Probiotics, lactic acid bacteria and bacilli: interesting supplementation for aquaculture

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.


Introduction
For many years, antibiotics and chemotherapeutics were supplemented in animals' diets at subtherapeutic levels, to promote benefits by enhancing growth rate, reducing mortality and improving reproductive performance. In 2003, the European Union stated in Regulation (EC) No. 1831/2003; 'Antibiotics, other than coccidiostats or histomonostats, shall not be authorized as feed additives'. Consequently, this banning urgently made the scientific community to seek for alternatives to reduce the abuse of antibiotics, and one of the promising feed additive was probiotic. Probiotics/fermented milk has a very long history as Genesis 18:8 stated, New Living Translation; 'When the food was ready, Abraham took some yogurt and milk and the roasted meat, and he served it to the men. As they ate, Abraham waited on them in the shade of the trees'. According to Bottazzi (1983), the Roman historian Plinius in 76 BC recommended administration of fermented milk products for treating gastroenteritis. However, the modern history of probiotics started more than a century ago, as the Russian Nobel prizewinner, Elie Metchnikoff, performed the observation that the regular consumption of some fermented milk products containing viable bacterial species may have a beneficial role in the maintenance and reestablishment of microbiota and consequently intestinal homoeostasis. The term probiotics, 'to be used for substances that favours the growth of micro-organisms' was first proposed by Lilly and Stillwell (1965), but more recently, Hill et al. (2014) suggested a more correct definition of probiotics 'live micro-organisms that, when administered in adequate amounts, confer a health benefit on the host'.
Probiotic administrations mainly depends on several factors, that is the probionts, supplementation form, vector of administration, dosage level and duration of application and several different administration modes have been used: oral administration via diet or water/bath, administration of several probiotics in combination, inactivated bacteria, spores, administration-continuously or regular intervals, and co-administration of probiotics with prebiotics (synbiotics) or plant products. Important questions to be clarified when discussing probiotics are; species isolated from the host, host specificity vs strains from other species or commercial probiotics, as well as single or combined administration.
The mechanisms of actions of probiotics in aquaculture are divided into; antagonistic compound secretion, substances produced by probiotics; act as antagonist for quorum sensing mechanism, adhesion and colonization to the intestinal mucosa, competitive exclusion when probiotic bacteria colonize the intestine and thereby inhibiting adherence and colonization of pathogenic bacteria, improved functionality of the gastrointestinal (GI) tract, modulation of the GI tract microbiota, competition for iron, sources of nutrients and enzymes for digestion, enhancement of immune responses, antiviral effect and improve water quality through modulation of the water microbiota.
In order to avoid overlaps with previous review papers, the current review aimed to present an updated overview of recently published data, mainly from 2018 and 2019, on health benefits of LAB and Bacillus probiotics, on their effect on growth performance, modulation of the gut microbiota, the immune system and disease resistance in finfish and shellfish.

Methods of probiotic administration
To our knowledge, the first application of probiotics in aquaculture was carried out by Kozasa (1986), but since then the environment-friendly treatment has increased rapidly, and several comprehensive aquaculture reviews have been published (e.g. Gatesoupe 1999;Merrifield et al. 2010;Hai 2015;Hoseinifar et al. 2018;Ringø et al. 2018Ringø et al. , 2020Ringø 2020). However, it is essential to investigate the best way of administration, optimal dose, and the technical solutions required, especially to keep the probiotics alive in dry pellets (Gatesoupe 1999).
Probiotic administrations depends on several factors i.e. the probiotics used, supplementation form, vector of administration, dosage level and duration of application, and several different administration modes are proposed: i Oral administration via diet or water/bath. Inclusion to the diet is the most widely used administration method. Probiotics and cell wall components (parabiotics) are applied in the feed, added to the entire tank or pond water to confer protection against infection. In fish-and shellfish larvae, live food (e.g. Artemia) has revealed to be an efficient carrier of probiotics. ii Administration of several probiotics in combination.
In the review, "Probiotics in man and animals," Fuller (1989) wrote, "Probiotic preparations may consist of single strains or may contain any number up to eight strains." However, since the early 1990s most aquaculture studies used single administration, but during the last years, supplementation of multiple probiotics in the diets has gained interest. The advantage of multiple-strain preparations is; they are active against wider range of conditions and species. iii Inactivated bacteria. For example, oral administration of heat-inactivated Lactobacillus delbrueckii and Bacillus subtilis, individually or combined. v Spores help the bacteria to survive by being resistant to extreme changes in the bacteria's habitat including extreme temperatures, lack of moisture/drought, or being exposed to chemicals and radiation. Bacterial spores can also survive at low nutrient levels, and spore-forming probiotic bacteria have received increased scientific and commercial interest.
v Culturing, storing and administration. Probiotics are usually added to feed as freeze-dried cultures, and sometimes mixed with lipids to be added as top. vi Lyophilization or freeze drying, is a low temperature dehydration process, involving freezing of the product at low pressure, and removing the ice by sublimation. This method is used in probiotic studies of finfish and shellfish. vii Administrationcontinuously or regular intervals?
Most studies carried out have continuously fed the host fish for a wide range of time, varying from 15 to 94 days (Hai 2015). The continual application of LAB, bacilli, and certain Gram-negative bacteria increase colonization of the supplemented bacteria, and modulated the microbial population in the GI tract. However, an important question arises; are the probiotics permanently colonisers in the GI tract? viii Co-administration of probiotics with prebiotics or plant products.
Important questions when discussing probiotics are; species isolated from the host, vs. strains isolated from other species or commercial probiotics?

LAB as probiotics in finfish and shellfish
Improve feed utilization Numerous investigations have recently conducted the alternation of enzyme patterns as a consequence of the consumption of LAB in shellfish and finfish (Tables 1  and 2). Recently, dietary inclusion of Lactobacillus sp. and Lb. pentosus at concentrations of 10 7 and 5 9 10 8 CFU per g improved several digestive enzymes of Pacific white shrimp (Litopenaeus vannamei) Zuo et al. 2019). Similarly, an elevation in protease, amylase and alkaline phosphatase was observed in narrow clawed crayfish (Astacus leptodactylus) fed Lb. plantarum at concentrations of 10 7 , 10 8 and 10 9 CFU per g (Valipour et al. 2019). Dawood et al. (2019) reported that incorporation of heat-killed Lb. plantarum at 50, 100 or 1000 mg kg À1 significantly enhanced amylase, lipase and protease activity of Nile tilapia (Oreochromis niloticus). Significant increase in lipase, amylase, trypsin, alkaline phosphatase and protease activity also recorded in common carp (Cyprinus carpio), olive flounder (Paralichthys olivaceus) and rainbow trout (Oncorhynchus mykiss) fed LAB in combination with b-glucan, mana oligosaccharide, Bacillus sp. and Citrobacter (Jang et al. 2019;Mohammadian et al. 2019aMohammadian et al. , 2019b.

Promote growth performance
Probiotic is one of the most promising means to sustain the normal growth, health and well-being of farmed fish and shellfish because they serve as nutrients source, vitamins and digestive enzymes, and they will significantly contribute to feed consumption, nutrients uptake and host's growth rate (Nath et al. 2019). Probiotics consumption have been speculated to improve the host's appetite or boost organisms' digestibility by stimulating the excretion of digestive enzymes and maintaining the balance of intestinal microbes, which led to the improvement of nutrients absorption and utilization, as well as survival and growth of the host.
Most studies using LAB in shellfish focus on growth performance and survival rate. Lb. pentosus and Lb. plantarum inclusion in Pacific white shrimp diets significantly improved growth performance and feed utilization (e.g. Correa et al. 2018;Gao et al. 2018;Zheng et al. 2018). Recently, Zuo et al. (2019) revealed that supplementation of Lactobacillus at 10 7 CFU per g for 27 days significantly increased body weight of Pacific white shrimp. In contrast, no significant difference in growth parameters was recorded in narrow clawed crayfish fed Lb. plantarum for 97 days (Valipour et al. 2019). Incorporation of LAB with other probiotics or functional feed additives resulted in higher growth performance in shellfish. Dietary supplementation of Enterococcus faecalis and Pediococcus acidilactici significantly improved weight gain and specific growth rate of narrow clawed crayfish and mud crab (Scylla paramamosain) (Safari et al. 2017;Yang et al. 2019). Wang et al. (2019) revealed that dietary in combination of Lb. pentosus, Lactobacillus fermentum, B. subtilis and Saccharomyces cerevisiae significantly improved growth performance and survival rate of Pacific white shrimp, but no significant difference was revealed in carcass composition.
Most finfish studies focused on the effects of different LAB and combination with other probiotics and natural immunostimulants on growth performance. Dietary administration of Lactobacillus spp. at different concentrations significantly enhanced growth parameters of several finfish species (e.g. Abdelfatah  plantarum in combination with orange peel derived pectin, corncob-derived xylooligosaccharide, Cordyceps militaris or Bacillus velezensis significantly enhanced growth performance of Nile tilapia (Van Doan et al. 2017, 2018, 2020a. Similarly, dietary administration of Lactobacillus in combination with b-glucan or mananoligosaccharide significantly stimulated the growth performance and feed utilization of common carp (Mohammadian Increase disease resistance Probiotics have been proven as an effective tool for disease prevention in aquaculture Ringø et al. 2018). Probiotics can interact with or antagonize other enteric bacteria by resisting colonization or by directly inhibiting and reducing the incidence of opportunistic pathogens . They can also improve host's health and well-being via physiological or immune modulation (Butt and Volkoff 2019 Final weight (FW), Weight gain (WG), Specific growth rate (SGR), Food conversion efficiency (FCE), Food conversion ratio (FCR), Protein efficiency ratio (PER), Survival rate (SR), digestive enzyme and disease resistance of shellfish. N/A-no information available; ↑-positive effect; ↓-negative effect; ?-no effect. Journal Weight gain (WG), Specific growth rate (SGR), Food conversion efficiency (FCE), Food conversion ratio (FCR), protein efficiency ratio (PER), survival rate (SR), digestive enzyme, and disease resistance of shellfish. N/A-no information available; ↑-positive effect; ↓-negative effect; ?-no effect.

Immune effects of LAB on finfish and shellfish
The immune effects of LAB on finfish have been the most extensively studied. Therefore, only the recent studies, published in 2018 and 2019, regarding the immune functions of LAB on finfish and shellfish are highlighted in this review (Table 3).

Finfish
Juvenile common carp were fed for 56 days with a diet mixed with Lb. acidophilus, an isolate from chicken manure, in a three differential dosages, 0Á2, 0Á4 and 0Á6% (Adeshina 2018), and all groups significantly increased numbers of immune cells. When challenged with Pseudomonas aeruginosa (1 9 10 7 CFU per ml) or A. hydrophila (1 9 10 7 CFU per ml), the carp survived in a dosedependent manner: RPS in P. aeruginosa challenge survival rates were 42, 68 and 79% respectively; A. hydrophila challenge survival rates were 43, 83 and 78% respectively. Common carp were soaked in the water containing Ent. faecalis CgM36 (10 6 CFU per ml), a bacteria isolated from carp for 30 min (Mulyani et al. 2018). Following 12 days of maintenance, the carp were challenged with A. hydrophila (10 6 CFU per ml). The LAB-treated carp showed an increase in their survival rate 4 days postinfection (50%) compared to the control group (35%). Three strains of other carp commensal LAB (CcB7, CcB8, CcB15) were also tested for their immune effects (Shabirah et al. 2019). Carp fingerlings were immersed in the LAB-containing water (10 6 CFU per ml) for 24 h, and this process was repeated three times in a 7-day period. The fish were then challenged with A. hydrophila (10 8 CFU per ml). The LAB-treated groups demonstrated significantly increased survival rates (CcB7 72%, CcB8 56%, CcB15 83%) compared to that of the control (33%). Common carp were fed carp-isolated Lac. lactis strains (Q-8, Q-9 or Z-2) for 8 weeks at a concentration of 5 9 10 8 CFU each per 1 g of feed (5 9 10 8 CFU LAB per g) (Feng et al. 2019). The Lac. lactis-fed fish increased gene expression of both proinflammatory (TNF-a, IL-1b, IL-6, IL-12), and anti-inflammatory cytokines (IL-10, TGF-b). However, the Lac. lactis Z-2-treated group had a decrease in TGF-b levels. Smaller juvenile common carp fed P. acidilactici MA18/5M-containing supplementary diet (6 9 10 8 CFU per g) for 60 days , revealed increased total immunoglobulin (Ig) concentration, mucous protease activity and skin lysozyme gene expression. The same LAB fed to beluga (Huso huso) for 8 weeks at three concentrations (10 7 , 10 8 , 10 9 CFU per g) (Ghiasi et al. 2018), revealed significantly increased total serum Ig level, lysozyme activity and respiratory burst activity in a dose-dependent manner. The immune effect of a soil-origin Lactobacillaceae, Pediococcus pentosaceus SL001, was studied on grass carp (Ctenopharyngodon idella) . When grass carp were fed P. pentosaceus SL001 (1 9 10 9 CFU per g) for 30 days, the gene expression levels of IgM and C3 complement protein were increased in both the liver and spleen. However, the expression levels of lysozyme, IL-1b and IL-8 were varied, whereas challenged with A. hydrophila, the P. pentosaceus-treated group displayed a significantly decreased mortality rate during the 7 days postinfection (Con: 90%, Lb. pentosaceus SL001: 52%). Nile tilapia fed host-originated probiotics (Lb. plantarum N11 (10 8 CFU per g), B. velezensis H3.1 (10 7 CFU per g)) for 15 or 30 days (Doan et al. 2018), revealed that fish fed the mixture of the two probiotics significantly increased innate immune parameters in both the 15 and 30 days-feeding groups (lysozyme and peroxidase activities, complement phagocytosis and growth performance), compared to the singular formation-treated groups. When challenged with S. agalactiae (1 9 10 6 CFU) at the 30-day feeding time point, the combined form-treated group showed the highest survival rate (relative percent survival, RPS 58Á33%). The singular or combined form of Lb. rhamnosus JCM1136 and Lac. lactis JCM5805 were fed (5 9 10 7 CFU per g) to the juvenile Nile tilapia for 6 weeks (Xia et al. 2018). Fish fed LAB, significantly increased the transcript levels of IFN-c lysozyme, hsp70 and IL-1b in the intestine and liver. However, there were no significant differences between the single and combined form-fed groups. When challenged with S. agalactiae WC1535 (2 9 10 3 CFU), the fish fed Lac. lactis survived at the highest level (con. 19%, Lac. lactis 59%). Red tilapia (Oreochromis spp.) were fed a synbiotic supplementary diet that included Jerusalem artichoke (10 g kg À1 ) and dried Lb. rhamnosus GG (1 9 10 8 CFU per g) for 30 days (Sewaka et al. 2019). The red tilapia significantly increased mucin-secreting goblet cell numbers, lysosomal activity, alternative complement (ACH50) activity and total Ig concentration. The RPS of the synbiotic-treated fish was 76Á43 AE 23Á24 when challenged with A. veronii (10 7 CFU per fish).
Olive flounder fingerlings fed Lac. lactis I2 (10 8 CFU per g) isolated from olive flounder, for 8 weeks (Hasan et al. 2018), displayed significantly enhanced innate   immune parameters: respiratory burst and the activities of superoxide dismutase, serum lysozyme, myeloperoxidase and antiprotease. Furthermore, the LAB-treated fish increased the gene expression of pro-inflammatory cytokines: TNF-a, IL-1b and IL-6. When challenged with Streptococcus iniae (10 8 CFU per ml), higher survival (20%) was revealed compared to control fish (0%). Another olive flounder-originated bacteria (Lactobacillus sakei PO11, Lb. plantarum PO23) were fed (10 11 CFU per g) in a single form to olive flounder for 42 days (Feng et al. 2018), and fish fed LAB increased gene expression of immune genes in the gill and head kidney: IL-1b, TNF-a, MHC-Ⅱ, IgM and TCR-b. Lactobacillus lactis HNL12 isolated from humpback grouper (C. altivelis) were fed to humpback grouper juvenile at different concentrations (10 6 , 10 8 , 10 10 CFU per g) for 4 weeks , and all Lac. lactis-fed groups increased the activities of respiratory burst, serum acid phosphatase and serum lysozyme up to 2 weeks of feeding. However, those innate immune parameters were diminished thereafter for the remainder of the 4-week experimental time period. When challenged with V. harveyi QT520 (1 9 10 5 CFU per fish), The RPSs of the 10 6 , 10 8 and 10 10 CFU per g-fed groups were 31, 53 and 50% respectively. Juveniles of Asian sea bass (Late calcarifer) were fed P. acidilactici MA18/5M (0Á9 9 10 7 CFU per g) for 42 days (Ashouri et al. 2018). The P. acidilactici-treated group significantly increased innate immune parameters in serum: respiratory burst, lysozyme and haemolysis activities. However, only the lysozyme activity was enhanced in mucosal immune parameters. When shabout juveniles (Tor grypus) were fed autochthonous Lb. casei PTCC1608 (5 9 10 7 CFU per g) for 60 days, the fish significantly increased haemoglobin concentration and white blood cell numbers ). In addition, gene expressions of IL-1b, TNF-a and IL-8 were also increased in the head kidney. Juvenile Caspian white fish (Rutilus frisii kutum) were fed a mixture of Lb. acidophilus, Lb. casei, Enterococcus faecium and Bifidobacterium bifidium (PrimaLac â , 1 g kg À1 ) for 45 days (Mirghaed et al. 2018). The fish fed PrimaLac â increased the enzyme activities of lysozyme, alkaline phosphatase and protease in the skin mucus.

Shellfish
A mixture of two autochthonous isolates, Lb. plantarum SGLAB01 and Lac. lactis SGLAB02, (1 : 1 ratio, 3 9 10 8 CFU per g each) was fed to Pacific white shrimp for 16 days (Chomwong et al. 2018), and LAB feeding significantly increased the enzyme activity of phenoloxidase and the gene expression of LvproPO1 and LvproPO2. When immersion-challenged with V. parahaemolyticus (1 9 10 4 CFU per ml), cumulative mortalities in the 10 days postinfection were significantly  reduced: Lb. plantarum SGLAB01, 50%; Lac. lactis SGLAB02, 40%; the mixture, 36Á7%); the control 90%. Juvenile white shrimp fed a commensal Lb. bulgaricus in two different concentrations (10 7 and 10 9 CFU per g) for 30 days (Roomiani et al. 2018). The LAB-fed shrimp significantly enhanced total haemocyte numbers, respiratory burst activity and prophenoloxidase activity. In addition, survival rates were increased significantly in a dose-dependent manner; control 33, 53 and 60Á00%, respectively, when challenged with V. parahaemolyticus PS-017 (10 7 CFU per ml). Three probiotics (Lb. pentosus BD6, Lb. fermentum LW2 and S. cerevisiae P13) were fed to juvenile white shrimp for 56 days in a single (10 6 CFU per g) or mixed formulation at three different concentrations (10 4 , 10 5 and 10 6 CFU per g; Wang et al. 2019). The shrimp fed with the probiotics in all cohorts increased phenoloxidase and respiratory burst activities. However, enhanced lysozyme activity was only observed in the groups fed LAB in the individual formulation, but not in the P13D group. When challenged with Vibrio alginolyticus (10 5 CFU per g shrimp), the shrimp showed an increase in survival rates: Lb. pentosus BD6, 59Á3%; Lb. fermentum LW2, 60%; S. cerevisiae P13, 47%; the control, 27%). However, the mixture-fed groups showed no improvement in survival rate. Japanese abalone (Haliotis discus hannai Ino) were fed Lb. pentosus, an isolate from abalone, for 8 weeks at various concentrations (10 3 , 10 5 and 10 7 CFU per g) (Xiaolong et al. 2018). Surprisingly, the natural death rates of the Lb. pentosus-fed groups increased in a dose-dependent manner, 2, 4 and 9%, respectively, although the rates were still lower than that of the control (11%). The LABfed groups significantly increased innate immune parameters: lysozyme, acid phosphatase, hepatopancreatic superoxide dismutase and catalase activities. When challenged with V. parahaemolyticus (5 9 10 8 CFU per abalone), mortality rates were decreased dose-dependently in 7 days postinfection: the control 100, 70, 55 and 50% respectively.
Four strains of LAB isolated from marine isolates (Lb. plantarum (LP), Weissella confuse (WC), Lac. lactis (LC) and Ent. faecalis (EF)) were fed individually (10 9 CFU per g) to juvenile sea cucumber for 30 days . All sea cucumber fed LAB (LP, WC, LL or EF) increased innate immune parameters: alkaline phosphatase, acid phosphatase, lysozyme, superoxide dismutase activities. When challenged with Vibrio splendidus immersion (10 8 CFU per ml), survival rates in the 10 days postinfection were significantly increased: the control, 48; LP, 67; WC, 63; LL, 65 and EF: 61%. The expression of immune-related genes varied depending on the types of LAB. Narrow clawed crayfish were fed Lb. plantarum KC426951, an isolate from rainbow trout, in various concentrations (10 7 , 10 8 and 10 9 CFU per g) for 97 days (Valipour et al. 2019), and probiotic administration significantly increased total haemocyte numbers in a dosedependent manner in response to an air-exposure challenge. Furthermore, the crayfish enhanced innate immune parameters following a post air-exposure challenge for 24 h: phenoloxidase, superoxide dismutase, catalase, lysozyme and total plasma proteins. However, phenoloxidase activity slightly decreased in all Lb. plantarum KC426951fed groups.
Testing the immune effects of the native commensal microbiomes on their own hosts appear to be a current trend in studies. Isolation of probiotics from the commensal microbiota may be a useful approach to enrich the pool of probiotics. Many studies demonstrated beneficial immunological effects when these autochthonous probiotics were administered to the hosts. However, when the native hosts were fed at high concentrations, some studies showed adverse effects. This may be due to the imbalance of the gut microbiota induced by the excessive feeding of autochthons bacteria. This possibility needs further investigation.

Bacillus as probiotics for finfish and shellfish
Genus Bacillus is one of the most frequently used probiotic genera in aquaculture, and in the recent review of Soltani et al. (2019b) information was presented on the potential of Bacillus as promising probiotics by producing bacteriocins, effect on growth performance, the immune system and disease resistance against pathogens in finfish and shellfish aquaculture. In order to avoid duplication, studies reviewed in the aforementioned review are not addressed in this paper.
An updated overview on the use of Bacillus as probiotics for finfish and shellfish are presented in Table 4. Under in vivo condition, B. subtilis, B. velezensis, Bacillus amyloliquefaciens, Bacillus circulans, Bacillus thuringiensis and Bacillus aerius increased resistance of finfish and shrimp to pathogenic bacteria including Streptococcus, Aeromonas, Vibrio, Enterococcus and Lactococcus (Meidong et al. 2018;Yi et al. 2018;Anyanwu & Ariole, 2019;Di et al. 2019;Jiang et al. 2019;Li et al. 2019;Lin et al. 2019;Mukherjee et al. 2019a;Peng et al. 2019;Soltani et al. 2019b;Vogeley et al. 2019). Bacillus species are also a natural resource for screening new quorum quenching bacteria and are commonly regarded as safe bacteria for the use in aquaculture as agents for improving water quality and disease control (Chen et al. 2020).  It has been demonstrated that use of Bacillus probiotics as the bioremediatory tools in the rearing water of aquaculture species and soil of aquaculture ponds have been exhibited as a feasible way of improving water quality through removing of toxic gases, for example ammonia, nitrite, nitrate and carbon dioxide that are harmful for aquatic organisms (Kewcharoen and Srisapoome 2019;Soltani et al. 2019b). Bacillus subtilis, B. licheniformis, B. cereus and B. coagulans are suggested as suitable bioremediatory tools for removing of organic detritus, but may not be naturally present in high enough concentrations in the aquatic ecosystems, that is water column and sediment (Soltani et al. 2019b). Bacillus subtilis and B. licheniformis are suggested as more suitable candidates for bioremediation of aquaculture rearing water (Soltani et al. 2019b). It has been shown that use of Bacillus to the rearing water can make a balance between the micro-organisms in the water column or in the pond soil through a bacterial competition with a consequence in the decreasing in load of secondary bacterial pathogens (Kumar et al. 2016). Bacillus also provide a suitable condition in the GI tracts of fish and shellfish, by improving digestion and absorption of the nutrients, which in turn improve the animal growth performance (Ghosh et al. 2019;Meidong et al. 2018;Li et al. 2019;Soltani et al. 2019b;Mukherjee et al. 2019b;Zhou et al. 2019;Vogeley et al. 2019;Tsai et al. 2019). However, further studies on mode of actions are needed. Probiotic bacilli can modulate the gut microbiota by bacterial competition, resulting in inhibition of pathogen adherence and colonization to intestinal mucosa (Meidong et al. 2018;Vogeley et al. 2019;Kuebutornyea et al. 2019;Soltani et al. 2019aSoltani et al. , 2019b. The modulation of finfish and shellfish innate immune responses, for example phagocytic and lysozyme activity, respiratory burst, antiprotease and peroxidase, superoxide dismutase and myeloperoxidase by Bacillus have been demonstrated (e.g. Yi et al. 2018;Zhou et al. 2019). For further information see Soltani et al. (2019b) Additionally, Bacillus probiotics can cause changes in animal cell physiology, for example neutrophil migration, plasma bactericidal activity and increasing of neutrophil adherence ability, that can eventually result in the improving of immune responses, for example increase in complement activity, immunoglobulin production and cell cytotoxicity (Di et al. 2019;Soltani et al. 2019b;Li et al. 2019). These immune-stimulatory effects by Bacillus occur in the gut-associated lymphoid tissue of finfish, although the detail mechanisms required further research works.

Other probiotics
Information on the use of other probiotics in finfish and shellfish aquaculture are less available. However, in a recent review, Ringø (2020) discussed the effects of Alteromonas, Arthrobacter, Bifidobacterium, Clostridium, Microbacterium, Paenibacillus, Phaeobacter, Pseudomonas, Pseudoalteromonas, Rhodosporidium, Roseobacter, Streptomyces and Vibrio on growth performance, immune response and disease resistance in shellfish. In order to avoid overlaps with above mention review, we recommend that readers with interest on this topic to have a closer look at the review of Ringø (2020), and the original papers discussed.

Commercial probiotics in shellfish aquaculture
Information on the use of commercial probiotics in shellfish aquaculture is available (Ringø (2020), and in order to avoid duplication readers with interest on the topic is recommend to have a closer look at the above mention review.

Conclusions
The importance of probiotic administration, their beneficial health effects has been discussed in several reviews. Falcinelli et al. (2018) discussed the effect of probiotic appetite control, glucose and lipid metabolisms. Even though there is numerous information available on the use of probiotics in aquaculture, there is no concrete evidence to conclude that probiotics are better than immunostimulants or vaccines, the beneficial effects upon the host and their environment ensure that probiotics will remain one of the most promising approaches used to control diseases and the subsequent environmental modifiers. In finfish and shellfish, manipulation of GI tract microbiota by probiotics have been revealed vs. control or inhibit adhesion and colonization of pathogenic bacteria in the GI tract, improve digestive enzyme activity and growth performance and enhance immune responses of the host against pathogenic infection or physical stress.
The functionality of gut microbiota, depends on the ability of micro-organisms to interact within the GI tract, which benefit the host through influence on inflammation, metabolism, immunity and even behaviour (e.g. Neuman et al. 2015;Boulange et al. 2016;Ram ırez and Romero 2017). When discussing disease resistance, a stable microbiota and its ability to adhere and colonize the intestine is of importance.
In the review of van Doan et al. (2020b) devoted to 'host-associated probiotics' in aquaculture, the authors presented the definition; 'bacteria originally isolated from the rearing water or the GI tract of the host to improve growth and health of the host', and revealed benefits of host-associated probiotics to include improved growth performance, feed value, enzymatic contribution to digestion, inhibit adherence and colonization of pathogenic micro-organisms in the GI tract, increase haematological parameters and immune response, and has gained attention within aquaculture. However, per se it is not clear, whether host-associated probiotics are more effective than probiotics from other origins, and this merits further research.
In addition to probiotics may also paraprobiotics (cell wall components; Taverniti and Guglielmetti 2011) serve as an alternative to the use of antibiotics in prevention and treatment of infections caused by pathogens. In this regard, it is of interest to notice that both probiotics and paraprobiotics can bind directly pathogenic bacteria, which limits adherence and colonization of the pathogen to gut cells.
The administration of autochthonous probiotics demonstrated to be beneficial on the immune response of both the finfish and shellfish. However, the high administration levels may be unfavourable to the host due to the ecological imbalance of the gut microbiota. In addition it is known that the microbiota and microbiota derived products influence the mucosal and systemic immune system in finfish and shellfish, however, the topic merits further investigations. The sporulation capacity of Bacillus gives them advantage due to their heat-tolerance and longer shelf-life in various environmental conditions compared to other probiotics, for example Lactobacillus spp. Production of digestive and antioxidant enzymes, and immune gene expression have revealed that probiotic Bacillus increase growth and resistance of fish and shellfish to pathogenic microbes.
Most probiotics studies per se have focus on different strains of LAB and Bacillus, however, the results of comparably limited studies on other probiotics revealed their potential to improve growth performance, physiological responses and disease resistance of different finfish and shellfish species. It seems that these probiotics merits future research. In addition, the importance of water quality management and available reports regarding probiotics bacteria with the ability to improve water quality highlight the importance of these probiotics in aquaculture. Interestingly, some probiotics bacteria which has not received much attention compared to LAB (e.g. Streptomyces sp.) are capable of producing chitinase which can resolve the issue of high levels of chitin in insect meal-based diet.
Compared to information available on the use of probiotics in endothermic animals, less information is available in aquatic animals, and several questions needs to be addressed. (i) Bidirectional signalling between the gut, its microbiome and the brain, and how can probiotics beneficially affect this interplay. (ii) How probiotics can improve behavioural-and GI disorder. (iii) Degradation of toxic organic compounds and production of bioactive compounds. Can probiotics degrade antinutritional factors like soybean b-conglycinin and soyasaponins? (iv) Use of bacteriocinogenic LAB strains. (v) Use of probiotics displaying antiviral effect, and evaluate the interactions between probiotics and viral infection. (vi) It is established that the gut microbiota plays a pivotal role in regulating host metabolism, but the effect of probiotic on metabolism of aquatic organisms' merits investigations. (vii) Adherence and colonization, is true colonization possible? VIII) Continuous vs pulse administration. (ix) Use of parabiotics vs probiotics. If we can clarify these questions, this will hopefully bring us a great step forward to clarify the role of probiotics in aquaculture.