Tissue Distribution and Metabolization of Ciguatoxins in an Herbivorous Fish following Experimental Dietary Exposure to Gambierdiscus polynesiensis

Abstract

Ciguatoxins (CTXs), potent neurotoxins produced by dinoflagellates of the genera Gambierdiscus and Fukuyoa, accumulate in commonly consumed fish species, causing human ciguatera poisoning. Field collections of Pacific reef fish reveal that consumed CTXs undergo oxidative biotransformations, resulting in numerous, often toxified analogs. Following our study showing rapid CTX accumulation in flesh of an herbivorous fish, we used the same laboratory model to examine the tissue distribution and metabolization of Pacific CTXs following long-term dietary exposure. Naso brevirostris consumed cells of Gambierdiscus polynesiensis in a gel food matrix over 16 weeks at a constant dose rate of 0.36 ng CTX3C equiv g^-1 fish d^-1. CTX toxicity determination of fish tissues showed CTX activity in all tissues of exposed fish (eight tissues plus the carcass), with the highest concentrations in the spleen. Muscle tissue retained the largest proportion of CTXs, with 44% of the total tissue burden. Moreover, relative to our previous study, we found that larger fish with slower growth rates assimilated a higher proportion of ingested toxin in their flesh (13% vs. 2%). Analysis of muscle extracts revealed the presence of CTX3C and CTX3B as well as a biotransformed product showing the m/z transitions of 2,3-dihydroxyCTX3C. This is the first experimental evidence of oxidative transformation of an algal CTX in a model consumer and known vector of CTX into the fish food web. These findings that the flesh intended for human consumption carries the majority of the toxin load, and that growth rates can influence the relationship between exposure and accumulation, have significant implications in risk assessment and the development of regulatory measures aimed at ensuring seafood safety.

Keywords: Gambierdiscus polynesiensis; bioaccumulation; biotransformation; ciguatera poisoning (CP); ciguatoxins; liquid chromatography–tandem mass spectrometry; metabolism; reef fish; tissue distribution; trophic transfer.

Figure 1

CTX-like activity (ng CTX3C equiv g⁻¹ TE) in tissues of N. brevirostris after 16 weeks’ dietary exposure to G. polynesiensis. All values are given as mean ± SD (n = 3–7). Concentrations in all tissues of control fish were below limits of detection (LOD: 0.32 μg CTX3C equiv kg⁻¹; n = 3). Within each assay, significant differences among tissues (p < 0.05, Table 2) are depicted with letters for r-RBA and by asterisks for CBA-N2a. GI = gastro-intestinal tract.

Figure 2

Relative toxin accumulation, i.e., percent of total body burden, in tissues of N. brevirostris after 16 weeks’ dietary exposure to G. polynesiensis. Values represent means of both screening assays (CBA-N2a and r-RBA). Tissues are labeled as follows: Sp = spleen, E = eyes, G = gills, GI = gastro-intestinal tract, L = liver, M = muscle, C = carcass. The brain and gallbladder are not labeled because the percentage is too small to be seen visually.

Figure 3

Percent of total dose of toxin retained in each tissue type as determined by r-RBA (n = 3–7) and CBA-N2a (n = 3).

Figure 4

Total Ion Chromatograms (TIC) acquired in positive multiple reaction monitoring (MRM) mode representing (a) a mixture of Pacific ciguatoxin standards and (b) the toxin profile of a muscle extract of N. brevirostris after 16 weeks’ feeding with G. polynesiensis. Extract Ions Chromatograms (EIC) of (c) the m/z transitions of 2,3-dihydroxyCTX3C standard and (d) the corresponding peak of interest at 5.50 min from the muscle extract.