First assessment of microplastic accumulation and characteristics in the zoanthid Palythoa caesia across coral reef ecosystems in Thailand

Microplastic (MP) pollution has become a critical concern for coral reef ecosystems, yet its presence and impact on zoanthids remain poorly understood. This study provides the first detailed assessment of microplastic accumulation in the zoanthid Palythoa caesia , a common benthic organism in coral reef environments. Samples were collected from three distinct reef sites in Thailand: Chan Island in the upper Gulf of Thailand, Kong Sai Daeng Island in the western Gulf, and Ao Jak of Mu Ko Surin, the Andaman Sea. A total of 624 MP particles were extracted from 38 P . caesia colonies, with MPs detected in every sample. The average abundance was 4.11 ± 1.38 particles·cm - ² or 2.37 ± 1.13 particles·g - ¹ wet weight, with Chan Island showing significantly higher concentrations than the other sites. This spatial variability appears to reflect differences in local pollution sources, particularly from tourism and coastal activity. MPs were present in both the surface and inner tissue layers, with 61% found on the surface. This suggests that mucus secretion may play a key role in trapping particles from the surrounding water. Fibers were the most common morphology (>80%), and black and blue particles dominated the color spectrum. FT-IR analysis identified polypropylene (PP) and polyethylene (PE) as the most abundant polymers, while Chan Island exhibited higher proportions of polyethylene terephthalate (PET) and polystyrene (PS), likely linked to single-use plastic waste. The presence of MPs within the inner tissue layers indicates active uptake by the polyps, likely due to their suspension-feeding behavior and the capacity of their coenenchyme to trap sediments. These findings highlight P. caesia as a promising bioindicator for microplastic contamination in reef ecosystems. Addressing this issue will require localized management strategies, including strengthened waste controls in tourist areas, improved regulation of recreational activities in protected zones, and long-term biomonitoring. Integrating microplastic mitigation into marine spatial planning will be essential for preserving coral reef resilience in the face of growing anthropogenic pressures.