Transcription
Translator: Hani Eldalees
Reviewer: Shimaa Nabil
Over the past two decades, jellyfish have begun to proliferate in the oceans. In New Zealand, divers have caught millions of stinging jellyfish, each no larger than a peppercorn. In Sweden, a massive swarm of moon jellyfish has shut down one of the world's largest nuclear reactors due to blockages in the plant's essential pipes. In the Sea of Japan, thousands of Nomura's jellyfish, weighing two hundred kilograms and with bells two meters in diameter, attack fishermen, tear nets, and devour local fish.
Around the world, these jellyfish voraciously consume fish eggs and sea worms, periodically disrupting aquaculture efforts and outcompeting adult fish for resources to survive. If things continue on their current trajectory, we could be heading towards a future where the entire ocean is filled with jellyfish. So, is there anything that can control these gelatinous creatures? Enter the scene, the humble sea turtle.
There are a large number of marine animals that feed on jellyfish, but sea turtles are among the oldest predators. While all known species of sea turtles prey on jellyfish at some point in their lives, none consume such a large number as the leatherback turtle. Leatherback turtles are the largest species of sea turtle, and they eat jellyfish almost exclusively, devouring over 1,000 metric tons of them over their approximately 50-year lifespan. This is particularly remarkable because 95% of a jellyfish's composition is water and it is very low in calories. To maintain a healthy weight, one of these turtles, which averages 500 kilograms, needs to consume nearly 400 kilograms of jellyfish daily. This is roughly the same weight as a grand piano.
While some sea turtle species have been documented to selectively eat the protein-rich gonads of their prey, this species of turtle eats jellyfish whole, meaning they consume large amounts of the unpalatable jelly. Normally, jellyfish are not this vulnerable. Most species possess tentacles covered in cells called nematocysts, which contain coiled, ready-to-fire venomous harpoons. These stinging structures, known as cnidocytes, are released upon contact. Jellyfish often use this sting to paralyze and kill their food, and it can also irritate the skin of potential intruders.
But they are completely useless against sea turtles. Most of these reptiles have thick shells covered in keratin. The same substance that makes up fingernails and claws. This leathery armor protects their skin while hunting, and any prey that is caught and tries to escape is impaled on the keratinous spines lining these turtles' esophagi. For most sea turtles, these adaptations make jellyfish an easy meal.
However, the jellyfish's true defense mechanism is how quickly they can reproduce. Almost all species of jellyfish have evolved to reproduce both sexually and asexually, allowing them to increase their numbers with or without a partner. In tropical environments, jellyfish reproduce continuously year-round. But in more temperate climates, species often reproduce once in a massive event, with jellyfish breeding at alarming rates. And humanity has caused this to happen at an even more accelerated pace. Fertilizer runoff from farms introduces chemicals that kill other fish at the same time and cause jellyfish to proliferate. Rising water temperatures from climate change speed up jellyfish reproduction and lengthen the breeding season. Meanwhile, both marine construction and pollution lead to a significant increase in the surface area for jellyfish polyps to attach, grow, and mature.
All of these issues require a wide range of policy-based solutions. But one key way to prevent jellyfish populations from getting out of control is to protect their natural predators – many of whom are currently under threat. Small-scale fisheries that are vital to local communities in Mexico and Peru often use gillnets, which accidentally catch and kill hundreds of sea turtles every year. These practices could drive these turtles to extinction in the Eastern Pacific within the next 60 years.
Fortunately, some researchers have already developed inexpensive tools to mitigate these risks. Installing green LED lights on gillnets has proven useful for allowing sea turtles, dolphins, and even seabirds to avoid the fishing gear. Solutions like these should allow small-scale fishermen to support their communities while reducing their impact on the ocean's defenders, the turtles.