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Three freshwater shrimp (Gammarus minus), showing the dramatic size difference between the smallest and largest. Photo credit: Douglas Glazier.

At first glance, the fact that large animals have higher metabolic rates makes sense, but when you look at the metabolic rates of animals scaled to their size, the smallest have the fastest pace of life. ‘Many biologists, including me, have been interested in what determines the metabolic rate of organisms’, says Doug Glazier from Juniata College, USA, adding that the relationship between size and metabolic rate was attributed by some to the way in which oxygen is delivered throughout animal bodies. However, Glazier was increasingly sceptical that the explanation was so straightforward. Environmental factors such as temperature and lifestyle can affect how the metabolism of animals varies when they grow, and few had investigated how combinations of factors influence the variation of metabolism, from small to large animals, across populations. So, Glazier decided to find out how the presence of predatory fish might impact the effect of temperature on the metabolism of freshwater shrimp (Gammarus minus), from diminutive youngsters to seniors more than 100 times their size.

Fortunately, Glazier had a natural ‘laboratory’ – a series of local springs inhabited by the shrimp – close at hand in the Appalachian Mountains to investigate how predators impact the effect of temperature on their metabolism. ‘We took advantage that some springs contain fish predators, whereas others do not’, says Glazier, who was able to collect shrimp ranging from 30 mg monsters down to minute 0.2 mg youngsters to measure their metabolic rates. ‘The hardest thing was to collect amphipods from Williamsburg Spring with a sufficient body-size range’, says Glazier, explaining that the crustaceans are very secretive, hiding in sediment at the bottom of the spring to evade the predatory sculpins that inhabit their water course.

Back in the lab, Glazier, Jeffrey Gring, Jacob Holsopple and Vojsava Gjoni, measured the metabolic rates of over 700 shrimp. ‘Each metabolic rate measurement involved a 6-hour incubation period, which was preceded by a 24-hour fasting period’, explains Glazier, adding that each crustacean also spent a week adapting to one of three temperatures (4, 10 or 16°C) prior to the measurement. ‘Our work took three years to complete’, Glazier adds.

However, when the team plotted the metabolic rates of the shrimp residing with, and without, predatory fish against their dry mass and drew a straight line through the points at each temperature, Glazier was amazed to see a complete reversal of the effects of temperature on the slope of the line depending on the presence or absence of the predators. The slope was steepest for the warmest crustaceans that had lived in fish-free streams but the slope became much shallower for the warmest shrimp that had shared their home with predatory fish. In addition, the graph for the cold shrimp that had originated in streams inhabited by fish always had a steeper line slope than hot and intermediate temperature fish from the same waterway. Fish predators dramatically affected the range of the shrimp's metabolism at different temperatures as they grew.

Glazier suspects that the crustaceans’ metabolic differences may be a result of predatory fish picking off the largest shrimp, which are most active at higher temperatures, to flatten the slope of the graph. However, the absence of predatory fish in other streams leaves larger cannibalistic shrimps free to devour smaller active members of the community, steepening the slope of the graph at higher temperatures.

‘In this way, we believe that differences in predation regime can have marked effects on how temperature affects metabolic rate and its scaling with body size’, says Glazier, who is keen to test his idea on farther afield amphipods and on isopod crustaceans closer to his Pennsylvania home.