Microplastics (MPs) are the most abundant type of debris in the marine environment, creating a significant threat to aquatic ecosystems due to their persistence, ability to absorb organic pollutants and potential ingestion by marine fauna. Shellfish are particularly vulnerable to MPs accumulation as they filter large volumes of seawater, and they become an important route for human exposure to these particles. This study, the first to examine MPs in European flat oyster (Ostrea edulis) populations, aimed to quantify these particles in the gill and digestive tissues of oysters from the Solent region (southern England). Enzymatic digestion using Proteinase-K was used in this study and MPs were identified in every oyster sampled to determine whether differences in abundance, type and size of MPs exist between locations. Oysters near urban areas contained significantly more MPs than those near rural areas. Fibres were the most prevalent type of MPs, with sizes varying across locations. The study found no significant differences in MPs presence between gills and digestive tissues, and an inverse correlation between the size of MPs and oyster size. The presence of MPs in wild O. edulis could be an additional threat to the survival of an already threatened species and may pose health risks for predatory species and human consumers of seafood. The use of O. edulis as a biomonitoring species for marine MPs pollution could help determine the extent, distribution and sources of MPs, potentially informing management strategies to reduce pollution.

Panulirus ornatus, the ornate spiny lobster, is a stenohaline weak hyper-osmoregulator, yet its osmoregulatory response to salinity stress remains poorly understood. This study investigated six osmoregulatory genes—Na⁺/K⁺-ATPase (nka), V-type H⁺-ATPase (vhe), Na⁺/HCO₃⁻ exchanger (nbc), Na⁺/K⁺/2Cl⁻ co-transporter (nkcc), Na⁺/H⁺ exchanger (nhe), and carbonic anhydrase (ca)—in juvenile gills exposed to 25 ppt, 34 ppt (control), and 40 ppt salinities during acute (48 h) and chronic (>38 d) phases. Transcriptome analysis revealed that all genes were unresponsive following either 25 ppt or 40 ppt salinity acute exposure. However, nkcc showed a tendency toward for upregulation under 25 ppt salinity during acute exposure. Additionally, glutathione S-transferase and putative ferrous reductase 1 were upregulated under 25 ppt salinity, suggesting increased metabolic demand. In contrast, glutathione peroxidase and an ammonia transporter were upregulated in 40 ppt salinity, indicating protein catabolism. Quantitative PCR confirmed nkcc- and nka upregulation under chronic 25 ppt salinity. Vhe, nbc, nhe and ca showed no response, and 40 ppt salinity did not affect the six target genes. These findings suggest P. ornatus relies on nkcc- and nka-mediated ion transport and lacks mechanisms to tolerate high salinity, resulting in reduced growth and survival. These findings define optimal salinity range for aquaculture (25–34 ppt), highlighting the need to avoid high-salinity stress in lobster water quality management