## 2 Ciguatoxins Structures and Reference Standards
### Availability of Ciguatoxin Reference Standards  
The availability of different giguotoxins is dependent on the amount that can be extracted from fish and/or produced by dinoflagellate cultures. However, the isolated amounts are extremely low, concentrations are in the ppb range, which is a major drawback \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**CTX** & **Other Names** & \
\begin{tabular}{c} **Molecular** \\ **Formula** \\ \end{tabular} &
\begin{tabular}{c} **Known Source** \\ \end{tabular} & **Structural Characteristics** \\ \hline \multicolumn{5}{c}{Type 1 (60 carbons, A to M rings, DEF rings size 7/7/9)} \\
**P-CTX-1** & CTX-1B, CTX, CTX1 & C\({}_{60}\)Hg\({}_{80}\)O\({}_{19}\)[98] & 1110.6 & Fish \\
**P-CTX-3** & 54-deoxy-CTX-1B & C\({}_{60}\)Hg\({}_{80}\)O\({}_{18}\)[122] & 1094.6 & Fish and \\ P-CTX-2 & 52-epi-54-deoxy-CTX-1B & C\({}_{60}\)Hg\({}_{80}\)O\({}_{18}\)[125] & 1094.6 & _Gambierdiscus_ spp. & at C54, compared to P-CTX-1 \\
**GT-4B\({}^{\text{a}}\)** & CTX-4B, CTX-4B, & C\({}_{60}\)Hg\({}_{80}\)O\({}_{16}\)[98] & 1060.6 & _Gambierdiscus_/_Fukugau_ & Two epimers at C52 differing from \\
**CTX-4A\({}^{\text{a}}\)** & P-CTX-4A & C\({}_{60}\)Hg\({}_{80}\)O\({}_{16}\)[126] & 1060.6 & spp. and herbivovous & P-CTX-1 by the absence of 2 -OH on \\ \hline \multicolumn{5}{c}{Type 2 (57 carbons, A to M rings, DEF rings size 7/8/9)} \\
**CTX-3C\({}^{\text{a}}\)** & P-CTX-3C & C\({}_{57}\)Hg\({}_{80}\)O\({}_{16}\)[123] & 1022.6 & _Gambierdiscus_/_Fukugau_ & Two epimers at C49 \\ CTX-3B & P-CTX-3B, 49-epi-P-CTX-3C & C\({}_{60}\)Hg\({}_{80}\)O\({}_{16}\)[119] & 1022.6 & spp. \\ \hline \multicolumn{5}{c}{Type 3 (62 carbons, A to M rings, DEF rings size 7/8/9)} \\
**C-CTX-1** & \multicolumn{5}{c}{C\({}_{62}\)Hg\({}_{20}\)O\({}_{19}\)[35]} & 1140.6 & Fish \\ C-CTX-2 & \multicolumn{5}{c}{C\({}_{62}\)Hg\({}_{20}\)O\({}_{19}\)[35]} & 1140.6 & Fish \\ C-CTX-3 & C-CTX-1143 & C\({}_{62}\)Hg\({}_{40}\)O\({}_{19}\)[120] & 1142.6 & Fish \\ C-CTX-4 & C-CTX-1143 & C\({}_{62}\)Hg\({}_{40}\)O\({}_{19}\)[120] & 1142.6 & Fish \\ C-CTX-5\({}^{\text{b}}\) & \multicolumn{5}{c}{C\({}_{62}\)Hg\({}_{90}\)O\({}_{19}\)[43]} & 1138.6 & _G. sirlvae_ & C-CTX-1 with the OH at C3 reduced \\ \hline \hline \end{tabular}  
* The more energetically favourable epimers (lower energy) are shown in bold. \({}^{\text{a}}\) isolated from a _Gambierdiscus_ spp. strains from French Polynesia [98; 99; 100; 123; 126]. \({}^{\text{b}}\) isolated from _Gambierdiscus_ and _Gambierdiscus_ arxinatus_ strains from the Caribbean [43].  
\end{table} 
Table 1: Names, molecular formulae, molecular mass (in Da), known sources (_Gambierdiscus_ spp. or fish), and structural characteristics of the three major types of ciguatoxins from the Pacific and Atlantic Oceans. Figure 1: Structures of ciguatoxins from the Pacific Ocean and Caribbean Sea. These structures belong, from top to bottom, to three types of congeners: Type 1 (60 carbons, A to M rings, DEF rings size 7/7/9), Type 2 (57 carbons, A to M rings, DEF rings size 7/8/9), and Type 3 (62 carbons, A to N rings, DEF rings size 7/8/9). The structures shown are more energetically favourable (lower energy) epimers at carbon C52 for P-CTX-1 [98], P-CTX-3 [122], GT-4B [98], P-CTX-3C [123], at carbon C56 for C-CTX-1, C-CTX-2 [35], and C-CTX-5 [43]. The less energetically favoured epimers are shown in parentheses, while the unstable open (reduced) forms of the hemiketals C-CTX 3/4 [120] are not shown for clarity. Brevetoxin (PbTx-2) is shown for comparison [124]. A progressive numbering from 1 to 60 (Type 1), 1 to 57 (Type 2) or 1 to 62 (Type 3) was attributed for each contiguous carbon of the continuous chain of each type of CTXs and was completed at the end by numbering of methyl (Me); however, for clarity only some carbons were marked in rings of this figure: as an example in Type 1 it is pointed 5 for C5 of ring A; 25 for C25 of ring F; 52 for C52 of ring L.  
for the development and validation of biological and analytical detection methods for CTXs.