Marine litter is a notable challenge to crabs in highly impacted mangrove areas: a case study in Guanabara Bay - SE Brazil

<title>Abstract</title> Mangroves represent a particularly vulnerable coastal ecosystem to the problem of marine litter. However, research addressing the impact of macroscopic litter on mangrove crabs remains limited. The aim of this study was to characterize the semi-terrestrial crabs in a mangrove swamp heavily affected by macroscopic marine litter, in South-Eastern Brazil. It also aimed to verify possible relationships between crab parameters and the percentage of substrate covered by marine litter. Quadrats were used in three subzones of the intertidal zone (lower, middle, and transitional). The litter was characterized after collection, and substrate coverage was assessed using 1.0 x 1.0 m photographs, which were analyzed using CPCe software. The fiddler crabs were analyzed by catch and release and, the crab <italic>Ucides cordatus</italic>, by examination and counting of burrows. The concentration of marine litter ranged from 69.6 to 120.3 items/m², and the substrate coverage varied between 26.0 and 100.0%. The diversity and abundance of crab species was found to be low. The substrate coverage by macroscopic marine litter had a significant negative effect on crab abundance (R = -0.63; p = 0.03). In addition, a strong and significant influence on the size of both <italic>U. cordatus</italic> and <italic>Minuca</italic> spp. (only females) was observed.