Microplastics in crabs in Hong Kong

Abstract

Microplastics, refers to plastics with particle size smaller than 5 mm, have been directly emitted into (primary microplastics) or indirectly produced in the marine environment (secondary microplastics). Not only certain regions were found to be polluted by microplastics, but the entire globe. Upon entrance into the marine environment, marine water, sediments and even organisms are subjected to the pollutants. Worldwide surveys and researches were conducted to understand the distribution, impacts and interactions of microplastics with the biotic and abiotic environment. Similar researches were also conducted on local seawater, sediments and organisms in recent decades. Yet, this is the first study utilizing local marine organisms, mangrove crabs, as biological assessment tool for assessing microplastics pollution situation in Hong Kong from their uptake through ingestion and inhalation. Samples of 49 mangrove crabs comprising of four species, including Parasesarma bidens, Paraleptuca splendida, Metopograpsus frontalis and Thalamita crenata, were collected from Yung Shue O, Pak Tam Chung and Ha Pak Nai for microplastics analysis retained in their stomach and gills respectively in late October and early November 2018. All the collected mangrove crabs were found ingested and inhaled with microplastics with an average of 61 suspected items (excluding debris) in their bodies. Crabs from mangrove at the western side of Hong Kong (Ha Pak Nai) were found with significantly higher microplastics abundance (p=0.013) than the eastern mangroves (Yung Shue O and Pak Tam Chung). Nearby river inputs including Pearl River discharge could possibly account for the difference. Individuals of Parasesarma bidens were found uptook more microplastics than others, whilst Paraleptuca splendida uptook least (p=0.001). The difference could be explained by variation in their diet and living habits. Female individuals were found intook significantly more plastic debris than male (p=0.009). Male fiddler crabs were reported to be more selective on food as to increase their feeding efficiency to compensate their loss in one feeding chela. Moreover, microplastics were more commonly found to be ingested than inhaled by mangrove crabs (p=0.000). Suspected microplastics were categorized into six groups based on their shape, namely debris, fragment, fiber, bead, pellet and others. Largest proportion (78%) of the suspected microplastics were debris, followed by fragment (16%), bead (2.7%), fiber, pellet and others. Slight difference in categorical compositions were found among factors, especially between microplastics in their stomach and gills due to difference in uptake mechanism. The size of microplastics identified in crabs are seemingly smaller than those from local water and sediments recorded recently. One of the identified microplastics was verified to be Low-Density Polyethylene (LDPE) through Fourier Transform Infrared (FTIR) Spectroscopy, whilst other pieces were yet to be verified due to size limitation of the analytical instruments. Undoubtedly, microplastics have been undeniably intruding Hong Kong water, sediments and even organisms like mangrove crabs despite numerous conventions and actions have been adopted internationally and locally. In light of that, it is suggested for the globe to develop circular plastic economy and to build up international centralized database for microplastics. Measures such as microbeads-related product ban, plastic beverage bottles recycling and long-term monitoring program are suggested to be adopted locally. These recommendations would help mitigate microplastic pollution problem, as well as minimizing impacts imposed on human, organisms, their habitats and the marine ecosystem.