Microplastics: revealing the true impact they have on elasmobranchs

Over the last decade, the increased presence of microplastics has become a major threat to the marine environment worldwide. These tiny pieces of plastic are easily spread through food webs and are vectors for many harmful toxins. With plastic being so ubiquitous it is inevitable that marine organisms will ingest them. Microplastics have been found in turtles, marine mammals and many fish species with numerous publications highlighting the negative impacts they have on ecosystems. But microplastics in elasmobranchs (sharks and rays) has been largely understudied. These animals are particularly susceptible to ingesting high levels of microplastics due to their feeding strategies, preferred prey and overlapping habitat with pollution hotspots. Understanding the effects of microplastics on elasmobranchs is critical to their conservation, as many species are listed as globally threatened by the IUCN and should be prioritised for conservation.

Microplastics found on Blouberg beach in Cape Town

To understand why these plastics pose such a huge issue, we must recognise where they come from. It is estimated that between 4.8 and 12.7 million tonnes of plastic enter the oceans every year from a variety of sources. Plastic is a popular material due to its wide range of uses, durability and low production cost. It is these same properties, alongside its disposable nature, that has lead to an abundance of plastic in the marine environment. Microplastics are defined as small plastic particles, usually less than five millimetres in size, and they can be classified as either primary or secondary. Primary microplastics are intentionally added to a product but unintentionally realised into the environment. They can be found in personal-care products like facial cleansers and toothpaste (microbeads), but also from wear and tear of synthetic products such as car tyres, clothing and plastic based paints. Secondary microplastics are formed by the fragmentation of larger plastic items such as fishing gear and household waste. These larger pieces of plastic can disintegrate over time due to UV radiation, salt water, microorganisms and mechanical degradation from wave and wind action.

How do microplastics harm elasmobranchs?

Elasmobranchs are a highly varied group of marine vertebrates, encompassing an amazing range of forms and lifestyles. The vast majority of sharks and rays are predatory but a few species, such as the basking shark, whale shark and manat rays, have adopted a grazing lifestyle. There are two main ways in which microplastics can be ingested by these megafauna. The predatory sharks will be affected indirectly through ingesting contaminated prey, whereas the filter feeding grazers will be impacted directly from consumption of polluted waters.

Filter feeding megafauna have an increased exposure to plastic pollution because many of them travel through pollution hotspots including the Gulf of Mexico, the Mediterranean Sea, the Bay of Bengal, and the Coral Triangle. The nature of their feeding strategies means that these animals must filter hundreds to thousands of cubic meters of water daily to obtain the nutrients they need to survive and their longevity increases the risk of pollutant bioaccumulation. The consumption of indigestible plastics can lead to a significant reduction in nutritional uptake and causes damage to their digestive tract. The secondary effect is much greater and has a more long-term effect on the animals. Microplastics hold high levels of toxins and persistent organic pollutants (POPs). These toxins can bioaccumulate over many years, progressing to disruption of biological processes and potentially altering their reproductive fitness. Additionally, toxins can also be transferred or offloaded from mother to offspring which has the potential to influence growth, survival, and reproduction of the offspring.

What does the future look like?

As plastic production is projected to increase globally, more sufficient research and a commitment to monitoring microplastics in the marine environment is needed. Scientists must gain a clearer understanding of how elasmobranchs may be impacted long term by microplastics. Methods should be incorporated into their conservation to reduce the risks that they face. Strategies should be implemented to minimise the initial entry of plastics into the oceans which would help to dramatically reduce the occurrence of microplastics across food webs and in the marine environment. No single action can end the microplastics problem, but educating people and raising awareness about the true impact microplastics have on our oceans, will hopefully continue to lead to small actions by all of us as well as larger actions by governments around the world.