Over the last two decades, the proliferation of jellyfish has become a significant concern for our oceans. In New Zealand, divers grapple with millions of tiny, stinging jellyfish. These are no larger than grains of pepper. Meanwhile, in Sweden, an enormous cluster of moon jellyfish led to the shutdown of one of the world’s largest nuclear reactors by blocking essential pipes. In the Sea of Japan, massive Nomura’s jellyfish, weighing up to 200 kilograms and boasting bells two meters in diameter, swarm fishermen, wrecking nets and consuming local fish. Globally, these creatures voraciously consume fish eggs and larvae, thwart marine farming efforts, and outcompete adult fish by depriving them of essential resources. If things continue this way, we might face a future where the entire ocean is teeming with jellyfish.
So, is there a solution to keep these gelatinous creatures in check? Enter the humble sea turtle. While various marine animals feed on jellyfish, sea turtles stand out as their ancient predators. Every known species of sea turtle consumes jellyfish at some point in their lives, but none do so as extensively as the leatherback. Leatherbacks are the largest species of sea turtles, and they predominantly feed on jellyfish, devouring well over 1,000 metric tons of them over their approximately 50-year lifespans. This is especially noteworthy because jellyfish are 95% water and low in calories. To maintain a healthy weight, an average 500-kilogram leatherback needs to consume roughly 400 kilograms of jellyfish daily. That’s approximately the same weight as a grand piano. Unlike some sea turtle species that selectively target the protein-rich gonads of their prey, leatherbacks consume jellyfish whole, decimating large swaths of unsuspecting jellies.
Ordinarily, jellyfish aren’t quite defenseless. Most species have tentacles armed with cells called cnidocytes, housing venomous harpoons ready to strike. These harpoons, known as nematocysts, are discharged on contact. Jellyfish frequently employ this sting to paralyze and kill their prey, and it can also irritate the skin of potential intruders. However, it proves entirely ineffective against sea turtles. Most of these reptiles possess thick scales covered in keratin— the same substance found in nails and claws. This leathery armor shields their skin while hunting, and any captured prey attempting to escape is impaled on the keratinized spikes lining the leatherback’s esophagus. For most sea turtles, these adaptations render individual jellyfish easy targets.
Yet, the true defense mechanism of a jelly population lies in their rapid reproduction. Almost all jellyfish species have evolved to reproduce both sexually and asexually, enabling them to multiply with or without a partner. In tropical environments, jellyfish reproduce continuously throughout the year. However, in more temperate climates, species often engage in a synchronized mass reproduction event, resulting in a staggering increase in jellyfish numbers. Unfortunately, human activities are making these events more frequent. Fertilizer runoff from farms introduces chemicals that simultaneously harm other fish and trigger mass reproductions. Rising water temperatures due to climate change accelerate jellyfish reproduction and prolong the reproductive season. Meanwhile, both marine construction and pollution significantly expand the surface area for jellyfish polyps to attach, grow, and mature.
Addressing these issues requires a diverse set of policy-based solutions. However, one crucial approach to preventing jellyfish overpopulation is safeguarding their natural predators, many of which are currently endangered. Small-scale fisheries, crucial to communities in Mexico and Peru, often employ gillnets that unintentionally capture and kill hundreds of sea turtles annually. In the Eastern Pacific, these practices could lead to the extinction of leatherbacks within the next 60 years. Thankfully, some researchers have already devised cost-effective tools to mitigate these risks. Attaching green LED lights to gillnets has proven effective in helping sea turtles, dolphins, and even seabirds avoid fishing gear. Solutions like this should enable small-scale fishers to support their communities while minimizing their impact on our resilient ocean defenders.