New Insights into the Occurrence and Toxin Profile of Ciguatoxins in Selvagens Islands (Madeira, Portugal)

Ciguatoxins (CTXs), endemic from tropical and subtropical regions of the Pacific and Indian Ocean and the Caribbean Sea, have caused several human poisonings during the last decade in Europe. Ciguatera fish poisonings (CFP) in Madeira and Canary Islands appear to be particularly related with consumption of fish caught close to Selvagens Islands, a Portuguese natural reserve composed of three small islands that harbor high fish biomass. In this study, fish specimens considered as potential vectors of CTXs were caught in Madeira and Selvagens archipelagos for toxins determination via sensitive liquid chromatography with tandem mass spectrometry detection (LC–MS/MS). CTXs were found in most of the fish samples from Selvagens and none from Madeira. Caribbean ciguatoxin-1 (C-CTX1) was the only toxin congener determined, reaching the highest value of 0.25 µg C-CTX1 kg−1 in a 4.6 kg island grouper (Mycteroperca fusca). This study indicates that a diversity of fish from different trophic levels contains CTXs, Selvagens appear to be one of the most favorable locations for CTXs food web transfer and finally, this study highlights the need of further research based on intensive environmental and biological sampling on these remote islands.


Introduction
Ciguatera fish poisoning (CFP) is a human food-borne illness caused by consumption of tropical and subtropical fishes harboring ciguatoxins (CTXs). Coral reef fish are frequently reported to accumulate CTXs, which are products of fish metabolism resulting from biotransformation of precursor compounds produced by the benthic dinoflagellates, Gambierdiscus and Fukuyoa, and transferred through the marine food web [1][2][3].
CTXs have been classified according to their geographic distribution through tropical regions, as Pacific (P), Caribbean (C) and Indian Ocean (I) CTXs-group toxins. CTXs are lipophilic polyether C-CTX1 was detected in fish of different size ranges and different trophic levels (Table 1). Due to the limited amount of C-CTX1 pure standard, C-CTX1 content was expressed in CTX1B equivalents and samples where C-CTX1 was detected were quantified as CTX1B and converted to C-CTX1 equivalents taking into account the conversion factor previously obtained for the pure standard of C-CTX1. Calibration was therefore carried out by using the pure standard of CTX1B in the concentration range of 0.28-27.88 ng mL −1 (n = 5, R 2 = 0.999). Limits of detection (S/N >3) and quantitation (S/N >10) were of 0.0045 and 0.0150 ng g -1 in matrix matched samples spiked with CTX1B standard solution. The highest concentration level, reaching 0.25 µ g C-CTX1 kg −1 , was determined in a 4.5 kg Island Grouper (sample 4), followed by a 1.6 kg Barred Hogfish (sample 6), which contained 0.11 µ g C-CTX1 kg −1 . The toxin concentration determined in the >19 kg top predator and known vector of CTX, the Dusky Grouper (sample 1), only accounted for a toxin concentration of 0.05 µ g C-CTX1 kg −1 . C-CTX1 was detected in fish of different size ranges and different trophic levels (Table 1). Due to the limited amount of C-CTX1 pure standard, C-CTX1 content was expressed in CTX1B equivalents and samples where C-CTX1 was detected were quantified as CTX1B and converted to C-CTX1 equivalents taking into account the conversion factor previously obtained for the pure standard of C-CTX1. Calibration was therefore carried out by using the pure standard of CTX1B in the concentration range of 0.28-27.88 ng mL −1 (n = 5, R 2 = 0.999). Limits of detection (S/N >3) and quantitation (S/N >10) were of 0.0045 and 0.0150 ng g −1 in matrix matched samples spiked with CTX1B standard solution. The highest concentration level, reaching 0.25 µg C-CTX1 kg −1 , was determined in a 4.5 kg Island Grouper (sample 4), followed by a 1.6 kg Barred Hogfish (sample 6), which contained 0.11 µg C-CTX1 kg −1 . The toxin concentration determined in the >19 kg top predator and known vector of CTX, the Dusky Grouper (sample 1), only accounted for a toxin concentration of 0.05 µg C-CTX1 kg −1 . Kyphosus sectatrix Bermuda sea chub <LOD S10 K. sectatrix Bermuda sea chub <LOD S11 Sphyraena viridensis Yellowmouth barracuda <LOD

Discussion
Several outbreaks reported in the Canary Islands since 2004 derived from consumption of fish caught close to the Selvagens Islands [20]. The cause of CFP in Portuguese waters has also been related to contaminated fish from Selvagens [18,19]. In the present study, CTXs was not detected in samples caught along the Madeira Island coast but were detected in most fish samples obtained from the further south Selvagens Islands. With the data available at the present day, reinforced with results of this study, one may argue that Selvagens islands are a hotspot of CTXs in Europe.
In this study, Caribbean CTXs was detected not only in top predators, as is the big size >19 kg dusky grouper, Epinephelus marginatus, but also in smaller sized fish from intermediate levels of the trophic chain. The highest concentration was registered in a 4.5 kg Island grouper (Mycteroperca fusca). This species is a near rocky or sandy-rocky sea-beds inhabitant that preys on fish, crustaceans, and cephalopods. Its abundance has been positively correlated with upright seaweed cover, and individuals may reach a longevity of 30 to 40 years and up to 7.8 kg [23]. It is common to associate higher CFP risk with larger fish and indeed, fishing amberjacks in Madeira is restricted to specimens not exceeding 10 kg. However, in addition to the fish size, longevity should also be considered. While island grouper may not reach a similar high size/weight as amberjacks, this species may live longer to accumulate high CTXs levels. In fact, after the first report of CFP in Portugal, due to consumption of 30 kg amberjack by crew members of a fishing boat, CFP was retrospectively identified in nature wardens of Selvagens Islands Nature Reserve that consumed parrotfish (Sparissoma cretense), blacktail comber (Serranus atricauda), barred hogfish (Bodianus scrofa), grey triggerfish (Balistes capriscus) and red porgy (Pagrus pagrus) and suffered from neurological disorders between 2 and 6 weeks [18]. However, LC-MS/MS analyses for the detection of CTXs had never been carried out in these fish species from Selvagens until the present study.
While several CTXs congeners have been indicated by [19] in fish caught from Selvagens after the 2008 incident, in the present study only C-CTX1 was detected, which is in agreement with the toxin profile described by [20] during outbreaks in Canary islands and it is typically observed in the Caribbean Sea [24]. Further studies need to be carried out to confirm the presence of other CTXs congeners, having in mind that LC-MS/MS analysis of CTXs is complex and interfering compounds leading to misidentification may occur. This fact was observed in some samples of pink dentex from Madeira (samples M3 to M5), which contained an interfering compound that was initially misidentified as a potential CTXs analogue.
According to EU directives, there are no limits for CTXs in fish, but the presence of CTXs as determined by any detection method is enough to ban the fishery products from the markets [15,16]. However, the toxic potential of C-CTX1 is assumed to be lower than most of other CTXs congeners, in particular to those from Pacific Ocean. EFSA stated that until better data is available, the following toxicity equivalency factors (TEFs) should be used to express fish toxicity as CTX1B equivalents: CTX1B = 1 and C-CTX1 = 0.1 [21]. EFSA estimates that fish containing 0.1-5 µg CTX1B kg −1 has been related with cases of CFP and that a concentration of 0.01 µg CTX1B kg −1 of fish is a level expected to not exert effects in sensitive consumers. In the present study, the highest concentrations reached in sample S4 and S6 are, according to EFSA, as well as according to the guidance levels established by the Food and Drug Administration (FDA) of the USA, low and around the safety levels respectively.

Conclusions
The evaluation of the risk of CFP in Europe is a very challenging task for scientists, seafood safety authorities and environmental managements. There are still unresolved issues related to the toxicology of the different CTXs analogues and metabolites, as well as with analytical methods, in particular due to the lack of reference materials, seasonal and spatial variability of toxin vectors and of the toxin precursor producers.
This study highlights Selvagens Islands as a key location to carry out studies on CFP incidence as an example of the emergence of this toxin in the EU coasts. Indeed, Selvagens may be used as a sentinel site for comparative CTXs occurrence in the Canary Islands and even Madeira. Selvagens Islands are a healthy ecosystem where fishing pressure is minimal, thereby fish may grow and reach high longevity and consequently harbor greater levels of CTXs. However, further studies involving intensive sampling in these remote Islands should be carried out to understand the Gambierdiscus dynamics, the toxin transfer in the food web and fish toxin metabolism.

Madeira and Selvagens Islands
Selvagens Islands are a very remote and isolated group of Portuguese oceanic islands, located in the Northeastern Atlantic, 293 km southeast from Madeira Island, Portugal, 180 km north from Tenerife Island, Spain and 600 km west from the African coast ( Figure 2). They are comprised of three islands of volcanic origin: Selvagem Grande, Selvagem Pequena and Ilhéu de Fora, and several islets, shaped specially by marine abrasion. Classified as a nature reserve in 1971, with a total marine area of 92 km 2 and included within the Natura 2000 Network, with a total marine area of 1242 km 2 , the biodiversity of the Selvagens Islands has greatly benefitted from this protection and the successive conservation action, being an example of a high coastal species diversity occurring even in very small areas of the northeastern Atlantic Ocean.
Toxins 2018, 10, x FOR PEER REVIEW 5 of 10 toxicity equivalency factors (TEFs) should be used to express fish toxicity as CTX1B equivalents: CTX1B = 1 and C-CTX1 = 0.1 [21]. EFSA estimates that fish containing 0.1-5 µ g CTX1B kg −1 has been related with cases of CFP and that a concentration of 0.01 µ g CTX1B kg −1 of fish is a level expected to not exert effects in sensitive consumers. In the present study, the highest concentrations reached in sample S4 and S6 are, according to EFSA, as well as according to the guidance levels established by the Food and Drug Administration (FDA) of the USA, low and around the safety levels respectively.

Conclusions
The evaluation of the risk of CFP in Europe is a very challenging task for scientists, seafood safety authorities and environmental managements. There are still unresolved issues related to the toxicology of the different CTXs analogues and metabolites, as well as with analytical methods, in particular due to the lack of reference materials, seasonal and spatial variability of toxin vectors and of the toxin precursor producers.
This study highlights Selvagens Islands as a key location to carry out studies on CFP incidence as an example of the emergence of this toxin in the EU coasts. Indeed, Selvagens may be used as a sentinel site for comparative CTXs occurrence in the Canary Islands and even Madeira. Selvagens Islands are a healthy ecosystem where fishing pressure is minimal, thereby fish may grow and reach high longevity and consequently harbor greater levels of CTXs. However, further studies involving intensive sampling in these remote Islands should be carried out to understand the Gambierdiscus dynamics, the toxin transfer in the food web and fish toxin metabolism.

Madeira and Selvagens Islands
Selvagens Islands are a very remote and isolated group of Portuguese oceanic islands, located in the Northeastern Atlantic, 293 km southeast from Madeira Island, Portugal, 180 km north from Tenerife Island, Spain and 600 km west from the African coast ( Figure 2). They are comprised of three islands of volcanic origin: Selvagem Grande, Selvagem Pequena and Ilhéu de Fora, and several islets, shaped specially by marine abrasion. Classified as a nature reserve in 1971, with a total marine area of 92 km 2 and included within the Natura 2000 Network, with a total marine area of 1242 km 2 , the biodiversity of the Selvagens Islands has greatly benefitted from this protection and the successive conservation action, being an example of a high coastal species diversity occurring even in very small areas of the northeastern Atlantic Ocean. Selvagens have been designated by National Geographic Society as one of the pristine sites in the oceans today [25]. Human pressure is very low in Selvagens, where limited number of visitors is Selvagens have been designated by National Geographic Society as one of the pristine sites in the oceans today [25]. Human pressure is very low in Selvagens, where limited number of visitors is allowed each year. In contrast, Madeira Island is heavily populated and a high popular tourism destination. As a result of the remote location of Selvagens Islands and the high anthropogenic pressures at Madeira, total fish biomass was estimated to be 3.2 times higher at Selvagens than at Madeira, and when considering only the fish top predators biomass it reach values 10 times higher at Selvagens [26]. Previous studies describing the marine fish diversity found that 34.1% of the ichthyofauna observed in Selvagens also occurs in the Canary Islands and 47.3% in Madeira Island [27].

Fish Samples
Specimens of fish species previously identified as vectors or potentially vectors of CTXs and representative of different trophic levels from both Madeira and Selvagens Islands were opportunistically obtained for this study ( Table 2). A portion of fish flesh (>100 g) were dissected from muscle close to fish head and stored at −20 • C until analysis.

Ciguatoxins Extraction
CTXs sample pretreatment was carried out following the conditions proposed by [29] with modifications [30,31], briefly: Fish flesh samples (15 g) were extracted twice by homogenizing 2 min at 9000 rpm in 45 mL of acetone (Ultra Turrax ® T25 basic IKA ® WERKE, Staufen, Germany). The combined extracts were concentrated to an aqueous residue and extracted twice with 15 mL of diethyl ether and further evaporated to dryness. The organic residue was dissolved in 4.5 mL 90% MeOH and defatted with 9 mL of hexane evaporating the aqueous layer using a multievaporator under reduced pressure (Syncore ® Polyvap, Barcelona, Spain). The remaining solid residue was dissolved in 2 mL of ethyl acetate and further purified through a solid phase extraction (SPE) cleanup including two different SPE mechanisms. The first normal phase Florisil SPE was used to remove polar interferences whereas non-polar and semipolar interferences were removed by using C18 SPE reverse phase. Cleanup conditions are described as follows: The 2 mL of ethyl acetate extract from extraction were passed through a Florisil cartridge (J. T. Baker, 500 mg, Center Valley, PA, USA) conditioned with 3 mL of ethyl acetate, and washed consecutively, with 3 mL of ethyl acetate and 5 mL of ethyl acetate-methanol (9:1) and ethyl acetate-methanol (3:1) [32]. Previous work carried out by this group on the optimization on the sample pretreatment for the LC-MS/MS analysis of CTXs allowed to conclude that the toxin mainly elutes in the second fraction with a recovery around 80% [30]. The residue containing the toxin was further dried under nitrogen at 50 • C and then dissolved in 2 mL of MeOH 60% and applied to a C18 cartridge (SUPELCLEAN, Supelco, 500 mg, Bellefonte, PA, USA) conditioned with 3 mL of MeOH 60%. The cartridge was washed with 3 mL of MeOH 60% and CTXs were eluted with 5 mL of MeOH 90%. The final eluate was dried and dissolved in 0.5 mL of MeOH LC-MS grade filtering (Syringe Driver filter Unit, Millex ® -CV 0.22 um, 13 mm, Millipore, Billerica, MA, USA) prior to LC-MS analysis.

LC-MS/MS Analysis
LC-MS/MS analysis was carried out following [29] conditions with modifications introduced in the LC-MS instrument in order to improve sensitivity [30,31]. LC-MS/MS analyses were performed by using an Agilent 1290 Infinity LC system coupled to an Agilent 6495 Triple Quadrupole LC-MS (Agilent Technologies, Waldbronn, Germany) equipped with an Agilent Jet Stream electrospray ionization source (iFunnel).
Analytes were separated in a Poroshell 120 EC-C18 (3.0 × 50 mm, 2.7 µm, Agilent Technologies, Waldbronn, Germany) with column temperature set at 40 • C. LC mobile phase was: 5 mM ammonium formate and 0.1% formic acid in water (A) and MeOH (B). Gradient used was 78% B to 88% B in 10 min and held for 5 min, increased to 100% B at 15.01 min and held 3 min returning to 78% B at 18 min, and 4 min of equilibration before the next injection. The injection volume was 1 µL and the flow rate 0.4 mL/min. The mass spectrometer was operated in positive mode monitoring [M+Na] + as precursor and product ions with collision energy of 40 eV. This approach allows a sensitive detection of the CTXs compared to other strategies that monitor CTXs water losses [33,34]. The selection of a single stable [M+Na] + as precursor ion, with high collision energy and methanol as mobile phase, allows the removal of the background noise monitoring the same [M+Na] + as product ion without any additional fragmentation due to the high stability of this adduct under the above mentioned conditions. LC-MS/MS system settings were: Drying gas, 15 L min −1 of N 2 at 290 • C; sheath gas flow, 12 L min −1 of N 2 at 400 • C; nebulizer gas, N 2 at 50 psi; capillary voltage, 5000 V; nozzle voltage: 300 V; fragmentor potential 380 V. CTXs were monitored by MRM as follows: CTX1B (m/z 1133.6 -> m/z 1133. 6