The effects of diet on microbiota in cod larvae

Live feed is not a major determinant of the microbiota associated with cod larvae

J. Hinchcliffe

As described in the hologenome theory, all animals live in close symbioses with bacteria. The gastrointestinal (GI) microbiota in vertebrates is diverse and contributes to the host’s health and development in a number of various processes. For example, the colonization of the GI system of animals has been shown to be important for the development and function of the gut system and as defence mechanism against pathogenic bacteria.

For aquatic animals, there is usually no contact between parents and offspring, but eggs and newly hatched fish larvae are exposed to bacteria in the water. The shaping of the animal GI microbiota is a complex process, and various factors influence the composition of the GI microbiota. For fish, the water is a likely source of bacteria for the initial GI microbiota. In newly hatched larvae, before feeding, there is an active uptake of bacteria and microalgae, at a rate that exceeds the drinking rate by two orders of magnitude. A number of factors could potentially influence the establishment of the GI microbiota in fish larvae; these include, but are not limited to, water quality, temperature, salinity, diet, larval developmental stage and gut structure. It is generally accepted that diet has one of the largest impacts on the GI microbiota of individual fish larvae.

The objective of this study was to investigate the impact of diet on the microbiota associated with cod larvae. Larvae were fed three different live feed diets (Copepods and two differently cultivated Rotifer cultures), and reared in three replicate tanks for each feeding regime. So except for diet, the rearing conditions were identical among tanks. The microbial communities were assessed for individual larvae, live feed and water using a PCR / DGGE analysis. Analysis of the DGGE profiles showed that different diets did not cause major changes to the composition of the microbiota associated with cod larvae. However, rearing of larvae with identical diet, but in separate tanks, often resulted in significant differences in larval microbiota. This led Bakke et al (2013) to speculate that diet does not entail major changes to the composition of cod larval microbiota, but the meduim does.

Bakke et al acknowledge that developmental changes in the digestive system could influence the GI microbiota, for example a more developed gut could provide more niches for the gut bacteria. Therefore changes observed in the microbiota of developing fish larvae may be explained by factors other than diet.

There is a possibility that diets more distinct in nutritional value could cause larger effects on the larval microbiota. Bakke et al state that cod larva require live zooplankton as feed, and the potential to manipulate the nutrient composition of live feed is limited. The natural feed for cod larvae is copepods, which are considered to be superior in nutritional value to the Rotifer used in this experiment.  But in cultivation of marine fish larvae rotifers are often used because they are simpler to cultivate. Bakke state that Rotifers and Copepods represent the extremes in feed that results in normal development of cod larvae, so this gives a good comparison.  Bakke et al also tested the microbiota associated with the different live feed cultures used in the present study and found that they were very different from each other. This is striking because the live feeds used in this study were put on different diets also (Rotifers cultivated in a standard yeast/lipid diet and the algae (Rhodomonas baltica) which also were used for cultivation of copepods). So the results presented in this study do not support the hypothesis of live feed culture as a major cause of change in microbiota of cos larvae. But the microbiota in the live feed cultures were different, to me there could be so much more done with this. Was this difference in the micro biomes of the live feed affecting the cod larvae in any way? This could serve as an opportunity for opportunistic detrimental bacteria, but this is only speculation.