MUDSUCKERS ACCLIMATISE TO HOT WATER BUT AT A COST

Abstract

A hot summer's day might be the perfect thing for a day trip to the seaside, but for fish, rising temperatures are not quite so welcome. As most fish cannot control their own body temperature, soaring temperatures cause their body temperature, and consequently their metabolic rate, to escalate. This can then eventually lead to an unsustainable increased demand for oxygen. With our climate heating up, how will fish cope? Nishad Jayasundara from Stanford University, USA, decided to investigate this as part of his PhD in George Somero's lab. He focused on the longjaw mudsuckers, explaining that as they frequently get caught in tidal pools where temperatures can reach 35°C, they might already have a few tricks for coping with higher temperatures and the increased need for oxygen (p. 2111).Using previously caught mudsuckers, Jayasundara divided them into four groups: three groups were slowly acclimated to a different temperature (9°C, 19°C, 26°C) and the fourth group was kept at a more ambient 13°C. ‘One question we were interested in was to see if they can actually change their upper and lower thermal limits…it's a pretty good index of how well a fish can deal with a short change in environmental temperature’, says Jayasundara. After the fish had spent 4 weeks at their acclimated temperature Jayasundara tested their thermal tolerance by slowly increasing the temperature until the fish lost their balance – a sign they'd reached their upper limit. While fish housed at 9 and 13°C were feeling faint by 33.3 and 35.1°C, respectively, fish acclimated to warmer water extended their tolerance by at least 2°C, with those acclimated to 26°C only feeling dizzy at 38.9°C.So how were the acclimated fish able to extend their thermal limits? Jayasundara reasoned that warm-acclimated fish might have found ways to compensate for and/or counteract the increased need for oxygen caused by the heat. He explains that returning resting metabolic or heart rates to normal levels seen at ambient temperatures would give them more scope for when temperatures got even hotter. However, when he looked at resting metabolic rate, they remained elevated in the warm-acclimated fish. While he did find that oxygen-carrying haemoglobin levels increased in warm-acclimated fish, it wasn't fully compensatory. Resting heart rate remained higher in warm-acclimated fish to ensure enough oxygenated blood was supplied to meet demand.Surprised that acclimated fish didn't change their metabolic or heart rates, Jayasundara decided to look at heart and metabolic rates during an acute increase in temperature, an event that is likely to occur on a hot summer's day in an estuary or tidal pool. All the fish showed the characteristic heat-induced increase in metabolic and heart rates. Fish acclimated to warm water, however, managed to raise their heart and metabolic rates even further. Heart rate rose to between 120 and 135 b.p.m., compared with the maximum 100 b.p.m. the ambient-acclimated fish could muster. So, although their resting metabolic and heart rates hadn't returned to ambient levels, they had still managed to increase their scope or capability to withstand further increases. However, this wasn't without cost, as the metabolic rate of fish acclimated to 26°C took twice as long to recover to their pre-stress levels. While mudsuckers might be able to acclimatise to future warmer climates, Jayasundara points out that: ‘It is important to look at not only how fish respond to a change in their water temperature but also how they recover from it and the costs involved.’