Can Slipper Snails and Sea Squirts Reveal Levels of Microplastic Pollution in the Sound?

New UConn Study investigates if these bottom-dwelling filter feeders can be used to track the amount of microplastics in Long Island Sound.

By Cheyenne Ellis

PhD candidate Kayla Mladinich sets up an experiment to expose bivalves to different sized plastic particles.
PhD candidate Kayla Mladinich setting up an experiment to expose bivalves to different sized plastic particles. Credit: Tyler Griffin.

Plastics first emerged during the early-to-mid-twentieth century as a much-desired alternative to scarcely available natural resources. Since then, these synthetic materials have permeated our environment, including places we never intended such as the waters of the Long Island Sound. 

In addition to large floating pieces of plastics, tiny broken-down particles called microplastics have also infiltrated our waterways. In many recent studies, bivalve mollusks (i.e., oysters, clams, mussels, and scallops) have been used in attempts to quantify microplastic pollution levels, but some researchers including Evan Ward and Sandra Shumway, marine scientists at the University of Connecticut, have raised concerns about their accuracy. Bivalves are selective particle feeders; in other words, they are picky eaters and may not consume all the microplastics to which they are exposed. 

“What this means is they capture a lot of different particles, but don’t ingest them all,” said Ward, Professor and Head of the Marine Science Department at the University of Connecticut in Groton. “If you just take a bivalve out of the environment and look at what’s in there, you’re going to find microplastics, but it’s not going to tell you much about the microplastic levels in the environment.” 

In their upcoming study, which has been awarded $301,150 by the Long Island Sound Study with $150,575 matching funds, Ward, Shumway, and their team will evaluate the effectiveness of two new potential bioindicators, Crepidula fornicata (also called slipper snails) and Mogula manhattensis (also called sea squirts or sea grapes), in quantifying microplastic distribution, to determine if they are an improvement over bivalves.

Measuring concentration levels with tunicates and slipper snails 

Ward and Shumway have opted to test the suitability of slipper snails and tunicates based on their previous research in 2019 which showed that these species, unlike bivalves, are indiscriminate suspension feeders, meaning they filter in all particles around them and do not have the ability to selectively reject particles.  

Mogula manhattensis, commonly known as "sea grapes" or "sea squirts".
Mogula manhattensis, commonly known as “sea grapes” or “sea squirts,” is a type of tunicate native to the Sound It takes in water through a process called filter feeding, which allows it to absorb nutrients. Credit: Wikipedia.

“The only time you see tunicates reject material back out of their mouth is when the concentration of particles gets really high,” said Ward. It is the same deal with slipper snails—while they can mass-reject particles, they cannot individually sort out specific particles while eating. 

The research, which is set to begin in the late summer of 2021, will test the viability of these organisms in both a lab and field setting, where the investigators will be heading into the Sound to find locations where both of the suspension feeders live in the same relative area.  

Crepidula fornicata, commonly known as "slipper snail".
Crepidula fornicata, commonly known as “slipper snail,” is native to the area. Like the tunicate, the snail gets nutrients from filter feeding. Credit: Wikipedia.

If the research shows that tunicates or slipper snails are good monitors for microplastics, future research and monitoring efforts could focus on them instead of bivalves, hopefully getting more accurate data from their stomach contents.     

In the lab, the team will expose the snails and tunicates to microplastics at a level similar to that which they recorded in their previous studies of microplastic distribution in the Long Island Sound. They will measure the feces, pseudofeces (particles that are spit out prior to ingestion), and the tissues which will help them to determine the gut retention time and the proportion of microplastics ingested. Knowing this information will potentially make it possible to measure microplastic pollution. 

Are microplastics harmful to shellfish? To us? 

An individual is pictured collecting slipper snails and oysters from Jimmy Bloom’s boat in Norwalk for a previous study.
Collecting slipper snails and oysters from Jimmy Bloom’s boat in Norwalk for a previous study (Copps Island Oysters, Norm Bloom and Son). Credit: Kayla Mladinich.

Research suggests that microplastics, at the current concentration levels, have no harmful effects on shellfish, or the humans who eat them.  

While there are a few studies that do note effects from microplastics, Ward cautions they may not have the most realistic lab conditions. His previous research has found microplastic concentration levels to be around one particle per liter in Long Island Sound, but the literature suggesting that shellfish may be impacted by microplastics often have concentration levels in the thousands, or even hundreds of thousands. “There’s a real disconnect between laboratory studies that are using these really unrealistic concentrations and what the animals are being exposed to currently [in the natural environment],” said Ward. 

That is not to say that shellfish are in the clear—they are already facing a multitude of threats, both anthropogenic and natural in origin.  

“One major threat to commercially important species of shellfish is disease,” said Shumway, Research Professor of Marine Biology at the University of Connecticut who is also currently an Editor-in-Chief for the Journal of Shellfish Research. “Warming seawater will also impact species in different geographic regions, and in some cases may result in the movement of species ranges.” 

Without seeing drastic action to reduce plastic usage and an improvement in waste disposal and recycling efforts, National Geographic predicts that plastic concentration levels in the ocean could triple by the year 2040. Could we potentially get to a point where the microplastic concentration is high enough to start seeing effects? Ward believes it is certainly possible and for that reason, focusing on prevention methods now, before we have reached that threshold, is of the utmost importance.  

How You Can Help 

A bag full of plastic balloons, water bottles and monofilament fishing line were and other trash picked up at a volunteer cleanup in Lighthouse Point Park in New Haven in 2018.
Plastic balloons, water bottles and monofilament fishing line were among the trash picked up at a volunteer cleanup in Lighthouse Point Park in New Haven in 2018. Credit: Judy Benson/ CT Sea Grant.

The creation of plastics has bettered our lives—there is no question about that. Disposable plastics have revolutionized the modern medical industry. Plastic polymers are sturdy, plentiful, and moldable. Without them, many simple objects we take for granted such as chairs, toothbrushes, and even combs, would look very different. Many would not be able to afford those things at all without the cheap price tags that often accompany plastic. So how did something with so many benefits also become something of such concern? 

Single-use plastics took off shortly after the introduction of plastic in the early twentieth century, for no reasons other than they were cheap to use and easy to throw out afterwards. Once a disposable object was discarded, it was forgotten. Companies were profiting and their customers were happy. Yet none of those plastics really disappeared—they have continued to sit in our landfills and oceans for decades, breaking into smaller pieces, but never really breaking down. In fact, the World Wildlife Fund estimates that it can take anywhere from 20-500 years for plastic to fully decompose, with most objects trending towards hundreds of years. 

Lowering the influx of ocean plastics will not only immediately benefit wildlife, but will also help lower the concentration of microplastic particles in the future. While widespread policy changes such as the Connecticut 2019 plastic bag ban have helped eliminate some pollution, we still have much further to go. Try taking some of these steps to reduce your individual plastic waste: 

  • Invest in reusable bottles, bags, and straws. 
  • Consider buying clothing made from natural materials like cotton instead of those with synthetic fibers. 
  • Recycle everything your local recycling center accepts and try to reuse the rest— the bottom of a plastic bottle makes the perfect seedling starter. 
  • While washing clothes, use a microfiber-catching ball or filter to prevent small particles from getting into the wastewater. Check out the Cora Ball website to learn more.
  • Spread the word! Get involved in beach clean-ups and encourage others to ditch single-use plastics. 

Ward and Shumway believe developing an awareness about how much plastic we use, and how much plastic is around us, is an important first step. They challenge everyone to start paying attention to the types of plastic they see while driving on the highway. 

 “If we could at least cut back on those, I think that would do a lot,” Ward said. 

The Long Island Sound Study Says No to Single-Use 

Credit: Long Island Sound Study/ Lucy Reading-Ikkanda.

With single-use plastics being the largest and most preventable source of plastic pollution, targeting these makes the most sense for reducing marine plastic debris and limiting future microplastic pollution. To emphasize this, the Long Island Sound Study created the #DontTrashLISound campaign on social media in 2017, posting information about alternatives to single-use on their online platforms. Since 2018, the Study has partnered with several other environmental organizations to hold beach clean-ups and distribute thousands of “Protect our Wildlife” stickers which could be displayed on reusable water bottles.

Cheyenne Ellis is a communications intern at NEIWPCC for The Long Island Sound Study (summer 2021). She received her Bachelor of Arts degree in Environmental Studies from Mount Holyoke College in 2021.

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