Tiny pieces of plastic may endanger Pacific oysters by adversely affecting their reproduction, according to a new study. They may have similar effects on other marine bivalves, raising questions about their impacts on marine ecosystems more broadly.

The plastic pieces are known as microplastics are, which are defined as being anywhere from 5 mm in size to just 1 nanometer (0.000001 mm). Scientists refer to primary microplastics and secondary microplastics: the former are intentionally manufactured super-small, primarily used in cosmetics and personal care products, industrial scrubbers used for abrasive blast cleaning, microfibers used in textiles, and pellets used in plastic manufacturing processes; the latter are the result of larger pieces of plastic disintegrating over time.

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Imagery of plastic pollution in the ocean often focuses on more visible impacts, such as trash that has become entangled around the neck of a marine mammal, or the appalling sight of vast amounts of plastic in the stomachs of seabirds on Midway Atoll. It is of course far harder to demonstrate the impacts of pollution that can not be seen, but those impacts are very real.

One of the great problems with microplastics is their ubiquity: It has been estimated that the ocean contains 5 trillion particles, totaling 250,000 tons, while a study last yearconcluded that 100,000 microbeads entered the ocean with each use of a personal cosmetic product that contained them. (The United States recently banned the production of personal care products containing microbeads from July 2017.) Just one cubic meter of ocean water may contain as many as 100,000 particles.

This is a problem particularly for filter feeding organisms such as mussels, sea cucumbers and some zooplankton, which may unintentionally consume large amounts of microplastics, which are often approximately the same size as their phytoplankton prey. Studies have shown that this can have adverse effects on those species’ energetics — unsurprisingly, as eating food-sized plastic is no substitute for eating actual food — as well as, in some cases, having immunological and neurological impacts. An additional concern is the leaching of chemical additives and pollutants from the microplastics.

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The latest study, published today in the Proceedings of the National Academy of Sciences, examined the effects of microplastic exposure on reproductively active Pacific oysters — a species the study’s authors chose “because of its worldwide production, economic importance as seafood, and important role in estuarine and coastal habitats.”

The authors established a number of tanks of oysters, which they fed phytoplankton, and in half of the tanks also introduced microplastics. Oysters that were exposed to microplastics readily ingested particles that were similar in size to the phytoplankton and, after two months of exposure, produced fewer and smaller oocytes (cells from which ova grow) and slower sperm, compared with those that weren’t.

Furthermore, exposed oysters produced 41 percent fewer larvae, and those larvae grew at a slower rate and ultimately reached a size 18 percent smaller than larvae from the unexposed tanks.

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The authors note that, “assuming no waste management infrastructure improvements, the cumulative quantity of plastic waste available to enter the marine environment from land is predicted to increase by an order-of-magnitude by 2025, especially in estuaries and coastal waters where oysters live and where waters are greatly influenced by increased human expansion.”

Accordingly, they say, their study is something of an “early warning system” to help “limit the impact of the microplastic legacy in the decades to come.”