We define co-introduced parasites as those which have been transported with an alien host to a new locality, outside of their natural range, and co-invading parasites as those which have been co-introduced and then spread to new, native hosts. Of 98 published studies of co-introductions, over 50% of hosts were freshwater fishes and 49% of parasites were helminths. Although we would expect parasites with simple, direct life cycles to be much more likely to be introduced and establish in a new locality, a substantial proportion (36%) of co-introductions were of parasites with an indirect life cycle. Seventy-eight per cent of co-introduced parasites were found in native host species and can therefore be classed as co-invaders. Host switching was equally common among parasites with direct and indirect life cycles. The magnitude of the threat posed to native species by co-invaders will depend, among other things, on parasite virulence. In 16 cases where co-introduced parasites have switched to native hosts and information was available on relative virulence, 14 (85%) were more virulent in native hosts than in the co-introduced alien host. We argue that this does not necessarily support the naïve host theory that co-invading parasites will have greater pathogenic effects in native hosts with which they have no coevolutionary history, but may instead be a consequence of the greater likelihood for parasites with lower virulence in their natural host to be co-introduced.

Species of Bacillus are spore-forming bacteria that are resistant to aggressive physical and chemical conditions, with various species showing unusual physiological features enabling them to survive in various environmental conditions including fresh waters, marine sediments, desert sands, hot springs, Arctic soils, and the gastrointestinal tract of finfish and shellfish. They are able to rapidly replicate and tolerate a multitude of environmental conditions, giving a wide range of beneficial effects in the aquaculture sector. Application of Bacillus spp. as probiotics in feed or for bioremediation of aquaculture rearing water has great potential for sustainable aquaculture. Species of Bacillus may play a desirable role in removing waste products from aquaculture environments, maintaining optimum water quality, and reducing stress, which can lead to an improved immuno-physiological balance, better growth and enhanced survival in target aquatic animals. Application of probiotic Bacillus spp. can enhance growth and immune function of aquatic organisms. Probiotic Bacillus can also assist in maintaining a higher density of beneficial bacteria and a lower load of pathogenic agents in aquaculture ponds. Much is still unknown, however, about how the probiotic efficacy of specific Bacillus species is affected by different aquatic animal species, age and growth condition, water quality, and diet. Also, the details of mode of actions of Bacillus spp. on the immune-physiological functions of aquatic organisms as well as their functions as bioremediators of water quality need further studies. This review addresses the presence of Bacillus spp. in the gastrointestinal tract of finfish and shellfish, their ability to produce enzymes and antibacterial compounds, and their efficacy and potency as probiotics in aquaculture.