So Long, and Thanks for All the Plastic

In 2020 scientists named a newly discovered deep-sea creature (amphipod) Eurythenes plasticus. Yes, the species name is plasticus! This 15 cm ruler-sized, alien-looking crustacean was discovered 6,000 metres below the sea, in the dark depths of the Mariana Trench. It was named for the tiny microplastic fibres found in its gut despite living so far away from the nearest human civilisation.

Sadly, the occurrence of plastic inside animal gut has been recorded since the early 1960s. Leach’s storm petrel, a small gangster-lookalike seabird, was the first to claim fame from ingested plastic. While small crustaceans and fish are the preferred choice of prey, petrel bellies were also found with hard white plastic pieces back in 1962. Today over 300 different marine species have been documented with ingested plastics.

The unfortunate fact is that plastic is everywhere. In the oceans at all depths — it’s on the seafloor, mixed into the sediment, entangled in mangroves, curled up on coral reefs, trapped in sea ice. No part of the ocean can be considered pristine and devoid of human impact.

Another unfortunate fact is that marine life eats this plastic, intentionally or unintentionally. Just like plastic plays havoc in the bellies of India’s stray cows, plastic causes serve damage to any organism’s health. By eating large amounts of plastic, any animal, no matter how big or small, will get a false sense of satiation, and the plastic will likely block gut passages, reduce stomach capacity, lead to poor appetite, cause indigestion and bloat. Hard or sharp plastics can tear the gut, cause lesions, ulcers and ruptures. Furthermore, plastics leach toxins and persistent organic pollutants into the body, causing poisoning, affecting organism health and reproduction.

Unintentional Intake

A large majority of marine animals filter feed, i.e. they indiscriminately sieve out particles from seawater for their nutrition. This tactic has evolved across taxa, from sponges to copepods, barnacles to baleen whales, and is an efficient way to consume the countless organic particles and plankton that prevail in seawater. Animals can control the size of the particles they consume, but not the make.

Similarly, deposit feeders, another common group of animals, feed on whatever gets deposited on their feeding apparatus or the seafloor. Coral, anemones, sea cucumbers are some examples of deposit feeders.

Feeding mechanisms have evolved over millions of years and aren’t designed to cope with the recent advances in materials science. The smaller the filter or deposit feeder is, the greater its chances are of inadvertently eating microplastics. Microplastics are microscopic fragments of larger plastics or tiny pellets used in plastic production. These are pervasive particles, impossible to clean up once they have entered the oceans. With over five trillion plastic particles in surface waters, intentional or unintentional plastic ingestion is unavoidable. If not direct, there is also indirect consumption as these indigestible materials move up the food chain.

Plastic resembles the prey

Plastics are deliberately consumed by animals because they are not able to distinguish them from their normal food. Plastic may look like food, sound like food, smell like food and or taste like food.

Sights & Sounds

Beached whale and dolphin carcases across the world have one thing in common — plastic-filled bellies. It’s awfully depressing to recover 100s of kilos of plastic from their stomachs, and it raises the question – why would some of the most intelligent animals on the planet do this? Again, the answer lies in the evolution of their feeding strategies, honed over millennia, to expertly target prey in dark, dense waters. They aren’t able to distinguish prey from plastic, which is a relatively recent invention arising within the last 100 years of Earth’s billion-year history. Often plastics like shiny wrappers resemble silvery fish leading to devastating consequences for all — fish, turtles, and mammals who consume them.

Cetaceans like whales and dolphins use both sight and sound to find food. They use echolocation, sending out sound pulses and analysing the reflected signal for the presence and absence of their favourite food like fish and squid. Unfortunately, the signal that bounces off a plastic bag or a piece of tarpaulin is much like that of their prey. The cetacean misunderstands it for food, consuming one large piece after the other until the stomach gets chock-a-block full of plastics.

Smells & Tastes

Turtles are thought to mistake plastic bags for jellyfish, which is very likely given how well they resemble the dome-shaped cnidarians. Recent studies have also shown that turtles are drawn to plastics soaked in seawater because of the smell. Often plastic that’s been in the ocean for more than a few hours starts getting fouled by bacteria, algae, or minute animals. These bio-fouled plastics smell and taste like food, making it difficult for the animal to distinguish fake from the real until it is too late. 

DMS (dimethyl sulphide), a compound usually produced by marine algae, especially when eaten by krill, is an important olfactory clue used by many animals, including seabirds, to locate food. To make matters more complicated, DMS is also produced when plastic breaks down, leading to further confusion and poor food choices.

The situation worsens when organisms start eating plastic preferentially over their normal food. It is unclear why this happens, but it has been observed for some species. In an experiment, cold-water coral chose to eat plastic pellets over brine shrimp eggs. Unfortunately, the plastics were covered in bacteria, and their ingestion caused infections in the coral and finally death. This sounds all too similar to the junk food problems humans face.

Coping with Plastic

While there is no silver lining, certain animals have found interesting ways to work around the plastic present in their food supply. Some birds and marine invertebrates can regurgitate consumed plastics. Others pass them out with their excreta with no problem at all. Particular species of soil bacteria are demonstrating their ability to grown on, consume and degrade plastic, a surprising solution to our waste management problems!