A turtle who is very interested in the smells that the pipe has to offer.
The pressure against disposable plastic has some emotional mascots, including the legendary turtle with a straw on its nose. The working hypothesis about why turtles are so attracted to plastic is that plastic floating in the ocean can look a lot like jellyfish. But how does that explain turtles that are trapped in other types of plastic like this straw and eat it?
An article published in Current Biology this week has an alternative explanation: it's the smell of plastic, not the look, that attracts turtles. Rather, it is the smell of organisms that adhere to the plastic. The authors found that turtles were attracted to the smell of plastic coated with goopy ocean organisms as much as the smell of food. This is useful information to find out how to mitigate the effects of ocean plastic on predators like turtles.
Delicious smelly plastic
Plastic floating in the ocean is pretty quickly attacked by all kinds of organisms that grow on it. It was found that this “biofouled” plastic emits dimethyl sulfide, an organic compound that releases plankton in large quantities. Animals use this smell as a signal that there may be food, and seabird species that rely on it as a food reference have been shown to ingest more plastic than species that use the odor signal less.
However, this is only evidence that smell is an important explanation for the plastic consumption of the oceans. Turtle biologist Joseph Pfaller and his team wanted to look for a second piece of evidence by experimentally testing whether turtles reacted to plastic the same way they reacted to food.
So they took a couple of plastic water bottles to Walmart and let them swim in the ocean for five weeks to get really biofouled. At the same time, they collected 15 loggerhead turtles to keep in tanks for a few months. When the turtles were a few months old, they used their feeding times to expose them to different smells and see how they reacted.
Turtles use a number of clues in the air – including dimethyl sulfide – to alert them of the presence of food. If they get a hint of something that smells like lunch, keep their "nostrils" (turtle nostrils) out of the water longer to help them identify the source of the smell.
Pfaller and his colleagues used a piping system that gave different smells to the tank area of each turtle and exposed each turtle to four different smells: deionized water, clean plastic, biofouled plastic and turtle food. Each turtle was exposed to one smell a day, with the smells presented in random order.
To measure how the turtles reacted, the researchers examined two different measures: the time each turtle spent with its nostrils out of the water and how many breaths they took. They found that there was no difference in the reaction between clean plastic and deionized water and no difference between biofouled plastic and food. The turtles also spent significantly more time with their nostrils out of the water when they smelled both the food and the biofouled plastic than when they got the aroma of clean plastic and water. This suggests that biofouled plastic smelled of food for the turtles.
The results for the number of breaths were not so clear – the turtles breathed more frequently for food and biofouled plastic than for clean plastic, but they also had a high number of breaths for the control of the deionized water. This could raise doubts about the researchers' conclusions, but it could also mean that turtles don't change how much they breathe in response to food smells – or it could even be a result of the noisy data provided with small samples.
Not just jellyfish
Together with the evidence of bird plastic consumption, this result provides second evidence that smell plays an important role in the attraction of animals to plastic. Because the researchers used only a small number of turtles, further studies will have to confirm their results in the future to ensure that the data was not too noisy to provide a reliable result.
Still, the results have the advantage of explaining something that doesn't make sense otherwise: why turtles and other marine animals eat such a wide variety of plastic, not just the species that might look like prey. Further work to explore this explanation could help develop a better understanding of how animals interact with plastic – and how to prevent them from being killed by it.
Current Biology, 2020. DOI: 10.1016 / j.cub.2020.01.071 (About DOIs).