Scientists refine method to trace the complicated diets of Japanese scallops

Have you ever heard the phrase: “You are what you eat”?

Well, it may be true in more ways than you think. Through many scientific advances, we have developed methods to tell what an animal has been eating by comparing the composition of their tissues and potential food sources in their environment. This may sound really unimpressive when you’re thinking about large animals like cows that just eat grass, but think about animals whose diet may be a little harder to identify. Scallops, for example, feed plankton, which could be anything floating nearby. To know the complicated diets of scallops, we rely on a concept called isotope fractionation.

Hold on. You might be asking, “what are isotopes”?

Put simply, sometimes elements in nature are just heavier than they usually are. A single atom of carbon for example usually weighs around 12 atom mass units. However, you can sometimes find carbon atoms that weigh somewhere around 13. This is what we call an isotope. In this case, isotope ¹³C. Some isotopes are radioactive and decay over time, but isotopes like ¹³C are pretty stable.

Scientists have found out that these stable isotopes interact differently with the environment compared to the same atoms of normal weight. This finding has led to methods involving isotopes that let us know details about what an organism eats. For example, certain animals’ tissues have consistent amounts of ¹³C­ in them. If that animal is eaten by another animal, a certain amount of ¹³C in the prey’s tissues get integrated into its predator’s body. By comparing the amount of regular ¹²C and ¹³C in an animal’s tissues, we can tell what it has been eating.

Now, back to scallops.

Isotope fractionation values have been used to determine the diets of shellfish before, but scientists have found that the numbers vary considerably because shellfish end up digesting a wide variety of particles. This would make it hard to use the method when you’re analyzing the diet of commercially grown shellfish like Japanese scallops. To make the method for analyzing the diet of these scallops more reliable, Dr. Frolan Aya, from SEAFDEC/AQD, and Prof. Isao Kudo, from the Faculty of Fisheries Science in Hokkaido University, sought to determine how the isotope fractionation values are affected by the quality of the scallop food and by how fast they grow. They also wanted to know if the values would be different depending on what kind of scallop tissue was analyzed.

They found that tissue-diet isotope fractionation values were higher in muscle tissues compared to digestive gland tissues. This implies – among other things – that the scallops’ digestive glands integrate what it eats faster than their muscles do. They also found that isotopic fractionation values of nitrogen in younger scallops were different from older scallops because of their growth rate. The results of their study would be helpful in estimating the food utilization of scallops under field conditions.

Further details of the study can be found in the article, “Effect of diet isotopic ratios on the δ13C and δ15N signatures of scallop-gut contents in a natural setting,” which was published this year in the journal Plankton and Benthos Research. Download a copy here: https://www.jstage.jst.go.jp/article/pbr/18/3/18_B180303/_article