Host-parasite-sediment partitioning of metals in Octolasmis-infested Scylla mud crabs from Sarawak mangrove rivers, Malaysian Borneo.

Mud crabs (Scylla spp.) can accumulate metals due to their benthic and deposit feeding nature. Octolasmis spp. are pedunculate barnacles commonly found in the gill chambers of mud crabs and traditionally regarded as ectocommensals. However, high infestations can obstruct airflow, trap organic waste, impair respiration, cause oxidative stress, tissue inflammation, and reduced immune responses in hosts, thus, supporting their consideration as ectoparasites. Octolasmis may also accumulate higher metal concentrations than their hosts, potentially revealing contaminant exposure in aquatic environments. We hypothesized that Octolasmis would accumulate higher metal concentrations than host tissues, supporting its potential as a bioindicator. This study aimed to evaluate Cd, Pb, Cr, Cu, Mn, and Fe partitioning among mud crab tissues (i.e. hepatopancreas and gills), Octolasmis, and sediments from four mangrove rivers in Sarawak during dry and wet seasons (2024-2025). Results showed that Octolasmis accumulated significantly higher metal concentrations (notably Fe, Cr, and Pb) compared to the hepatopancreas and gills of mud crabs (p < 0.001) in all sites. Mud crabs tissue-to-sediment bioaccumulation factors (BAFts) indicated Cu and Pb enrichment in both tissues (hepatopancreas and gills), and Cr enrichment in the hepatopancreas at Kuap River (BAFts = 35.5). Parasite-to-host gill ratios (BAFph > 1) confirmed efficient metal accumulation for the Octolasmis. Hazard indices (HI > 1) at all sites indicated potential non-carcinogenic risk. These findings establish Octolasmis as a robust bioindicator for monitoring metal contamination within mud crabs and estuarine systems.