Effect of dietary Bacillus spp. and mannan oligosaccharides (MOS) on European lobster ( Homarus gammarus L.) larvae growth performance, gut morphology and gut microbiota

Abstract

The effect of dietary application of a commercial probiotic ( Bacillus spp.) and mannan oligosaccharides (MOS), used singularly and combined, on the survival, growth performance and feed cost-benefit of larval Homarus gammarus was assessed. Un-supplemented Artemia (control) or Artemia enriched with probiotics (100 mg l −1 Bacillus spp.), MOS (12 mg l −1) or probiotics + MOS (100 mg l −1 Bacillus spp. + 12 mg l −1 MOS) was fed to four replicate groups of zoeal I lobsters for 30 days. Carapace length and weight of five H. gammarus from each replicate was recorded on 1, 3, 9, 13, 18 days post hatch (dph) and for post-larval condition at 18 dph. Additionally, moulting success was recorded from 14 to 30 dph with survival calculated at 30 dph. Morphological analysis of the posterior intestine was also conducted on larval and post-larval H. gammarus using light and electron microscopy. In a secondary experiment the effects on gut microbiota were assessed using both culture-dependent and culture-independent methods. After 18 dph, larval H. gammarus fed a diet containing Bacillus spp + MOS had significantly ( P < 0.01) improved weight gain, carapace length, weight to carapace length ratio, specific growth rate (SGR), food conversion ratio (FCR) and post-larval condition, compared to all other groups. The individual supplementation of either Bacillus or MOS also significantly improved growth parameters, survival and post-larval condition compared to the control group, but to a lesser extent. Survival of all groups was significantly ( P < 0.01) elevated after 30 days compared to the control group. Light microscopy demonstrated no significant increases in gut absorptive surface area in larvae or post-larvae receiving biotic supplemented diets. However, electron microscopy revealed significant increases in microvilli length and density in larval and post-larval H. gammarus fed biotic supplemented diets compared to the control group ( P < 0.05). Culture-based analysis of gut microbiota demonstrated probiotic Bacillus spp. colonisation in Bacillus and Bacillus + MOS fed larvae. Denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA revealed that microbial species richness and diversity was reduced in Bacillus + MOS fed post-larval lobsters. Subsequently the microbial profiles of Bacillus + MOS were the most dissimilar to the control group. Improvements seen in the present study appear to be an amalgamation of effects highlighted with the individual use of Bacillus and MOS, and so are probably additive rather than synergistic in nature.