Tough and Stretchy: Mechanical Properties of the Alimentary Tract in a Fish Without a Stomach.

The mechanical properties of intestinal tissues determine how a thin-walled structure exerts forces on food and absorbs the force of food as it enters and travels down the gut. These properties are critically important in durophagous and stomachless fish, which must resist the potential damage to foreign bodies (e.g., shells fragments) in their diet. We test the hypothesis that the mechanical properties of the alimentary tract will differ along its length. We predict that the proximal region of the gut should be the strongest and most extensible to handle the large influx of prey often associated with stomachless fish that lack a storage depot. We developed a custom inflation technique to measure the passive mechanical properties of the whole intestine of the stomachless shiner perch, Cymatogaster aggregata. We show that mechanical properties differ significantly along the length of the alimentary tract when inflated to structural failure, with 25-46% greater maximal stress, strain, extension ratio, and toughness at the proximal (25%) position. We also find that the alimentary tissues (excluding the heavily muscular rectum) are generally highly extensible and anisotropic, and do not differ in wall circumference or thickness along the alimentary tract. These findings contribute to our knowledge of the mechanical properties of fish intestinal tissues and guide future studies of factors influencing the evolution of fish alimentary systems.