Figure 1.

Terrestrial animals are cultured on flat surfaces (left). As feed is normally supplied to the animals in cratches, the feed associated-microbiota has little direct interaction with the environmental microbiota as opposed to the host intestinal microbiota. Defecation will modify the environmental microbiota on a local scale, but not necessarily on a regional scale (including the animal's intestine) because of dispersal limitation, or limited active uptake in the case of grazers (Bissett et al., 2010). Aquatically cultured animals live in water that completely surrounds them (right). As feed is dosed directly in the water, the feed microbiota may have a profound effect on the composition of the environmental microbiota. Similarly, the composition of the environmental microbiota may be dependent on the host intestinal microbiota because of defecation in the water. Microorganisms in the water easily spread on a regional scale, including to the animal's intestine because of drinking for osmoregulation, during feeding or by active uptake. The (in)dependency of the host microbiota from both the feed microbiota and the environmental microbiota is thus a relevant question (Wong and Rawls, 2012).