Paper Reviewed
Madeira, C., Madeira, D., Diniz, M.S., Cabral, H.N. and Vinagre, C. 2016. Thermal acclimation in clownfish: An integrated biomarker response and multi-tissue experimental approach. Ecological Indicators 71: 280-292.

Marine fish living in tropical waters tend to experience much smaller seasonal variations in temperature than fish inhabiting other latitudes. As such, there are concerns that tropical fish maintain a much narrower temperature tolerance and that they might presently be close to their optimal temperature limit. If temperatures were to therefore rise in the future in response to CO2-induced global warming, many tropical fish species might experience widespread decline and possible extinction. But does this hypothesis hold any water?

Examining this possibility, the five-member Portuguese research team of Madeira et al. (2016) examined the cellular stress response of a tropical clownfish species (Amphiprion ocellaris) exposed to elevated temperatures over a period of one month. Their experiment was conducted in a controlled laboratory setting in which they subjected juvenile A. ocellaris to either ambient (26°C) or elevated (30°C) temperatures, while examining several biomarkers (e.g., stress proteins and antioxidants) in several tissue types (brain, gills, liver, intestine and muscle) at 0, 7, 14, 21 and 28 days of temperature treatment. And what did their measurements reveal?

According to Madeira et al., "results showed that exposure time significantly interacted with temperature responses and tissue-type, so in fact time influenced the organisms' reaction to elevated temperature." First, at day 7 they observed significantly higher levels of biomarkers in fish in the high temperature environment that was indicative of a typical thermal stress response. Thereafter, however, they report that biomarker levels stabilized, showing either "a significant decrease in comparison with controls or no significant differences from the control" through the end of the experiment, which observations they suggest are indicative of temperature acclimation.

Commenting on their findings, Madeira et al. write that "A. ocellaris probably lives far from its upper thermal limit and is capable of adjusting the protein quality control system and enzymes' activities to protect cell functions under elevated temperature, adding that "these results suggest that this coral reef fish species presents a significant acclimation potential under ocean warming scenarios of +4°C." And that is very good news for those concerned about the impact of global warming on tropical reef fish.