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Kathryn Knight

‘You are what you eat’, is an old adage, and according to Sora Kim from the University of California, Santa Cruz (UCSC), USA, you can learn a lot about an animal's diet by analysing the accumulation of heavy isotopes in its tissues. Kim explains that heavy carbon and nitrogen isotopes (13C and 15N), which occur naturally at very low levels in the diet, tend to accumulate in the diner's body at fractionally higher levels than they occur naturally: the heavy isotopes become enriched relative to the lighter – and more common – 12C and 14N

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isotopes. By knowing the rates at which these isotopes are incorporated into an animal's tissues and the relative abundance of nitrogen and carbon isotopes in the predator and its prey, it is possible to learn about an animal's ecology. ‘Stable isotope ratios can tell you that it is more likely to be eating squid than fish and they can also give you a sense of whether they are feeding near or off shore, or in an estuary or pelagic environment’, Kim says. However, before you can begin to learn about an animal's diet from its accumulated heavy isotope ratios, you have to know how fast the animal accrues the isotopes and the impact of different diets on the isotope ratios; and in order to establish those, you need to do a long-term feeding study (p. 2495).

Backed up by a loyal army of enthusiastic undergraduate researchers and the UCSC vet, David Casper, Kim and her thesis advisor, Paul Koch, collected nine leopard sharks from the Marine Science Institute, California, and relocated the animals to the Long Marine Lab at UCSC. Then the team fed the sharks on a diet of chopped squid three times a week and collected plasma and red blood cell samples and muscle biopsies every 3 weeks. Kim then analysed the carbon and nitrogen isotopic ratios for all three tissues.

‘Other isotope incorporation rate studies have only lasted for 3–6 months’, says Kim, who had expected that the ratios of the heavy and light isotopes would reach steady-state levels in a year at most. However, 12 months in, Kim was still seeing alterations in the isotope ratios. The shark's incorporation rates were incredibly slow, but after 18 months all three of the sharks' tissues reached stable isotope ratios for both elements. Then she switched the leopard shark's diet to tilapia – which had different 13C/12C and 15N/14N ratios – repeating the same laborious feeding routine while continuing to take tissue samples until the sharks' isotopic ratios readjusted to the new diet.

Calculating the final enrichment values and incorporation rates for red blood cells, plasma and muscle with Carlos Martínez del Rio, Kim realised that they were very different for each tissue and significantly higher than the values that had previously been estimated. ‘The carbon enrichment value that a lot of people use based on the literature was more like 1‰ or 1.5‰. However, for the tilapia diet the enrichment value for carbon in plasma is 3.7‰, for red blood cells is 2.8‰ and for muscle is 3.5‰’, she says. Also, the different tissues reached equilibrium at very different rates. ‘Those give different ideas of time and diets because the plasma has a much quicker incorporation rate than red blood cells and muscle, so the plasma would give an idea of what the shark was eating more recently but the muscle would give an impression of what the shark was eating over a longer period of time’, explains Kim.