Size-specific vulnerability of hermit crab (Coenobita spp.) to entrapment in marine plastic litter: Evidence from the Nicobar Islands, India

Hermit crabs (Coenobita spp.) plays a critical role in nutrient cycling, seed dispersal, and cleaning of coastal ecosystem. The coastal habitats are increasingly polluted by anthropogenic marine debris, such as plastic bottles and containers, which pose a substantial entrapment hazard to hermit crabs. However, the role of container and cavity size in hermit crab entrapment remains an unexplored aspect. In this study, we investigated the potential effects of container and cavity size (bottle opening diameter) on hermit crab entrapment at remote beaches of the Nicobar Islands. We surveyed 28 beaches across the Nicobar archipelago to evaluate the intensity of hermit crab entrapment in marine litter. We recorded 4783 entrapments with 4102 crabs trapped in 198 containers (plastic & glass bottles), 681 trapped in non-bottle containers (Styrofoam boxes, helmets, fishing floats, etc.). Only 0.52 % of entrapped hermit crabs were found alive. Our findings indicate that container size is critical for hermit crab entrapment with 80 % of entrapment occurred in bottles with volumes of 5000 ml, 2000 ml, 1500 ml, 1250 ml, 1000 ml, 900 ml, and 800 ml. The bottle opening diameter acts as a size-selective filter, allowing entry of hermit crabs whose shell widths are smaller than the bottle opening diameter (τ = 0.163, p = 0.000); while larger shells are associated with larger cavity openings. This size-selective entrapment could disproportionately impact hermit crab population, potentially leading to decline of smaller hermit crabs. Such declines may cause cascading effects on beach ecosystem health, necessitating further rigorous investigation.