For cold-blooded icefish, even slight changes in water temperature have serious consequences. So, increasing the water temperature immediately increases both their metabolic rate and the production of any by-products of cellular metabolism, like reactive oxygen species. The problem with reactive oxygen species is them interacting unpredictably and easily with all sorts of cellular components. These interactions cause damage to the cell by deactivating or even changing the function of important structures like proteins and cell membranes.
