National Geographic

How leatherback turtles grow huge on a diet of jelly

Imagine eating nothing but jelly all your life. It’s hardly the richest source of food, and you might expect an animal with such a menu to be small and feeble.  You’d be wrong. The leatherback turtle eats jellyfish and little else, but it grows up to 640 kilograms in weight, and can migrate over thousands of kilometres. How can such a powerful giant subsist on such ethereal meals?

This is not an easy question to answer. Leatherbacks feed well below the ocean surface, and the jellyfish they seek are found in dense but sparse patches. Finding the animals, and studying their eating habits, is very challenging. Mike James from Dalhousie University, Canada, did it by attaching small video cameras to the shells of 19 turtles.

She tracked the animals off the eastern coast of Canada, where they gather in large numbers during the summer. As they came up for air, James dleftly pasted the cameras onto their backs. The devices recorded continuously for a few hours and James tracked them via GPS. When the turtles resurfaced, she retrieved the cameras.

The videos revealed that, in Canada at least, the turtles mainly feast upon the largest species of jellyfish – the lion’s mane. The jellyfish’s bell can reach several inches across, and its long trailing tentacles are packed with stinging cells. But the turtle swallows it nonetheless. Its sharp pointed lip snags the jellyfish, while nightmarish, backwards-pointing spines in its mouth and throat prevent the victim from escaping.

Each individual jellyfish may be relatively short of nutrients, the turtle grows fat upon them for several reasons. For a start, the turtles’ attacks never fail. While a gazelle will regularly manage to escape from a lion, and a seal will often outrun a shark, jellyfish can never evade a hungry leatherback. With a 100% success rate, all the turtles have to do is to find their prey. And that’s easy – the lion’s mane is found in large swarms in the summery Canadian waters, so a diving turtle runs into a couple of them every minute.

The jellies aren’t particularly fast, so it takes just 22 seconds to catch up to one, and another minute to eat it. The turtles can swallow dozens in quick succession, and they leave nothing behind. The jellyfish may not be an especially nutritious meal, but none of it goes to waste.

Heaslip estimated that during the summer months, the turtles eat around 73 per cent of their own body weight every day, packing in around 16,000 calories. That’s around 3 to 7 times more than they actually need to survive.

This summer binge allows them to dramatically increase their body weight before heading off on the long voyage to the Indo-Pacific, in search of mates and find breeding spots. It’s a 9,000 kilometre trip, fuelled by the unlikeliest of diets.

Reference: Heaslip, Iverson, Bowen & James. 2012. Jellyfish Support High Energy Intake of Leatherback Sea Turtles (Dermochelys coriacea): Video Evidence from Animal-Borne Cameras. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0033259

There are 8 Comments. Add Yours.

  1. Dan Milton
    March 19, 2012

    Impressive journeys by leatherback turtles have been tracked, but what I’ve found on the Web shows them crossing the Atlantic or Pacific routinely but not going from one ocean to another.
    Could you give your source for migration from Canada to the Indo-Pacific?

  2. MattK
    March 19, 2012

    I agree with Dan, I think there is some confusion somewhere as the Atlantic population of leatherbacks (merely endangered) and the Pacific population (imminent extinction) are separate to the best of my knowledge.

  3. Ed Yong
    March 20, 2012

    Hi folks,

    A quick holding statement – I’m at a conference, but I’ll check this when I get the time. Entirely possible I got this wrong. More soon.

  4. Bobby LaVesh
    March 20, 2012

    Could you give your source for migration from Canada to the Indo-Pacific
    ———————————————————————————————————-

    All these centuries have passed and we could have followed the turtles to find the North West passage all along.

  5. ChasCPeterson
    March 20, 2012

    The problem with a diet of ‘jelly’ isn’t that it’s non-nutritious. The ‘jelly’ in question isn’t Welch’s Grape, it’s the mesoglea of the jellyfish, the dry matter of which is mostly collagen and other perfectly nutritious proteins. The problem is that it’s mostly not dry matter, but water, and seawater at that. My guess is that the major costs to eating such a densely distributed food are osmoregulatory.

  6. Ed Yong
    March 20, 2012

    @Dan Milton + MattK – Folks, you’re right. I got confused in the reporting and corrected the piece. Sorry about that.

  7. rdiac
    March 20, 2012

    I was under the impression most tetrapods and fish would have trouble surviving on jellyfish owing to the relative paucity of several nutrients, this also being the reason jellyfish can thrive in oceanic deserts. *couldn’t find a reference sorry*
    So, does this mean that these guys know something about personal biomes making up the difference that the rest of us don’t? Anyhow, good to know that when we’ve eaten all the fish we can still go jellyfish and turtles instead of just plain jellyfish. I was almost worried there…

  8. div
    March 22, 2012

    “dense but sparse”

    I know what you meant, but that is confusing! Both words are essentially antonyms and can function equally well in either role you’ve placed them in. Perhaps rich/thick/(actually dense is pretty good as long as its twin isn’t of the same order) and scattered/sporadic/widely separated.

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