We all know that multitasking is challenging, but imagine the multiple physiological obstacles faced by fish as the environments to which they are adapted begin shifting. For example, as CO2 levels and water temperatures rise, oxygen levels can fall, and pollution often exacerbates the situation. Many aquatic species have to juggle these simultaneous challenges. However, Patricia Schulte and colleagues from the University of British Columbia, Canada, explain that little is known about how fish respond when posed with several simultaneous physical threats. ‘Few studies have considered the possibility that the physiological changes that organisms undergo to cope with chronic exposure to one stressor could alter sensitivity to another’, the team says. Knowing that fish that have adapted to warmer conditions can modify their gills to satisfy the increased oxygen demands associated with a rise in metabolic rate, Schulte and her colleagues wondered whether these warm-acclimated fish may also be better prepared to cope when oxygen levels fall.

Schulte, Tara McBryan, Timothy Healy and Kristen Haakons turned to their favourite species, the Atlantic killifish (Fundulus heteroclitus) – which resides in salt marshes, where water temperature and oxygen saturation can vary wildly from day to day – to find out how they respond to a reduction in oxygen having previously adjusting to warmer conditions. Transporting fish from their Atlantic coast homes to the Vancouver lab, McBryan allowed the animals to acclimate to life at 15°C. Then she warmed some of the fish to 20, 23 and 30°C before testing how well they reacted when she lowered the oxygen levels to 2% air saturation.

The 15°C fish managed reasonably well, appearing to pass out after over an hour, but the fish that had been warmed to 23°C struggled, passing out after just 2.6 min, and the fish in the warmest conditions collapsed even before the oxygen reached its target. Then McBryan allowed the fish to adjust for 6 weeks to the temperature at which their oxygen response had been measured, before lowering the oxygen again. This time, all of the fish fared much better, holding out significantly longer before they passed out. And when McBryan investigated the fish's gills, she found a noticeable increase in the surface area that absorbs oxygen. Schulte says, ‘Our experiments show that the changes fish make in their gills at high temperature also help them to tolerate hypoxia. This ability should help them to cope with the complex combination of stressors associated with human-caused climate change.’

T. L.
T. M.
K. L.
P. M.
Warm acclimation improves hypoxia tolerance in Fundulus heteroclitus
J. Exp. Biol.