Combined effects of surface seawater variation and heavy metal contamination on Javanese medaka in Port Dickson coastline, Malaysia

Global climate changes may cause variations in the temperature of seawater and chemical alteration of the toxic pollutants in the tropical marine ecosystem. However, limited studies were conducted on these combined impacts on tropical marine organisms. Therefore, this study was conducted to provide a comprehensive assessment of the potential biological effects of temperature-dependent metal toxicity (TDMT) testing on Javanese medaka as indigenous fish model organisms. The temperature and other water parameters of the Port Dickson coastal area were recorded from the year 2020-2022 through monitoring activities. The surface seawater temperature of the Port Dickson coastal area ranged between 28.68°C and 30.17°C in March 2020 and October-November 2021, respectively. Also, the heavy metal pollution was evaluated, and the obtained results showed that the concentration (mg/L) of heavy metal in seawater was in the order of Zn < Pb < Cd < Al < Cu. There are significant differences in Cu between sampling sites and other heavy metals at p < 0.05. The copper (Cu) metal pollution in Port Dickson exceeded Class III (> 0.008 mg/L) of Malaysia Marine Water Quality Standards (MMWQS). Therefore, metal Cu was chosen to be used in the next objective of the study of temperature-dependent metal toxicity (TDMT) testing of embryos and adults of Javanese medaka. The climate impacts in the tropical aquatic ecosystem can be bio-monitored with the practical bioassay method of fish embryo toxicity testing (FET) and tissue histopathology techniques. In this study, the median lethal concentration of Cu (14 days-LC50-Cu) on the Javanese medaka embryo at the temperatures of 25.0°C, 33.0°C, and 35.0°C were 0.289 mg/L (p < 0.001), 0.195 mg/L (p = 0.001), and 0.144 mg/L (p = 0.003), respectively. The morphological abnormalities of the embryo of Javanese medaka from 14 days of acute exposure at the temperatures of 25.0°C, 33.0°C, and 35.0°C include lateral body curvature, abnormal somite, tail curvature, dent body cavity, shrunken yolk, and shrunken body cavity. While the 96-hour LC50-Cu of adults of Javanese medaka at the temperatures of 25.0°C, 33.0°C, and 35.0°C were 0.825 mg/L (p < 0.001), 0.434 mg/L (p < 0.001), and 0.291 mg/L (p < 0.001), respectively. In addition, it showed that the lower concentration of 21 days of chronic exposure exerted similar histopathological effects to the high concentration of 96 hours of acute exposure. The semi-quantitative analysis showed a significant difference (p < 0.05) between mild, moderate, and severe at the sublethal level as compared to the control treatment. The most evident and observable severity of the lesions in the gills was epithelial lifting and lamellar fusion and necrosis; liver lesions included vacuolation and congestion in blood sinusoids, and the kidney showed severity in tubule degeneration, glomerulus shrinkage, and tubule degeneration. The most affected organs during temperature-dependent sublethal exposure of Cu were kidney > gills > liver at the temperatures of 25.0°C, 33.0°C, and 35.0°C. Also in this study, metal Cu was proved to be one of the endocrine-disrupting chemicals (EDCs) that can interfere with the normal functions of the endocrine system of fish. In addition, metal Cu can cause a decrease in gonadal staging and maturity of germ cells in oogenesis and spermatogenesis of female and male Javanese medaka. Generally, depending on the rate of increases in temperature of global climate change, they may have impacted the tropical marine organisms in the coastal and estuarine populations.

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