Why Do Sea Turtles Eat Plastic? Bags That Look Like Jellyfish

Plastic bags that look like jellyfish

Many sea turtles eat jellyfish, and the leatherback sea turtle eats almost nothing else. To a hunting turtle, a clear or white plastic bag floating in the water can look very much like a jellyfish: soft, pale, see-through and pulsing gently in the current. Balloons and their ribbons resemble jellyfish with tentacles.

Research supports this. In a 2014 study, Qamar Schuyler and colleagues compared the plastic found in turtles’ guts with the debris floating in the sea around them. Turtles did not swallow plastic at random. They tended to pick soft, flexible, clear or white pieces that resembled jellyfish, and avoided many other kinds of debris. Because leatherbacks specialize in jellyfish, they are especially likely to swallow bags.

Plastic that smells like food

Looks are only part of the story. Within days to weeks in the sea, plastic becomes covered with algae, bacteria and tiny animals, a coating called biofouling. These organisms release odor chemicals that marine animals use to find food.

A 2020 study by Joseph Pfaller and colleagues, published in Current Biology, tested this with young captive loggerhead turtles. The researchers released different smells into the turtles’ tank. The loggerheads reacted to the odor of biofouled plastic just as strongly as to the odor of their normal food, sticking their noses out of the water and becoming more active, while they ignored clean plastic and plain water. This suggests that old, weathered plastic may actually attract turtles, not just fool them when they see it.

Why turtles cannot bring it back up

Once a turtle swallows something, it has trouble getting rid of it. Sea turtles, and leatherbacks in particular, have backward-pointing spines called papillae lining the throat. These help them hold onto slippery jellyfish, but they also stop plastic from being coughed back out. The plastic travels down into the stomach and intestines.

How swallowed plastic harms sea turtles

Plastic can kill a turtle in several ways:

• Blockage: a bag or knot of debris can block the gut so food cannot pass.
• Injury: sharp fragments can tear or puncture the gut wall.
• False fullness: plastic takes up space in the stomach, so the turtle feels full and eats less, which slows growth and weakens it. Scientists call this nutrient dilution.
• Floating: trapped gas from blocked guts can make a turtle float and struggle to dive.
• Chemicals: plastics can carry and release chemical additives and pollutants into the body.

A 2018 study of dead turtles in Australia by Chris Wilcox and colleagues estimated that a turtle that had swallowed 14 pieces of plastic had a 50 percent chance of dying from it. Some turtles survive with plastic in their gut and pass it, but even a single piece in the wrong place can be fatal, especially in a small, young turtle.

How common is plastic in sea turtles?

The numbers are striking. A review of leatherback necropsies (animal post-mortems) published by Mrosovsky, Ryan and James in 2009 found plastic in the guts of about one-third of the leatherbacks examined, from records stretching back to the 19th century. Large analyses since then have estimated that around half of all sea turtles worldwide may have eaten plastic at some point, with young turtles in the open ocean most at risk.

Not all species and places are the same. Juvenile green turtles and loggerheads often carry the most plastic, because young turtles feed among floating seaweed and debris that ocean currents collect in the same places. Loggerheads in the Mediterranean, one of the most plastic-polluted seas, frequently have debris in their guts. Microplastics, fragments smaller than 5 mm, have also been found in sea turtles of all seven species across several oceans.

What sea turtles should be eating

Sea turtles have varied diets. Leatherbacks eat jellyfish, green turtles graze seagrass and algae, loggerheads crush crabs and shellfish, and hawksbills eat sponges. Each of these feeding styles brings turtles into contact with plastic in different ways, from floating bags to fishing line on the seabed. See what do sea turtles eat for a species-by-species guide, and moon jellyfish for one of the leatherback’s favorite prey.

Reducing the problem

Plastic is one of several human threats to sea turtles, alongside fishing bycatch, egg collection and loss of nesting beaches. Unlike some threats, it is one that individuals can help reduce. Cutting down on single-use bags, bottles and balloons, disposing of rubbish properly and joining beach cleanups all keep plastic out of the sea. Many governments have also acted: California banned single-use plastic bags at stores in 2016, and many European countries charge for or ban them. For the bigger picture, see why are sea turtles endangered.