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The leatherback sea turtle is the largest turtle, and the third largest living reptile, in the world.

It’s a docile beast, just swimming around all day and eating jellies, which are the entirety of its diet. And these turtles get so big because they are so well fed. You see, having a diet of incredibly slow moving jellies means that once the turtle gets a target, it scores a meal every time with a 100% success rate. (Imagine how big a lion would get if it were so successful!)

Via the Not Exactly Rocket Science blog:

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.

…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 explains how these turtles can grow so giant on a diet of nothing but jelly and stinging tentacles.

There is simply no way for the jelly to outrun the turtle, so the leatherback is a jelly-eating machine. Little do you know, the evolutionary adaptation for keeping these doomed jellies from floating back out of their mouths during feeding is simply terrifying.

Backwards facing spines in the turtle's mouth keeps the ethereal jellies from escaping.

Leatherbacks consume everything from swarms of the smallest jellies to the most massive jellies in the sea like the Lion’s Mane Jelly (which for many is their primary prey). Either way, the spines catch these jellies on the way down, keeping a meal from escaping that often takes more than one bite or involves many small individuals.

A close up view of the jelly-killing spines

Good thing you’re not a jelly, right?

You can see that the spines continue all the way down the turtle's throat.

Learn much more about leatherback sea turtles here.

[Via Nature Wants to Eat You, Not Exactly Rocket Science, NOAA]