Abstract
Mangrove forests are recognized as plastic sequestration hotspots, particularly in urbanized areas, which may contain up to two orders of magnitude more plastic litter. Fiddler crab populations, known as ecosystem engineers, are thriving precisely in these expanding plastic‐pollution hotspots, prompting inquiry into how these creatures navigate the substantial plastic loads within sediments they have evolved to inhabit and feed upon. To investigate this, we released two types of unaged, fluorescently labeled polyethylene microspheres (hereafter, “microspheres”)—green (20–27 μm diameter) and red (75–90 μm diameter)—into polluted urban mangroves inhabited by the fiddler crab Minuca vocator . The microsphere sizes were chosen to mimic the natural range of food particle sizes encountered by these crabs. This field experiment tracked the uptake and fate of microspheres in the crabs' hepatopancreas, gills, hindgut, and the sediments over 66 days. Microspheres were incorporated by most crabs (91 out of 95), with uptake markedly higher in the hindgut (26.04 ± 1.19 SE microspheres g −1 ) and hepatopancreas (10.07 ± 1.19) than in the gills (0.86 ± 0.20). The average uptake of microplastics, 48.67 (±7.24 SE) per crab was more than 16 times higher than the density in the sediment (2.92 microspheres g −1 ), and represents one of the highest uptakes ever recorded in nature. Approximately 15% of this load, primarily located in the hepatopancreas, was found in a fragmented state, suggesting degradation through digestive processes. Microspheres fragmented by digestive processes were detectable in mangrove sediments within just 14 days, challenging the notion that plastic degradation is a long‐term process driven mainly by abiotic aging over decades. Combined with the widespread presence of fiddler crabs along coastlines acting as plastic sinks, these findings suggest that biotic fragmentation is an important pathway for the degradation of plastics entering the ocean.