects marine species and their habitats.
Ecosystems – Communities of living organisms interacting with their physical environment. – Coral reefs are complex ecosystems that support a wide variety of marine life and are sensitive to environmental changes.
Fisheries – Places where fish are reared for commercial purposes or areas where fish are caught. – Sustainable management of fisheries is essential to prevent overfishing and ensure the long-term health of marine populations.
Conservation – The protection and preservation of natural resources and environments. – Conservation efforts aim to protect endangered species and restore damaged ecosystems to their natural state.