The effects of plastic debris on bivalves

Holloway, Sasha

The effects of plastic debris on bivalves

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Plastics have become increasingly pervasive in marine ecosystems, comprising 60-80% of all marine debris encountered. Numerous studies have investigated the effects of microplastics on mussel physiology, function and aggregate formation, however, the effects of macroplastics remain largely unknown. In this thesis, I firstly synthesised existing knowledge about the effects of plastic on bivalve physiology though a comprehensive literature review, identifying patterns, trends and knowledge gaps. Microplastics induce various physiological changes, act as vectors for other pollutants, and amplify physiological effects when co-exposed with chemical pollutants. Although studies often report no or limited effects, these tended to be omitted in other reviews. Experimental studies often fail to generate realistic environmental conditions, limiting their practical relevance. Macroplastics have been shown to smother bivalves, reducing their abundance and altering species compositions of their associated fouling communities by restricting the flow of oxygen and organic matter. Additionally, macroplastics serve as settlement substrate or rafting material in otherwise unsuitable habitats. However, bivalves that settled on macroplastic experienced increased mortality and altered sex ratios, and the species composition of associated communities differed with polymer type. Overall, the literature review revealed that effects of both micro and macroplastics on various aspects of bivalve physiology and associated communities are highly variable, and that macroplastics remain greatly understudied. Secondly, I conducted an experiment to determine the effects of macroplastics on the physical structure, physiology and associated communities of mussel (Mytilus galloprovincialis) aggregates. Film (polyethylene plastic bags) and filament (plastic nylon fishing rope) shaped plastics were introduced into mussel clumps as they aggregated in the laboratory. The mussel aggregates were then suspended on platforms in a marina for 85-91 days before being retrieved and various attributes examined, including structural and physiological variables as well as associated infauna and epibionts. Results indicated that macroplastic exposure had no effect on particulate organic matter accumulation, respiration rate, spatial complexity, byssus strength, body condition index, or mortality of mussels in experimental aggregates. Infaunal abundance and species composition were also unaffected. Plastic shape and amount showed an interactive effect on the percentage cover of epibionts growing on the mussel aggregates. Epibiont cover increased with increasing amounts of filaments and decreased with increasing amounts of film shaped plastics. The presence, shape or amount of macroplastics in mussel beds had no clear and consistent effects on the physical structure of mussel beds, the physiological performance of mussels and the composition of epibionts and associated mobile fauna. This study thus suggests that macroplastics only have very limited effects on M. galloprovincialis and their epibiont communities. In conclusion, this thesis highlights the inconsistency of plastic effects on bivalves and their associated communities within the literature and through primary experimental investigation. While isolated effects existed, the literature is likely underrepresenting studies that have not found any effects, as was the case for the experimental part of my study. While certain limitations should be addressed in future studies, such as the lack of standardised methods and units, the variability of results and trends may indicate that plastics only have very limited effects on the performance of bivalves, many of which are highly adaptive and resilient to environmental stressors.

Keywords

plastics

bivalves

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Masters

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