Climate change is warming our world and is causing larger and more frequent temperature fluctuations. This could be a serious problem for fish and other ectothermic animals – which do not regulate their body temperature – because they are at the mercy of their variable environment. However, adult and juvenile fish have the capacity to acclimatize to challenging temperatures, altering their physiology to improve function and performance. Yet, relatively little is known about how being exposed to temperature extremes affects embryonic fish and how these effects might persist in later life. Graham Scott from McMaster University, Canada, in collaboration with Ian Johnston from the University of St Andrews, UK, set out to discover how fish are affected by their early thermal history and the duo's findings were published in a recent issue of PNAS.
First, Scott and Johnston wanted to find out how the temperature experienced by zebrafish embryos affected their swimming performance later in life. Swimming performance is an excellent measure of overall performance in fish because they swim to escape predators, and to find food, mates and suitable spawning grounds. The duo raised zebrafish embryos at three temperatures found in the wild during the breeding season (22, 27 or 32°C) until they hatched, then reared all of the fry to adulthood at 27°C. The researchers then tested the swimming performance of these adult fish after transferring them back to 22, 27 or 32°C and found that the animals performed better at the temperatures that they had been exposed to as embryos. In contrast, when the researchers acclimated fish for long periods at more extreme temperatures (16 or 34°C), the fish raised at the warmer temperature as embryos performed best at both extremes, indicating that they were overall hardier fish.
Next, the researchers determined whether the differences in swimming performance after acclimation to extreme temperatures were related to differences in muscle size and fibre type. Slow and intermediate muscle fibres are used for sustainable aerobic swimming, while fast muscle fibres are used for sprinting. Measuring the overall size of the fish's trunk muscles and using antibodies and histological stains to identify the slow and intermediate muscle fibre types, Scott and Johnston found that the fish that swum best at the hot extreme had remodelled their muscles to have a higher proportion of slow and intermediate muscle fibre types, while the fish that swum best at the cold extreme grew larger trunk muscles with more slow muscle fibres.
Lastly, the team wanted to determine which genes were responsible for the fish's enhanced swimming performance at extreme cold temperatures. They measured gene expression in the swimming muscles of adult zebrafish using whole-transcriptome shotgun sequencing (a novel technology for measuring gene expression). Whereas all of the zebrafish altered the expression of a large number of genes important for energy metabolism, oxygen supply and muscle remodelling after cold acclimation, the hardy fish raised at the warmest embryonic temperatures had an exaggerated response for several of these genes, explaining why these fish swum better and remodelled their muscle at extremely cold temperatures. The results also supported the researchers' previous finding that embryonic temperature can have long-lasting effects on physiology and swimming performance that persist into adult life.
This research suggests that the effects of environmental temperature in early life stages may differ from the effects on adult fish. Embryos raised in a warmed environment may sometimes grow to become hardier and better able to deal with both high and low temperatures – which is good news for fish facing an uncertain climate future!