Australia.- Why is there gold in our prawns? Have we struck it rich by developing a new prawn that can produce gold? Unfortunately not, but by intentionally including gold in prawn diets, we have learned many things about the biology of the prawn digestive system.
Gold is a rare earth metal, meaning we can detect gold (and other metals such as yttrium and ytterbium) at very low levels in our samples. Rare earth metals are commonly used in nutrition studies to determine how the ingredients in the diet are digested by the animal. The proportional increase of the indigestible metal markers from the diet as it comes out the other end is able to tell us how efficiently the proteins and lipids in the feed are being used by the prawn. This can define which raw materials are best for inclusion in prawn diets, such as those from our previous research (Glencross et al., 2018).
The innovative use of gold in this latest experiment was that it comes in very small, precisely sized nanoparticles (0.0001 mm and below). We used these nanoparticles to tell us not only how prawns are digesting their food, but also how we might best deliver innovative functional feed solutions.
This experiment was logistically complex, having to feed animals a standardised ration of one of three different diets (that contained different sized gold particles) over a 24 hours’ time series. We then sacrificed the animals at different times after eating, and analysed the amount of metal present in different sections of their digestive tract (stomach, digestive gland and intestine).
We discovered that the movement of food through the prawn digestive system is extremely rapid, with one meal moving completely through the animal within five hours. We also thought that prawns were messy eaters, and were also able to discover that approximately 20% of food is lost as they chew it. The most interesting part was that the smallest gold particles were retained in the digestive gland for longer than 24 hours, rather than moving rapidly through the intestine. This means that particles of this size that encapsulate water-soluble biological molecules (e.g. amino acids, nucleotides, peptides, antimicrobial compounds?) are potentially more suitable to deliver functional feed solutions.
Overall, this work demonstrated some unique aspects of the prawn digestive system. It is able to filter out very tiny (sub-micron) particles of food, far smaller than previously thought. The smallest particles (<50 nm) that get through might be best suited to delivering functional ingredients as they enter the site of intracellular absorption and digestion, i.e., the digestive gland. The movement of food through the digestive system is extremely fast, supporting data that prawns are extremely effective at pulling out the nutrients from their daily meals.
But they aren’t as efficient as we thought at getting the food into their mouth and some of the nutrients ingested are not ending up being absorbed. Each of these things can help us formulate new and improved diets that help prawns grow faster, use dietary ingredients more efficiently, and reduce environmental impact in aquaculture.
Dr Nick Wade
Senior Research Scientist, Breeding and Genomics
N.M.Wade, N.Bourne C.J.Simon. Influence of marker particle size on nutrient digestibility measurements and particle movement through the digestive system of shrimp. Aquaculture, Volume 491, 1 April 2018, Pages 273-280. https://doi.org/10.1016/j.aquaculture.2018.03.039 https://www.sciencedirect.com/science/article/abs/pii/S0044848617320069