Implications of Ecological Niche Differentiation in Marine Bacteria for Microbial Management in Aquaculture to Prevent Bacterial Disease

Aquaculture, breeding, and rearing of aquatic animals (mollusks, crustaceans, and finfish) in marine, brackish, and freshwater bodies is playing an increasingly important role with respect to food security for the growing human population and is predicted to dominate the seafood supply within a few decades [1]. However, the further sustainable expansion of the sector is currently hampered by a number of factors, amongst which diseases are playing a prominent role, especially in the early life stages of the animals (i.e., larviculture [2]). A major group of causative agents are bacterial pathogens, such as Vibrio spp. [3]. These bacteria cause huge losses in the aquaculture industry worldwide, with acute hepatopancreatic necrosis disease (AHPND) as a notable recent example [4]. This disease, caused by strains of Vibrio parahaemolyticus that acquired a plasmid encoding two toxin genes, was first reported in southern China in 2009,subsequently spread over Southeast Asia, and reachedMexico in 2013. The AHPND disease typically affects shrimp postlarvae,within 20–30 days after stocking, and frequently causes up to 100%mortality. Global losses in the shrimp farming industry because of this disease have been estimated to be over US$1,000 million per year [5]. Pathogenic vibrios are opportunistic pathogens (as opposed to obligate pathogens) since they are capable of surviving and multiplying in the absence of their host [6]. In a recent paper, De Schryver and Vadstein argued that the ecological r/K theory could serve as a foundation for the development of microbial management strategies to prevent diseases caused by opportunistic pathogens in aquaculture [7]. According to the ecological theory of r/K selection, an unstable environment containing high nutrient levels per individual selects for organisms with the capacity to exploit nutrients and increase population size, termed rstrategists. On the other hand, a stable environment where the resources per individual are scarce will select for slow-growing organisms, termed Kstrategists. Most bacterial diseases in aquaculture (and especially larviculture) are caused by opportunistic pathogens that are ubiquitous in the marine environment and that are capable of quickly increasing their population size in the aquaculture environment: i.e., r-strategists [2,7]. The triggers that induce mortality events are not yet completely understood (and are probably different for different pathogens), although increases in dissolved nutrients, temperature, and periods of hypoxia have been shown to be involved [8–10]. Management strategies aiming at disease prevention in aquaculture systems should be pursued at different levels, the first of which includes the implementation of hygienic barriers (e.g., the disinfection of incoming water in order to avoid those pathogens that are part of the