Macroscopic and microscopic features of natural and experimental Flexibacter maritimus infection, and epidemiological aspects of the disease, have been reported in a number of species of fish in Tasmanian aquaculture including Atlantic salmon, Salmo salar L., rainbow trout, Oncorhynchus mykiss (Walbaum), greenback flounder, Rhombosolea tapirina Günther, and striped trumpeter, Latris lineata (Bloch & Schneider). There is a great deal of consistency in the pathology in salmonids and non‐salmonid species, with erosive lesions of external surfaces being the most prominent clinical sign. Experimentally induced disease of salmonids and flounder is similar to natural infection. Mature lesions show dermal and gill erosion, with dermal bacterial invasion into the dense connective tissue and occasionally underlying musculature, but a remarkable lack of inflammatory response. The earliest lesions show consistent fragmentation and degeneration of the epithelium, with infiltration of amorphous protein‐like materials and occasionally intra‐epithelial cellular inflammatory cells, plus congestion and haemorrhage of the superficial dermis, but without visible bacteria in standard sections. Variable scale loss, oedema and a low level of inflammation in scale pockets, plus variable small adherent bacterial mats, are evident before full epithelial erosion.

29Bacillus are Gram-positive spore-forming bacteria that are resistant to aggressive physical and 30 chemical conditions with various species existing unusual physiological features enabling them 31 to survive in various environmental conditions including fresh waters, marine sediments, desert 32 83 beneficial bacteria such as genus Bacillus in the GI tract of aquatic organisms merits 84 investigations, as autochthonous bacteria, adherent to mucosa, rapidly colonise the digestive 85 tract at early developmental larval stages (Ringø et al. 1996; Hansen & Olafsen 1999; Vadstein 86 et al. 2018).87 4 SOLTANI ET AL.During the last 20 years, numerous studies have been published on Bacillus in the intestine of 88 finfish and shellfish, their potential as probiotics, pathogenicity and their effect on the immune 89 system (e.g. Abriouel et al. 2011; Cha et al. 2013; Kim et al. 2014; Mukherjee & Ghosh 2016; 90 Wu et al., 2016; Sreenivasulu et al., 2016; Dash et al., 2018; Bachruddin et al. 2018). 91Therefore, the current review aimed to present an overview of published data on Bacillus in the 92 GI tract of finfish and shellfish, antagonistic ability, health benefits as probiotics, pathogenicity 93 and their effect on immunostimulation. 94 Bacillus in the gastrointestinal (GI) tract of finfish and shellfish 95Optimal GI functionality is of importance for sustainable animal production, and three 96 components namely diet, mucosa and commensal microbiota are essential for gut health. 97 Evaluation of the gut microbiota of finfish dates back to late 1920`s and early 1930`s (Reed & 98 Spence 1929; Gibbons 1933) and sine then numerous studies have been published. The GI tract 99 microbiota in fish is divided into; the GI lumen microbiota (the allochthonous), and those that 100 adhere to the mucosal surface (the autochthonous microbiota), however, several factors can 101 modulate the gut microbiota (Ringø et al. 2016). The intestinal microbiota contributes to several 102 essential factors such as pathogen displacement, nutrient-and receptor competition, production 103 of antimicrobial factors, structural factors (induction of IgA, tightening of tight junction and 104 immune system development) and several metabolic functions. Of the commensal gut 105 microbiota of aquatic animals, phylum Firmicutes which include genus Bacillus is essential, 106 and the contribution of Sugita and colleges (Japanese scientists) and Indian scientists (Ghosh 107 and colleges, and Ray and colleges) can not be underestimated.108

Probiotics administration in aquafeed is known to increase feed consumption and absorption due to their capacity to release a wide range of digestive enzymes and nutrients which can participate in digestion process and feed utilization, along with the absorption of diet components led to an increase in host's health and well-being. Furthermore, probiotics improve gut maturation, prevention of intestinal disorders, predigestion of antinutrient factors found in the feed ingredients, gut microbiota, disease resistance against pathogens and metabolism. The beneficial immune effects of probiotics are well established in finfish. However, in comparison, similar studies are less abundant in the shellfish. In this review, the discussions will mainly focus on studies reported the last 2 years. In recent studies, native probiotic bacteria were isolated and fed back to their hosts. Although beneficial effects were demonstrated, some studies showed adverse effects when treated with a high concentration. This adverse effect may be due to the imbalance of the gut microbiota caused by the replenished commensal probiotics. Probiotics revealed greatest effect on the shrimp digestive system particularly in the larval and early post-larval stages, and stimulate the production of endogenous enzymes in shrimp and contribute with improved the enzyme activities in the gut, as well as disease resistance.