The One Thing Stopping Jellyfish from Taking Over

Over the past few decades, jellyfish have begun to dominate our oceans. In New Zealand, divers encounter thousands of stinging jellyfish, each no larger than a grain of pepper. In Sweden, a massive swarm of moon jellyfish shut down one of the world’s largest nuclear reactors by clogging the plant’s main intake pipes. And in the Sea of Japan, thousands of 200-kilogram Nomura’s jellyfish with bells two meters in diameter swarm fishermen, snapping nets and devouring local fish. All around the world, these creatures voraciously consume fish eggs and larvae, often undermining marine farming efforts, and outcompeting adult fish by consuming the resources they need to survive. If things continue on their current trajectory, we could be headed for a future where the entire ocean is teeming with jellyfish.

So, is there anything that can control these prolific creatures? Enter the humble sea turtle. There are many marine animals that feed on jellyfish, but sea turtles are among their most ancient predators. And while every known species of sea turtle consumes jellyfish at some point in their lives, none eat as many as the leatherback. Leatherbacks are the largest species of sea turtle, and they feed on jellyfish exclusively, consuming over 1,000 metric tons of them during 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 typical 500-kilogram leatherback needs to eat about 400 kilograms of jellyfish every day. That’s roughly the same weight as a grand piano. And while some sea turtle species have been documented selectively eating their prey’s protein-rich tentacles, leatherbacks devour jellyfish whole — taking down large portions of unsuspecting jellies.

Normally, jellyfish aren’t quite this defenseless. Most species feed using tentacles equipped with venomous spears coiled and ready to launch. These pointed structures, known as nematocysts, are deployed upon contact. Jellyfish frequently use this sting to immobilize and kill their prey, and it can also irritate the skin of would-be intruders. But it’s completely ineffective against sea turtles. Most of these reptiles feed on tough scales made of keratin — the same material that makes up nails and claws. This hardened armor protects their skin while they hunt, and any trapped prey that tries to escape is impaled on the keratinized spikes lining the leatherback’s throat. For most sea turtles, these adaptations make individual jellyfish easy prey.

However, a jelly population’s real defense mechanism lies in how quickly they can reproduce. Almost all jellyfish species have evolved to reproduce both sexually and asexually, allowing them to increase their numbers without a mate. In tropical conditions, jellyfish reproduce continuously year-round. But in more temperate environments, species often reproduce simultaneously in a large bloom, where jellies multiply at alarming rates. And humanity is making these blooms much more common. Fertilizer runoff from farms introduces chemicals that simultaneously kill off other fish and trigger blooms. High water temperatures caused by climate change accelerate jellyfish reproduction and extend the breeding season. Meanwhile, both marine development and pollution conveniently increase the surface area for jellyfish polyps to attach, grow, and mature.

These myriad issues require a wide range of policy-based solutions. However, one crucial way to prevent jellyfish populations from spiraling out of control is to safeguard their natural predators — many of whom are currently under threat. Small-scale fisheries that are essential for communities in Mexico and Peru often use gillnets, which unintentionally trap and kill many sea turtles each year. In the Eastern Pacific, these practices could drive leatherbacks to extinction in the next 60 years. Fortunately, some researchers have proactively developed low-cost tools to mitigate these risks. Attaching green LED lights to gillnets has proven effective in allowing sea turtles, dolphins, and even seabirds to avoid fishing gear. Solutions like this should enable small-scale fishermen to support their communities while minimizing their impact on our fragile ocean ecosystems.