Diagram from Chatterjee and Sharma (2019) showing how different size classes of plastic impact different sizes of marine life.
Since the first reports on the prevalence of small plastics fragments in the environment in 1970s (Carpenter & Smith 1972), numerous studies around the World have shown that plastics are pervasive in nearly every marine habitat (Auta et al. 2017; Law 2017). The relatively small size of most plastic debris (GESAMP 2015) facilitates their ingestion by a wide range of organisms, from zooplankton and bivalves to fish, seabirds, marine mammals, and even humans (Andrady et al. 2011; Cole et al. 2015; Lusher et al. 2017). Plastic consumption can have major adverse effects on the nutrition, biology, and physiology of organisms through damage or blocking of the digestive tract, false satiation, reduction in growth and reproduction, and potential toxicity from absorbed pollutants (Moore 2008; Cole et al. 2011; Sutton et al. 2016; Fossi et al. 2016). These impacts can, in turn, have cascading impacts on the structure and function of entire ecosystems through changes in the distribution and abundance of affected organisms and alterations of organismal interactions (Galloway et al. 2017).
Filter-feeding bivalves, such as oysters, have become a primary research focus for studying the impacts of plastic pollution on marine ecosystems given their abundance in heavily human-impacted coastal ecosystems and their ability to process large volumes of water during feeding (∼5-25 L seawater h-1; Korringa 1952), and their potential to serve as direct vectors of microplastics to humans through consumption (Van Cauwenberghe & Janssen 2014; Rochman et al. 2015). In experimental studies, ingestion of microplastics has strong negative effects on bivalve reproduction, physiology, and health through false satiation and mechanical damage to digestive systems (e.g. Sussarellu et al. 2016; Van Cauwenberghe & Janssen 2014). Filter feeders form the base of marine food webs and can connect microplastic pollution to higher trophic levels, both directly, through trophic interactions, and indirectly, through disruption to their growth, survival, and reproduction. However, there is an emerging divergence in opinion of the actual severity of impacts that bivalves face in the natural environment (e.g. Egbeocha et al. 2019; Lenz et al. 2016; Shumway et al. 2018).
Studies in the natural environment, or exposure experiments that use naturally occurring levels of microplastic remain a rarity, largely due to logistical challenges and lack of supporting field measurements. Many dose-response experiments have been conducted with microplastic concentrations that are several orders of magnitude higher than those reported within field studies. Contradictory studies are emerging that demonstrate sublethal, but potentially additive, responses when organisms are exposed to environmentally relevant concentrations of microplastics (e.g. Raineri et al. 2018; Revel et al. 2019; Weber et al. 2018). Such responses are perhaps not surprising given that many bivalve species have mechanisms to cope with the presence of non-food particles in the water column (particularly sediments), including rejection as pseudofeces prior to ingestion, preferential selection of particles, post-ingestive selection in the stomach, and differential absorption in the gut (Ward & Shumway, 2004).
Microplastics likely represent a significant stress-factor for bivalve species, due to both mechanical impacts from ingestion and impacts on their physiology (e.g. Kazmiruk et al. 2018; Smith et al. 2018). These direct impacts can alter growth, energy acquisition, and immune response, which may in turn lead to increased susceptibility to other stressors such as pollutants, disease, predation and climate change (Nelms et al. 2019). Microbial biofilms that rapidly colonize plastics in the marine environment may further exacerbate these impacts by facilitating the transfer of pollutants and pathogens from the environment to consumers (Kirstein et al. 2016). The rapid development of biofilms on plastics suggests that plastics may also play a critical, yet understudied, role in disease spread dynamics (Lamb et al., 2018; Curren & Leong 2019). The reduced fitness of organisms consuming microplastics may lead to lowered immunoresistance and higher potential for infection (e.g. Browne et al. 2008). Understanding the role that microplastics play in disease transmission and pathogenesis is critical to monitoring the health and function of oysters and the humans that consume them. Protozoan and bacterial pathogens (e.g. Perkinsus marinus Dermo, Juvenile Oyster Disease) have a long history of heavily impacting oyster populations and the ecosystem goods and services that oysters provide (Lafferty et al., 2015). Oysters, which are often consumed raw, can also carry the human pathogens, such as Vibrio parahaemolyticus and V. vulnificus, which cause an estimated 80,000 illnesses and 100 deaths in the US each year (https://www.cdc.gov/vibrio/faq.html).
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