The continuous increases in temperature and microplastic concentration have raised grave concerns worldwide. The influence of water temperature and microplastic concentration on microplastic ingestion and retention rates in Holothuria cinerascens (Brandt, 1835) was investigated. H. cinerascens were sampled by handpicking during low tide from Park Rynie beach on the south coast of KwaZulu-Natal, South Africa. They were then cleaned and transported to the control temperature room. Animals were placed in pairs in aerating tanks and acclimated for 48 h. For the exposure experiment, 50, 100, and 150 polyethylene fragments per litre of sizes (0.5–1 mm and 1–2 mm) were added to each tank at varied temperatures (19, 24, 28, and 32 °C) and were left for 48 h and 72 h. Thereafter, animals were transferred to tanks void of microplastics and left to depurate for 48 h for the retention experiment. Microplastic collected from the gut and faecal samples were used to calculate the ingestion and retention rates. Water temperature significantly influenced ingestion and retention rates, while microplastic concentration influenced the ingestion rate only. The highest microplastic particle number/g.h was recorded in high microplastic concentration for both ingestion and retention experiments. Also, the highest microplastic particle number/g.h was obtained at 28 °C in the ingestion experiment. More microplastic particle number/g.h was retained at 19 °C in the ingestion experiment. Ingestion rate increased with increased microplastic concentration, and temperature up to 28 °C and then decreased at 32 °C. More microplastics were retained at high temperature (28 °C) and low temperature (19 °C) at 48 h and 72 h exposure, respectively. Increase in water temperature and microplastic concentration will increase the susceptibility of H. cinerascens to ingest more microplastics from their natural habitat.
