Abstract:

Two experiments were conducted in order to investigate the effect of dietary mannan oligosaccharides
(MOS) on gilthead sea bream (Sparus aurata). Experiment I was designed to assess the effect of dietary MOS
(0%, 0.2% and 0.4%) on fish fed diets containing fishmeal (FM) as the only protein source. Experiment II was
designed to assess the effect of MOS (0% and 0.4%) on fish fed soybean meal (SBM) as a partial replacement
of FM (SBM inclusion 31% of diet). After 9 weeks feeding on the experimental diets growth parameters, body
composition, liver and intestinal histology and intestinal microbial diversity were assessed. The results
showed that mean final weight, specific growth rate (SGR), feed conversion ratio (FCR) and protein
efficiency ratio (PER) remained unaffected by MOS supplementation of fish fed FM or SBM diets.
However, compared to the control group (FM0), condition factor (K) and hepatosomatic index (HSI) were
significantly lower in fish fed 0.2% MOS (FM02) and 0.4% MOS (FM04), respectively. These parameters were
unaffected in SBM-fed fish. Body proximate composition remained unaffected by MOS supplementation in
fish fed either FM or SBM diets (P>0.05). Histological evaluation revealed that MOS had no effect on
glycogen deposition in liver and no effect on gross villi morphology in the anterior intestine in either
Experiment I or II. However, relative to the control groups (FM0) dietary MOS appeared to improve gross
morphological absorptive surface area in the posterior intestine in Experiment I. Electron microscopy
revealed that dietary MOS had a pronounced effect at the ultrastructural level in both experiments, as
microvilli density and length were elevated in both intestinal regions in fish fed both the FM and SBM based
diets. No significant histological differences were found between respective FM0 and SBM0 groups.
DGGE analysis revealed that both SBM and MOS affected the intestinal microbial species richness and
diversity. However, the effect of dietary MOS on the gastrointestinal microbiota was more pronounced in
FM-based diets (Experiment I) as was reflected by increased species richness and diversity and reduced
similarity between microbial profiles of the different FM groups. The effect of MOS in Experiment II on SBMfed
fish was marginal, as species richness and diversity remained unaffected and similarity between
microbial profiles of the SBM groups and replicates remained high (i.e. >80%). Dietary SBM exerted a greater
effect on gut microbiota than dietary MOS