The one thing stopping jellyfish from taking over - Mariela Pajuelo & Javier Antonio Quinones

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. 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.

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. And 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. 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 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 useless 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 jelly 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. And humanity is making these blooms much more frequent.

Fertilizer runoff from farms introduces chemicals that simultaneously kill 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.

But 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 in Mexico and Peru 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 leathery ocean defenders.