Atlantic bluefin tuna larvae, are them stochastically colonized by bacteria?

Bluefin tuna (Thunnus thynnus) is a species strongly impacted in the western Atlantic Ocean and Mediterranean Sea and the collapse of its recruitment put effort to develop methods for reproduction in captivity. Improving the survival of the early developmental stage is one of the major targets in larval hatchery. At the moment survival expectancy for tuna larvae drops at 2-3 days after mouth opening. Although more studies are required to understand the effect of environmental factors and nutritional requirements, there is growing concern about the microbiota associated with tuna larvae and their interactions.

Studies on the microbial communities structure of captive reared marine fish are more abundant on cold water species (Atlantic halibut and cod) than warm and temperate water species as tuna. Some studies on halibut reveal similarities between larvae sampled from different geographical regions. Other similarities in Bacterial Community Profiles (BCP) were found over several years in the same batches of larvae. Also feeds and change of diet can affect significantly BCP associated with larvae (this was reported in cod larvae). If we consider that for cod larvae BCP varied as much between individuals from the same tank as between different hatcheries.

Furthermore some studies demonstrate that the high temperature (around 24°C) required by tuna larvae may promote potential pathogen bacterial growth in the rearing tanks so “green water” system are commonly used. Green and pseudo-green methods are rearing systems that use microalgae to regulate microbial growth, and where light intensity, photoperiod and feed supply are also regulated. Another system is mesocosm: large tanks colonized by wild plankton populations and after the first phytoplankton bloom bacteria proliferate and then remain constant. This system is combined with regular addition of cultured feed as microalgae and rotifers.

The aim of this work was to compare BCPs of tuna larvae reared with three methods commonly used in Mediterranean hatcheries: clear water, pseudo green and mesocosm.  To evaluate how these three methods affected the bacterial colonization of individual tuna larvae, BCPs of individual were compared within and between rearing groups during two years using the PCR-DGGE method. The authors found high individual variability of microbiota associated with larvae and this result agree with previous result observed in cod larvae sampled from different hatcheries. Unexpected result was high diversity and individual variability after the mouth opening, when the larvae had not started exogenous feeding (in many species this is a crucial point for microbial colonization). No regular succession of bacterial communities after start feeding were found in this study. An apparent shift was found after the introduction of live prey (yolk sac sea bream larvae) but it was reported only in two individuals from one hatchery. No significant effect of rearing systems was detected on the bacterial communities composition. To explain these results authors do not exclude the implication of any factors as physiological status of individual, stochastic inoculation, effect of live feed. They point out the limited set of experiments on this species and the need of many individuals to obtain representative sample avoiding errors.

In my opinion this work lack of the identification of at least some specific group of bacteria. Is knew that in Sarda sarda larvae (bonito, another Scombridae reared in the same condition as T. thynnus) the most two abundant groups of associated microbes are Vibrionaceae and Rhodobacteriaceae. Should be interesting to see if these groups match with that from tuna. Anyway the author mentioned that the sequencing of band from the gel wasn’t possible for this work. Another point should be the sampling of different larval tissues as skin and gut after mouth opening using also in situ visualization methods. From this work seems that tuna larvae are stochastically colonized by bacteria and to clarify this, useful data in this work, should be the evaluation of BCPs of surrounding water. The study of tuna hatchery and larvae associated microbial communities seems to be a wide open field for the moment. 

Gatesoupe, F. J., Covès, D., Ortega, A., Papandroulakis, N., Vadstein, O., & Gándara, F. (2013). A spatiotemporal study of bacterial community profiles associated with Atlantic bluefin tuna larvae, Thunnus thynnus L., in three Mediterranean hatcheries. Aquaculture Research, 44(10), 1511-1523.