Why turtles have a fatal attraction to plastic

Study finds they mistake its smell for food.

Loggerhead turtle in experimental arena using nares to smell airborne odorants.

Joseph Pfaller

By Natalie Parletta

Researchers have shown that turtles mistake smelly plastic debris for food, shedding new light on their fatal attraction that results in frequent ingestion and entanglement.

Previously, it was thought that turtles eat these ubiquitous, synthetic by-products of human industry because they look like jellyfish prey – the jellyfish hypothesis.

But this doesn’t explain why turtles eat so many different kinds of plastic, many of which don’t resemble jellyfish, and get caught up in plastic nets and trash that they’re presumably not confusing for prey, says lead author Joseph Pfaller from the University of Florida, US.

In as little as a week, plastics become covered in communities of micro- and macro-organisms, particularly in areas with lots of plankton and nutrients, causing them to emit smells.

And marine predators like turtles use natural odours, such as dimethyl sulphide, to find productive foraging areas in the open sea, Pfaller explains.

“If marine plastics emit similar odours after becoming biofouled with bacteria, algae and small marine animals, then areas of concentrated plastic debris could become ‘olfactory traps’ that attract turtles from considerable distances away and cause normally adaptive foraging strategies to become detrimental or even lethal.”

To test whether turtles are attracted to the smell emanating from biofouled plastic, Pfaller and colleagues conducted experiments on 15 young, captive-reared loggerhead sea turtles (Caretta caretta), as described in the journal Current Biology.

In the lab, turtles keep their nose out of the water for longer when they smell food.

The experimenters therefore piped odours from food (containing fish and shrimp meal), biofouled plastic, clean plastic or clean, deionised water while keeping all other stimuli constant, and recorded their reactions to each condition on video.

They found that the marine critters kept their noses out of the water three times longer when exposed to odours from both their food and biofouled plastic, compared to the control smells.

“Because turtle’s responses to food and biofouled plastic were elevated and indistinguishable,” Pfaller says, “then this supports the idea that turtles can not only detect the odours emanating from biofouled plastic, but also that biofouled plastic elicits a foraging response in sea turtles.”

He was surprised that the response was so strong, given the animals’ familiarity with their own food, saying the finding underscores the danger posed by all kinds of plastics to turtles and other marine animals and their roles in the ecosystem.

“The plastic problem in the ocean is more complex than plastic bags that look like jellyfish or the errant straw stuck in a turtle’s nose. These are important and troubling pieces to the puzzle.

“Whether it’s through vision, smell or another sensory mechanism, any cue that attracts marine animals to plastics facilitates opportunities for fatal injection or entanglement.”

The study underscores the growing imperative to tackle the problem of marine plastic, which covers huge areas of the Pacific Ocean, says senior author Kenneth Lohmann from the University of North Carolina.

“These areas may draw in marine mammals, fish and birds because the area smells like a good foraging ground.

“Once these plastics are in the ocean, we don’t have a good way to remove them or prevent them from smelling like food. The best thing we can do is to keep plastic from getting into the ocean at all.”

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Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
  1. https://www.seeturtles.org/ocean-plastic
  2. https://www.iucn.org/resources/issues-briefs/marine-plastic
  3. https://bmcecol.biomedcentral.com/articles/10.1186/1472-6785-14-14
  4. https://www.cell.com/current-biology/fulltext/S0960-9822(20)30115-9
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