Influence of marker particle size on nutrient digestibility measurements and particle movement through the digestive system of shrimp

A range of inert digestibility markers, such as yttrium(Y) and ytterbium (Yb) oxides, are used to indirectly calculate the digestibility of nutrients in aquafeeds. Gross over-estimations of nutrient digestibility can be encountered if several assumptions are not met, including that nutrients move through the digestive system at the same rate as the markers. The present study sought to assess the effect of particle size on apparent nutrient digestibility, and traced the movement of several inert metal markers (Y, Yb and gold) of various sizes (> 3 mu m, 400 nm, 100 nm, 50 nm and 5 nm) through the shrimp digestive tract. The first of two experiments assessed the effect of particle size on nutrient digestibility using traditional faecal collection methods, while the second traced particle movement within the stomach, hepatopancreas and intestine at various times (1, 2, 3, 5, 10 and 18 h) after a set ration. This work showed that particle size had a significant effect on nutrient digestibility, and that 50 nm and 5 nm gold particles had relative particle retention coefficients of 37.3 +/- 2.7% and 48.9 +/- 1.8%, respectively. Gold particles were detected in the hepatopancreas of animals fed 5 nm gold for > 18 h, indicating that small particles remain within the animal for long periods of time. This is despite that the shrimp digestion was extremely rapid, with evacuation rate of the digestive tract calculated at 22.1% h(-1). Large Y and Yb particles (> 3 mu m) found within the animal 1-hour after feeding showed that the amount of true feed intake was lower than measured feed intake by 15-25%, and provided an estimate of manipulative losses during feeding. Overall, this study showed that the shrimp filter press was highly efficient at excluding small indigestible particles, to below 400 nm, and that extremely high rates of nutrient digestion were maintained despite rapid movement through the digestive system. This work may in part account for the disparity between high levels of apparent nutrient digestibility and low nutrient utilisation efficiency measured in shrimp.