Evaluation of the distribution of adipose tissues in fish using magnetic resonance imaging (MRI)

Abstract

Abstract The accumulation of excess fat in fish would impair fish health and has been a serious problem in cultured fishes. Adipose tissues are specific tissues for fat deposit in many fish species, therefore the knowledge of the anatomic distribution and total mass of adipose tissues is the basic premise to study the mechanism of fat accumulation in economic fishes. However, this issue has not been well documented. To investigate the morphology of adipose tissues in fishes, a T1-weighted magnetic resonance imaging (MRI) technique was developed to scan grass carp, turbot, tilapia, pompano and large yellow croaker. Three-dimensional (3-D) images that quantitatively integrated the total volumes of adipose tissues in these fish were constructed for the first time. From 2-D and 3-D MRI images, we identified two patterns of adipose tissue distribution in fish: visceral adipose tissue dominant (grass carp, tilapia, pompano and large yellow croaker) and subcutaneous adipose tissue dominant (turbot). The volumes of adipose tissues assessed by MRI were highly consistent with those obtained by traditional dissection. In a fasting experiment on tilapia, the MRI signal of mesenteric adipose tissues successfully distinguished between fish before and after 28-d starvation. Although our MRI technique has limitations for measuring lipid in organs not specifically dedicated to fat storage, e.g., liver, muscle and intestine, the method will help researchers to gain insights into the distribution, size, volume and shape of the adipose tissues in intact and live fish. This could be a powerful tool in future studies of fish lipid metabolism. Statement of relevance to aquaculture In aquaculture, the accumulation of excess fat in fish, which is mainly sourced from unbalanced diets or feeding strategy, would impair fish health and has been a serious problem in cultured fishes. Adipose tissues are specific tissues for fat deposit in many fishes, therefore the knowledge of the anatomic distribution and total mass of adipose tissues is the basic premise to study the mechanism of fat accumulation in economic fishes. However, this issue has not been well documented. Our present work investigated the distribution and content of adipose tissues in five economic fishes with different feed habits and farming environments by using the MRI technique. Through this work, we described the shape, distribution and mass of adipose tissues in different economic fishes for the first time. We also developed an ideal MRI scanning method for live fish, which largely expends the application of the MRI technique in nutrient physiological study in aquatic animals. Because the adipose tissue has been widely accepted to be closely related to nutrition status and diets in the most of farmed animals, including economic fishes in aquaculture, our present study has close relevance to aquaculture.