## 2 Distribution in Nature
### Terrestrial Animals  
Evidence supporting endogenous production of TTX has been obtained predominately from terrestrial organisms although the origin of this toxin in these species is very controversial [40]. TTX in terrestrial animals is limited to Amphibia as newts, toads and frogs [34]. Some authors argue that origin of TTX in terrestrial animals is endogenous because this toxin has a role in defence [34]. In particular, the levels of TTX and 6-epiTTX in news are higher in the skin rather than in the liver, supposedly TTX is biosynthetically produced by the creature as a protection mechanism for predators. It was suggested that the degree of coloration of frogs belonging to the _Brachycephalus_ family can be related to its toxicity. The authors found the highest TTX levels in the skin followed by liver \begin{table}
\begin{tabular}{c c c c} \hline
**Organisms** & **Species** & **Bacteria** & **Reference** \\ \hline Red algae & _Jania_ sp. & _Vibrio_, _Alteromonas_ and _Shewanella_ & [47] \\ \hline Crustacean: Copepods & _Pseudocaligus fugu_ and & & \\ (Parasites of Pufferfish) & _Taniacanthus_ sp. & & \\ \hline Crustacean: xanthid crab & _Atergatis floridus_ & _Vibrio_ strain & [14] \\ \hline Mollusc gastropod & _Niotha Clathrata_ & _Vibrio_ and _Pseudomonas_ & [49] \\ \hline Mollusc cephalopod & _Octopus maculosus_ & _Bacillus_, _Pseudomonas_ and & \\  & & _Vibrio_ spp. & \\ \hline Echinorms: starfish & _Astropecten polyacanthus_ & _Vibrio alginolyticus_ & [50] \\ \hline Vertebrates: pufferfish & _Takifugu snyderi_ & _Vibrio_ strain & [51] \\ \hline Pufferfish & _Takifugu obscurus_ & _Aeromonas_ & [52] \\ \hline Pufferfish & _Fugu poecilonotus_ & _Vibrio_, _Alteromonas_ and _Shewanella_ & [47] \\ \hline Pufferfish & _Takifugu niphobles_ & _Raoultella terrigena_ & [41] \\ \hline Pufferfish & _Fugu obscurus_ & _Bacillus_ spp. & [53] \\ \hline Pufferfish & _Fugu rubripes_ & _Bacillus_ and _Actinomycetes_ & [54] \\ \hline Pufferfish & _Chelonodon patoca_ &
\begin{tabular}{c} _Microbacterium arabingalactanoylticum_, \\ _Serratia marcescens_, _Vibrio alginolyticus_ \\ \end{tabular} & [55] \\ \hline Pufferfish & _Fugu vermicularis radiatus_ & _Vibrio_ strain _LM-1_ & [56] \\ \hline \end{tabular}
\end{table} 
Table 1: TTX-producing bacteria isolated from several marine organisms. and ovaries of bright yellow frogs. However, cryptic coloration frog was found to be non-toxic. Interestingly, the \"poison dart\" frogs are brightly colored and are well known for high toxicity, though this derives not only from TTX. Then, the bright coloration protects these frogs from predators who instinctively avoid them. This fact strongly suggests that toxic frogs naturally synthesise the toxin, although, studies have not been performed to definitively corroborate this theory [34]. In frogs of the genus _Ateolopus_, captive-raised individuals do not possess TTX, suggesting a dietary or other environmentally dependent origin of toxicity [33]. In captivity, TTX levels in the terrestrial newt, _Taricha granulosa_, lethal to almost all potential predators, increase with a TTX-free diet. Also, when it is induced to release TTX from their skin by electrical stimulation, regeneration of TTX apparently by secretion occurs within nine months, whereas captive-raised _Metopus_ toads lose TTX-toxicity [36]. Additionally, authors did not find evidence of bacterial symbionts (mtDNA signatures) in TTX-laden organs of _T. granulosa_. Although individual specimens of the newt _T. granulosa_ can contain up to the equivalent of 6 mg of TTX, efforts to isolate TTX-producing bacteria from them have so far been unsuccessful [33,40,57]. Using PCR primers that specifically amplify 16S rRNA genes of bacteria, tissues from newts, _T. granulosa_, were examined for the presence of bacteria which may produce TTX. No amplification of bacterial DNA was seen in samples taken from skin, liver, gonads or eggs. Amplification of bacterial DNA was only seen in samples taken from newt intestines, a tissue with low concentrations of TTX.