Microplastics
(MP; 1 μm to 1 mm) of various shapes and compositions are ingested
by numerous marine animals. Recently, proposals have been made to
adopt bivalve molluscs as bioindicators of MP pollution. To serve
as indicators of MP pollution, however, the proposed organisms should
ingest, without bias, the majority of plastic particles to which they
are exposed. To test this premise, eastern oysters, Crassostrea
virginica, and blue mussels, Mytilus edulis, were offered variously sized polystyrene microspheres (diameters
19–1000 μm) and nylon microfibers (lengths 75–1075
× diameter 30 μm), and the proportion of each rejected
in pseudofeces and egested in feces was determined. For both species,
the proportion of microspheres rejected increased from ca. 10–30%
for the smallest spheres to 98% for the largest spheres. A higher
proportion of the largest microsphere was rejected compared with the
longest microfiber, but similar proportions of microfibers were ingested
regardless of length. Differential egestion of MP also occurred. As
a result of particle selection, the number and types of MP found in
the bivalve gut will depend upon the physical characteristics of the
particles. Thus, bivalves will be poor bioindicators of MP pollution
in the environment, and it is advised that other marine species be
explored.
SummaryBenthic microeukaryotes are key ecosystem drivers in marine sandy beaches, an important and dynamic environment; however, little is known about their diversity and biogeography on a large spatial scale. Here, we investigated the community composition and geographical distributions of benthic microeukaryotes using high-throughput sequencing of the 18S rRNA gene and quantified the contributions of environmental factors and spatial separation on the distribution patterns of both rare and abundant taxa. We collected 36 intertidal samples at 12 sandy beaches from four regions that spanned distances from 0.001 to 12,000 km. We found 12,890 operational taxonomic units (OTUs; 97% sequence identity level) including members of all eukaryotic super-groups and several phyla of uncertain position. Arthropoda and Diatomeae dominated the sequence reads in abundance, but Ciliophora and Discoba were the most diverse groups across all samples. About onethird of the OTUs could not be definitively classified at a similarity level of 80%, supporting the view that a large number of rare and minute marine species may have escaped previous characterization. We found generally similar geographical patterns for abundant and rare microeukaryotic sub-communities, and both showed a significant distance-decay similarity trend. Variation partitioning showed that both rare and abundant sub-communities exhibited a slightly stronger response to environmental factors than spatial (distance) factors. However, the abundant subcommunity was strongly correlated with variations in spatial, environmental and sediment grain size factors (66% of variance explained), but the rare assemblage was not (16%). This suggests that different or more complex mechanisms generate and maintain diversity in the rare biosphere in this habitat.
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