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1. Introduction
Plastic constitutes the largest portion of marine litter in the world, it was estimated that up to 12.7 million tons of plastic debris enter the oceans every year [1]. Plastic debris has been found on the surface of open oceans from the poles to the equator [2,3,4,5], along shorelines, as well as in deep-sea sediments [6,7,8]. The larger plastic items that have been found in the ocean pose threats, including suffocation, entanglement, and ingestion, to wildlife such as birds, sea turtles, marine mammals, invertebrates, and fish. The potential biological consequences to wildlife include damage to moving, feeding and reproduction ability, ulcerations, and even death [9,10,11,12].
Microplastics are plastic debris with diameters less than 5 mm [13], and the major sources of microplastics can be categorised into primary and secondary sources. Primary microplastics refer to particles originally manufactured in a particular size for specific applications. They include pellets that were used to manufacture plastic products and microbeads that are used in cosmetic, personal cleansing, and household products [14]. These microplastics cannot be completely removed by wastewater plants and end up in the oceans [9,15,16]. The breakdown of larger plastic debris by photochemical, mechanical, and biological processes in the marine environment results in secondary microplastics [10]. As most plastics were designed to be non-biodegradable, they only break down into smaller sizes and exist in the natural environment for hundreds or even thousands of years [3]. At the same time, the wastewater from washing clothes also discharges synthetic fibres to the marine environment [17,18].
Previous research has suggested that the sizes of microplastics that are equivalent to the sizes of plankton, and thus have a high potential to be mistaken as food, are ingested by a wide range of marine species [19]. Evidence has shown that ingested microplastics may remain in the digestive tract, causing injury and internal blockages that hinder food digestion processes [10,20]. Microplastic ingestion may also pose risks of degradation in feeding, respiratory, and reproductive functions and could change molecular and cellular pathways [11,21]. Moreover, the accumulation of plastics in the digestive tract may provide a false signal of being full; thus, the organism may reduce its food consumption [22]. In addition, liver toxicity [23] and vertical migration due to increased buoyancy have...