Wide and discriminate use of antibiotics has resulted in serious biological and ecological concerns, especially the emergence of antibiotic resistance. Probiotics, known as beneficial microbes, are being proposed as an effective and eco-friendly alternative to antibiotics. They were first applied in aquaculture species more than three decades ago, but considerable attention had been given only in the early 2000s. Probiotics are defined as live or dead, or even a component of the microorganisms that act under different modes of action in conferring beneficial effects to the host or to its environment. Several probiotics have been characterized and applied in fish and a number of them are of host origin. Unlike some disease control alternatives being adapted and proposed in aquaculture where actions are unilateral, the immense potential of probiotics lies on their multiple mechanisms in conferring benefits to the host fish and the rearing environment. The staggering number of probiotics papers in aquaculture highlights the multitude of advantages from these microorganisms and conspicuously position them in the dynamic search for health-promoting alternatives for cultured fish. This paper provides an update on the use of probiotics in finfish aquaculture, particularly focusing on their modes of action. It explores the contemporary understanding of their spatial and nutritional competitiveness, inhibitory metabolites, environmental modification capability, immunomodulatory potential and stress-alleviating mechanism. This timely update affirms the importance of probiotics in fostering sustainable approaches in aquaculture and provides avenues in furthering its research and development.

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The skin, gills, and gut are the most extensively studied mucosal organs in fish. These mucosal structures provide the intimate interface between the internal and external milieus and serve as the indispensable first line of defense. They have highly diverse physiological functions. Their role in defense can be highlighted in three shared similarities: their microanatomical structures that serve as the physical barrier and hold the immune cells and the effector molecules; the mucus layer, also a physical barrier, contains an array of potent bioactive molecules; and the resident microbiota. Mucosal surfaces are responsive and plastic to the different changes in the aquatic environment. The direct interaction of the mucosa with the environment offers some important information on both the physiological status of the host and the conditions of the aquatic environment. Increasing attention has been directed to these features in the last year, particularly on how to improve the overall health of the fish through manipulation of mucosal functions and on how the changes in the mucosa, in response to varying environmental factors, can be harnessed to improve husbandry. In this short review, we highlight the current knowledge on how mucosal surfaces respond to various environmental factors relevant to aquaculture and how they may be exploited in fostering sustainable fish farming practices, especially in controlled aquaculture environments.

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