Predicting the weather is never easy. Governments spend fortunes on massive computers to forecast the next day's conditions. But cold-blooded (ectothermic) animals don't have the luxury of weather reports to help them prepare for unexpected highs, and, as climate change takes hold, the weather is likely to become increasingly unreliable. But how well prepared are aquatic animals to cope as the environment becomes more changeable? And are some populations of fish naturally better prepared for variable weather than others, thanks to their genes? Intrigued by this possibility, Suzie Currie and colleagues from Acadia University, Canada, decided to focus on an amphibious tropical fish, the mangrove rivulus (Kryptolebias marmoratus), which routinely haul themselves out of the water onto land. The small creatures also have the ability to reproduce by mating with themselves – allowing scientists to breed populations in which every individual is genetically the same – making them ideal creatures to begin teasing apart the details of how well animals deal with climate variability.
With three genetically distinct populations of mangrove rivulus (originating initially from the Bay Islands, Honduras; Dangriga, Belize; and Twin Cayes, Belize) living in the laboratory, Currie's students – Sarah Brown and Gabby Rivard (Acadia University) – set up miniature environments, where some members of each population experienced a regularly repeating heatwave, hitting 35°C every midday before gradually cooling back to 27°C by midnight. However, other members of the three populations experienced almost random temperature rises and falls, hitting 35°C at 3 pm on some days and 9 am on others, while only cooling to 29°C on some occasions and falling to 27°C on others.
After the fish had spent 5 weeks settling into their heatwave cycles, the researchers monitored their survival for a further 10 weeks as the heatwaves continued, to find out whether the varying temperatures weakened or strengthened the fish. Fortunately, the fish that had originated in Honduras and Dangriga survived reasonably well; however, the fish that had originated in Twin Cayes struggled. And when the researchers checked the fish's growth, the fish that were living in the randomly varying heatwave were smaller than the fish that were experiencing a more dependable climate; the unpredictable weather was limiting the fish's growth. In addition, the fertility of the fish was impacted by the unreliable heatwave: they produced no eggs. To find out how resilient the fish were, the team also checked the temperature at which they toppled over when they could no longer stand the heat, and found that the fish that had experienced a regular heating cycle were more resilient, withstanding temperatures up to 40.9°C.
Given that high temperatures reduce the amount of oxygen carried in the water for fish to breathe, Currie and colleagues suspected that the fish would increase their gill surface area to compensate for the reduction in oxygen supply. However, when Rivard and Glenys Gibson (Acadia University) scrutinised the gills, the fish that had experienced both types of heatwave had reduced the surface area of their gills and packed the gaps between the gill filaments with cells. Incidentally, this is what the fish do when preparing to emerge onto land, so it's possible that the fish that had experienced varying temperatures were preparing to jump ship and leave the water.
Concluding, Currie says, ‘We have to consider the natural thermal variation that the animal would experience when we are trying to understand how animals will respond to warming temperatures’. And, as the three populations were affected by the heatwaves differently, it seems that the fish's genetics will play a role in whether they are able to cope when the weather becomes seriously unpredictable.