## 2 Distribution in Nature
### Pufferfish  
The amount of TTX accumulated in pufferfish depends on the species and varies among organs in different seasons but it is concentrated mainly in ovary, liver, and other body parts, as the intestine. It was reported that juvenile cultivated pufferfish in aquaria or in cages suspended above the sea floor become non-toxic over time in captivity. They became toxic when they were grown in open water again or were fed with toxic puffer livers, whereas certain other species of pufferfish did not become toxic. For instance, pufferfish, _Takifugu rubripes_ becomes non-toxic when it is fed on a TTX-free diet. Also, when pufferfish were fed with a TTX-containing diet their toxicity increased significantly [41]. These experimental findings suggested that TTX can be acquired and accumulated from the food chain and that certain species of pufferfish may possess a functional ability to store or eliminate this toxin. Relevant evidence for this possibility is the observation of saturation uptake kinetics for TTX revealed by tissue of pufferfish liver which is indicative of a carrier-mediated membrane transport system for the toxin [44]. In fact, a binding protein for TTX as well as for saxitoxin (STX), another neurotoxin that blocks sodium channels, present in the plasma of pufferfish, may represent a possible mechanism for tissue delivery [45]. It has also been demonstrated that the source of TTX in pufferfish is an endosymbiotic bacteria that naturally inhabits the gut of the animal. It could be that pufferfish initially acquire the TTX producing bacteria via the food web and that these bacteria then persist in the fish.  
Thus, available evidence is consistent with the interpretation that pufferfish and certain other marine species accumulate TTX directly from marine bacteria that synthesize this toxin. Alternatively, TTX storage by a given species may occur by numerous pathways of dietary transfer through a complex community food chain involving bacteria, plankton, invertebrates and fish [16]. In addition, since TTX is widely distributed among a wide range of species, it was suggested that this toxin has an exogenous microbial origin, rather than being produced by pufferfish _per se_. At least 12 different species of TTX-producing bacteria have been isolated from various pufferfish tissues including the skin,intestine, ovaries, and liver [41,42]. A newly TTX-producing bacterium, _Raoultella terrigena_, was isolated from the intestines of a local toxic pufferfish _Takifugu niphobe_. The TTX production ability of this strain was investigated and the 16S-23S rDNA ITS region was sequenced. The toxicity of the strain was confirmed by mouse bioassay, ELISA and mass spectrometry (MALDI-TOF). These outcomes reiterate that the TTX found in pufferfish was likely produced by the associated bacteria among a diversity of bacterial species [41]. An interesting discussion regarding how bacteria could produce TTX under certain physiological parameters was recently reported by [PERSON] and [PERSON] [46]. The concept of multiple horizontal gene transfer and synergistic production of secondary metabolites could be possible in the case of TTX-producing bacteria [46].  
Table 1 represents most of the marine organisms, including invertebrates and vertebrates, as different species of pufferfish, in which bacteria were identified as TTX producers. Other TTX-producing bacteria and the host organisms are reviewed in [PERSON] and [PERSON] [46].