The effect of cadmium on plasma melanocyte-stimulating hormone and morphological changes of melanophores in the cichlid fish Oreochromis niloticus, at different salinity levels

The effects of cadmium concentration (0, 2.5 and 5 mg L ⁻¹ ) on melanocyte-stimulating hormone (MSH), melanophore index (MI), and melanophore number (MN), as well as a microscopic examination of scale melanocytes in tilapia (Oreochromis niloticus Linnaeus, 1757) was evaluated at different salinity levels (0, 5 and 15 ppt). The levels of MSH, MI, and MN were lower in Cd-exposed fish than in control fish (not exposed to Cd) at salinity level of 0 and 5 ppt. In ppt, however these levels of MSH, MI and MN in control and Cd-exposed fish were not significantly different. In the media without Cd, the levels of MSH, MI and MN were not significantly different at all salinities. The morphological changes of melanophores were higher in Cd-exposed fish than in control fish at salinity 0 and 5 ppt, respectively. These morphological changes were not significantly different in the control fish at all salinities as well as in fish exposed to 0–5 mg L ⁻¹  Cd at salinity of 15 ppt. This study therefore demonstrates that the toxic effect of Cd on MSH levels and melanophore morphology decreased with increasing salinity. Further, due to the sensitivity of chromatophores to Cd, melanophore morphology is proposed as a biomarker of Cd exposure in aquatic ecosystems.