Intestinal function and health in Ballan wrasse

Ballan wrasse has proven to be a very efficient eater of salmon lice, a parasite feeding on salmon. The Ballan wrasse is placed in the net pen together with the farmed salmon, where it provides its cleaning service to the salmon. To assure good access of Ballan wrasse for the salmon the wrasse itself is now being farmed for the purpose. Although important steps towards production of efficient and healthy B. wrasse have been taken, there are still many questions in need of answerers. A main challenge in B. wrasse production is to formulate diets that give good feed intake, growth and fish health in the ongrowing and broodstock stages, challenges related to the unique digestive system of the species. Wrasses have a short intestine (2/3 of body length) with no stomach or pyloric caeca. A better understanding of the basic biology of digestive functions is therefore vital to facilitate diet formulation and feeding regime to improve the overall production. This project has led to understanding of mechanisms underlying effects of dietary composition on passage rate, nutrient digestibility, and establish how the intestine regulates the passage of feed, and how appetite-signalling (hormones) may be involved. Further, the basic characteristics of the gut's immune responses, the volumetric development of organs from larvae to mature stages, and effects of dietary immune challengers and stimulators, and how intestinal inflammation affects digestion will be investigated. We have revealed that the digestion of protein, fat and glycogen is very efficient in this fish. Considering how short the intestine is, and that the passage time is between 10 and 14 hours, we might have a record of digestion in cm/h! Ballan wrasse has an efficient digestive system where up to 70% of protein has been digested and absorbed in the first part of intestine ? anterior bulbous. Besides, anterior bulbous is the main site for digestion of other macronutrients such as lipid and carbohydrate. About 50% of lipid and 80% of carbohydrate in the ingested food have absorbed in this section. The next part of intestine continues absorbing nutrients during the food pass through it. From another experiment, we learned that it took only 5 hours for the food to travel that distance. Based on the data, we have also created a data model that calculates the digestive, absorption and uptake efficiency of nutrients. The model is based on known but simplified physiological and hormonal principles, and also simulates the eating behavior of the species (which can best be characterized as nibbling). When we run the data model, we see that it slightly overestimates protein absorption in the anterior part of the intestine, but gives good data for the rest of the intestine. Both the model and the experimental data show that protein absorption continued along the entire intestine, with 80% absorbed by the hindgut. However, we have also investigated the intestine in a glass tube, in vitro. We simply dissect the intestine out of the fish and keep it alive in a special liquid. In this way, we can feed the intestine and observe how the food travels through. This experiment showed that the food does not simply move from one end to the other, but is pushed backwards and then forwards, and then towards the exit again! It looks like the peristalsis is actively churning the food and keeping it in the foregut to optimize digestion and absorption. This process is partly controlled by nutrient sensing in the intestine. Undigestible fibers are transported faster through the intestine compared to nutrients such as fat and protein. We have also found that the hormone CCK modulates the peristaltic movements in such a way that the food stays longer in the anterior part of the gut to optimize digestion and absorption. Intestinal immune responses were also investigated in a feeding trial in which Ballan wrasse was fed either a reference diet, or the identical diet supplemented with i) the antinutrient and immune challenger soya saponin ii) a commercial prebiotic or iii) a combination of soya saponin and prebiotics. Histomorphological analyses revealed clear structural alterations in the gut of fish fed saponin, both alone and in combination with prebiotics. Accordingly, the prebiotic was not able to prevent the alterations. The results indicated a progressing inflammation with increased infiltration by immune cells particularly into the distal parts of the intestine. Gene expression profiles obtained by RNA sequencing mirrored the histological and biochemical changes induced by the saponin load. The work has provided novel basic knowledge on the anatomy, digestive and immune function of the Ballan wrasse intestine. Additionally, the study demonstrated that Ballan wrasse gut health and digestive function may be markedly affected by diet composition. The results show that feed ingredients must be selected carefully to avoid those containing saponins.

Den viktigste effekten fra prosjektet er at vi nå vet hvordan spist fôr fordøyes og transporteres gjennom fordøyelsessystemet i berggylten. Dette har hjulpet industri og forskningsmiljøer til å tilpasse fôringsregimer på en mer optimal måte. En essensiell del av prosjektet var å utvikle metodikk for in vitro analyse av tarmens peristaltikk. Denne metodikken kan implementeres hos andre arter der tarmens respons på fôringridienser fortsatt er ukjent. Vi har også i dette prosjektet vist hvor stor effekt forskjellige fôringrdienser har på tarmens fordøyelsesprosess, dette er viktig kunnskap å ta med seg i komposisjon av nye fôr. Modellering av fordøyelse er et viktig verktøy for forståelsen av denne prosessen. I fremtiden håper vi slike modeller vil være presise nok til å predikere effekten av forskjellig sammensetning av fôr. Vi vet nå mye mer om årsak og sammenheng til inflammasjon i tarmen. Dette er viktig kunnskap for å finne årsak når disse problemene oppstår i oppdrett.

Although important steps towards production of efficient and healthy B. wrasse have been taken, there are still many questions in need of answerers. A main challenge in B. wrasse production is to formulate diets that give good feed intake, growth and fish health in the ongrowing and broodstock stages, challenges related to the unique digestive system of the species. Wrasses have a short intestine (2/3 of body length) with no stomach or pyloric caeca. A better understanding of the basic biology of digestive functions is therefore vital to facilitate diet formulation and feeding regime to improve the overall production. This proposal will investigate the digestive system by multiple approaches; in vivo, in vitro as well as modelling, methods ranging from histology to next generation deep sequencing. This project will lead to understanding of mechanisms underlying effects of dietary composition on passage rate and nutrient digestibility, and establish how the intestine regulates the passage of feed, and how appetite-signaling (hormones) may be involved. Further, the basic characteristics of the gut?s immune responses, their development from larvae to mature stages, and effects of dietary immune challengers and stimulators, and how intestinal inflammation affects digestion will be investigated.