Prebiotic Applications in Shellfish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 413

(92.3%). Using selective agar Zhou et al. (2007) reported that dietary scFOS increased the
proliferation of Streptococcus faecalis and Vibrio parahaemolyticus in the GI microbiota of
Pacific white shrimp L. vannamei .
Considering the fact that prebiotic effects are driven, in part at least, by influencing the
host microbiota, it is clear that this topic has not been sufficiently explored in shellfish. Future
studies should utilize quantitative techniques such as FISH, qPCR and metagenomics to elu-
cidate the impact of prebiotics on the complex microbial communities within the GI tract of
shellfish. The host-microbe interactions at the intestinal epithelium are a key determinant of
gastric morphology, activity and immunity.
15.3.4 GI morphology and digestion
Prebiotic induced improvements in the length, regularity and density of mucosal folds and/or
improvements in microvilli formations have been documented in a number of teleost species
(Pryor et al. 2003; Yilmaz et al. 2007; Salze et al. 2008; Sweetman et al. 2008; Dimitroglou
et al. 2009; 2010a; 2010b). It has been suggested that oligosaccharides can promote the
build-up of lactic acid (and other short-chain fatty acids (SCFAs)) in the GI tract and that
such acids induce mucosal cell proliferation (Lji et al. 2001), thus improving mucosal
structure. Moreover, SCFAs, mainly acetate, propionate and butyrate, are produced as a result
of prebiotic fermentation and these SCFAs are considered an important energy sources for
intestinal enterocytes (Scheppach 1994). It has also been suggested that the acids produced by
LAB can cause higher solubility of minerals, increasing the uptake of metal ions (Merrifield
et al. 2010a).
Despite the plethora of information in finfish, such effects in shellfish have only been inves-
tigated with the use of MOS in lobsters and shrimp (Genc et al. 2007; Daniels et al. 2010;
2013; Sang and Fotedar 2010; Sang et al . 2014). Genc et al. (2007) studied the effects of MOS
on the hepatopancreas histology in tiger shrimp and reported no effect on the morphology of
the hepatopancreas tissue. In a later study the application of MOS in larval European lobster
resulted in elevated intestinal microvilli length and density (Daniels et al. 2010; Figure 15.1).
The addition of MOS to juvenile spiny lobster diets also improved absorptive surface area,
as measured by the internal perimeter to external perimeter ratio of the gut wall using light
microscopy (Sang and Fotedar 2010). Increased epithelium thickness and epidermal cell den-
sity have also been reported in juvenile brown tiger prawn ( P.monodon ) fed diets supplemented
with MOS at lower concentrations (1-2 g kg -1 ) (Sang et al. 2014).These improvements in the
development of GI mucosal structure provide a larger surface area for the absorption of nutri-
ents, thus potentially allowing for improved feed conversion and growth (as documented in
Table 15.1).
However, the reports of enhanced feed conversion could also be due to enhanced diges-
tive enzyme activity. Prebiotics provide a nutrient source for bacteria and thus prebiotics can
enhance the presence of certain bacteria which may aid digestion or provide vitamins (Gib-
son 2004; De Vrese and Schrezenmeir 2008). For example, FOS can be fermented by bacteria
including lactobacilli and bifidobacteria (although bifidobacteria are not commonly reported
in fish) which express the enzyme β-fructosidase (Sghir et al. 1998; Manning and Gibson
2004). Bacillus spp., which can ferment oligosaccharides (Mahious et al. 2006), are also
known to generate enzymes that aid the digestion of proteins and starch via the production
of protease and amylase, respectively (Ellouz et al. 2001; Ben Messaoud et al. 2004; Konsula
and Liakopoulou-Kyriakides 2004; Liu et al. 2009). Stimulation of such exogenous enzymes