They don't look like your archetypal canary, but when mayfly larvae vanish from the waterways, you know the ecosystem is in trouble. ‘Mayflies are often important players in freshwater ecosystems and are widely used as indicators of ecological status’, says David Buchwalter, from North Carolina State University, USA, adding that the aquatic larvae, which are vulnerable to pollution and rising temperatures, can vanish without warning when conditions become harmful. However, Buchwalter was concerned about our limited understanding of the impact that high temperatures might have on these essential members of the freshwater ecosystem. ‘Studies of aquatic insect thermal limits have historically been done by heating larvae until they drop’, says Buchwalter. But few insects experience the steep and high temperature increases that are investigated in the lab in their natural surroundings. Concerned that scientists weren't building a realistic picture of mayfly thermal tolerance, Buchwalter decided to investigate the physiological impact of temperatures that mimic and exceed those that the larvae might genuinely experience on a hot summer's day.
Although many mayfly species are difficult to rear in the lab – the life cycle can be long, complex and often requires flowing water – David Funk, John Jackson and Bernard Sweeney from the Stroud Water Research Center, USA, had successfully isolated and reared a few species in the lab, including Neocloeon triangulifer, which reproduce asexually and have a much simpler life history. ‘We had to determine the chronic thermal limits of this species’, says Buchwalter, so Funk reared over 3000 larvae from eggs to adulthood across temperatures ranging from 14 to 30°C over several months to find out how they coped.
Although the growing larvae survived well at temperatures up to 26°C, something went drastically wrong at 28°C, when the death rate rocketed to 80%. However, when the team warmed 23-day-old larvae rapidly, they were able to cope with much higher temperatures (40°C) before succumbing to the effects. ‘Insects clearly can deal with relatively warm water on a short-term basis, but cannot sustain prolonged exposures’, says Buchwalter. So what was causing the insect's vulnerability?
One possibility was that the larvae simply could not supply enough oxygen for their tissues to sustain their suped-up metabolism at extremely high temperatures. If this was the case, Buchwalter reasoned that as the temperature increased, the larvae would activate genes at high temperatures that should help them to deal with a reduced oxygen supply (hypoxia), in addition to experiencing a significant reduction in the larvae's spare metabolic capacity for activities beyond those required for basic survival.
However, when Buchwalter measured the larvae's resting and maximum metabolic rates at 22, 26 and 30°C, he was surprised that their ability to provide sufficient oxygen at the highest temperatures was not compromised. And when Kyoung Sun Kim and Hsuan Chou painstakingly searched for evidence that the larvae were activating genes that would help them to deal with hypoxia as the temperatures rose, they found that the genes were only activated at temperatures that were in excess of those that the larvae naturally experience on a hot day. Whatever is killing overheated larvae on a hot summer's day, it is probably not their ability to supply enough oxygen to tissues as their metabolism rockets.
‘Insect respiratory systems are very efficient and may not be limited by higher temperatures under ecologically relevant conditions’, says Buchwalter, adding, ‘I think we need to get away from acute thermal challenge studies and focus on environmentally relevant thermal regimes to better understand how temperature imposes limits on species’.