Turtles have an unfortunate habit of eating plastic objects floating in the sea. These cannot be digested and may ultimately kill them. It is widely assumed that this fondness for plastics is a matter of mistaken identity. Drifting plastic bags, for instance, look similar to jellyfish (水母), which many types of turtles love to eat. Yet lots of plastic objects that end up inside turtles have no similarity to jellyfish. Joseph Pfaller of the University of Florida therefore suspects that the smell of marine microorganisms (海洋微生物) which grow on floating plastic objects fools turtles into feeding.
The idea that the smell of floating plastic objects might lure animals to their death first emerged in 2016. Researchers at the University of California noticed that certain chemicals, notably dimethyl sulphide (二甲基硫), which are released into the air by floating plastics, are those which many seabirds sniff (嗅) to track down food. These chemicals mark good places to hunt because they indicate plenty of the algae (海藻) and bacteria (细菌). The researchers also found that birds which pursue their food in this way are five or six times more likely to eat plastic than those which do not.
Since turtles are known to break the surface periodically and sniff the air when finding the way to their feeding areas, Dr Pfaller theorised that they are following these same chemicals, and are likewise fooled into thinking that floating plastic objects are eatable.
To test that idea, he and his colleagues set up an experiment. They arranged for 15 turtles to be exposed to four smells: the vapour from deionised water; the smell of turtle-feeding balls made of shrimp and fish meal; the smell of a clean plastic bottle chopped up into ten pieces; and the smell of a similarly chopped bottle that had been kept in the ocean for five weeks to allow algae and bacteria to grow on it.
Two of the smells proved far more attractive to the animals than the others. When sniffing both the smell of food balls and that of five-week-old bottles, turtles kept their nostrils out of the water more than three times as long, and took twice as many breaths as they did when what was on offer was the smell of fresh bottle-plastic or deionized -water vapour.
Though they have not yet tested whether dimethyl sulphide is the culprit, Dr Pfaller and his colleagues think it is the most likely candidate. In an unpolluted ocean, pretty well anything which had this smell would be eatable—or, at least, harmless. Unfortunately, five-week-old plastic bottles and their like are not.
