Spatial variability and depuration of tetrodotoxin in the bivalve Paphies australis from New Zealand

Abstract

Tetrodotoxin (TTX) is a potent neurotoxin responsible for many human intoxications globally. Despite its potency and widespread occurrence in taxonomically diverse species, the primary source of TTX remains uncertain. Paphies australis, an endemic clam found in New Zealand, has been found to contain TTX in several locations. However, it is unknown if this represents endogenous production or accumulation from an external source. To address this question, the concentrations of TTX in whole P. australis and dissected organs (siphons, foot, digestive gland and the 'rest') from thirteen sites around New Zealand were determined using liquid chromatography-tandem quadrupole mass spectrometry analysis (LC-MS/MS). Depuration rate of TTX was also investigated by harvesting and measuring concentrations in P. australis maintained in captivity on a toxin-free diet every three to 15 days for 150 days. The LC-MS/MS analyses of the spatial samples showed that TTX was present in P. australis from all regions tested, with significantly (p < 0.001) higher concentrations (15-50 μg kg^-1) observed at lower latitudes of the North Island compared with trace levels (0.5-3 μg kg^-1) in the South Island of New Zealand. Tetrodotoxin was detected in all the dissected organs but the siphons contained the highest concentrations of TTX at all sites analysed. A linear model of the depuration data identified a significant (p < 0.001) decline in total TTX concentrations in P. australis over the study period. The siphons maintained the highest amount of TTX across the entire depuration study. The digestive glands contained low concentrations at the start of the experiment, but this depurated rapidly and only traces remained after 21 days. These results provide evidence to suggest that P. australis does not produce TTX endogenously but obtains the neurotoxin from an exogenous source (e.g., diet) with the source more prevalent in warmer northern waters. The association of higher TTX concentrations in shellfish with warmer environments raises concerns that this toxin's distribution and abundance could become an increasing human health issue with global warming.

Keywords: Biotoxin; Clam; Emerging threat; Geographic variability; Marine bivalves; Neurotoxin.

Fig. 1

Paphies australis collection locations, dates and seasons from around New Zealand.

Fig. 2

Tetrodotoxin concentrations in populations of Paphies australis collected around the New Zealand coastline, determined using liquid chromatography-mass spectrometry (n = 5). Solid black line shows median, box shows 1st and 3rd quartiles, whiskers extend to the last data point within 1.5 times the inter-quartile range. Dots outside the whiskers are considered as outliers. Different letters indicate where significant differences occur between sites (one-way ANOVA with Tukey's HSD post-hoc test, p < 0.001). Sites are ordered by increasing latitude for each Island (i.e., North and South Island). Akaroa was not included in the statistical as the concentrations of toxin were under the limit of quantification of the method (<2.0 μg kg⁻¹).

Fig. 3

Tetrodotoxin concentrations in the organs and tissues of Paphies australis from different sites around New Zealand. Data are a composite of organs from 10 different individuals pooled together to enable sufficient mass for the toxin extraction. Organ dissections and extractions were not possible from sites where less than 15 P. australis were collected (i.e., Whangaruru, Auckland, Waihi and Akaroa sites). Sites are ordered by increasing latitude for each island (i.e., North and South Islands).

Fig. 4

Tetrodotoxin (TTX) concentrations in Paphies australis maintained in captivity and fed a TTX-free diet for 150 days. Data are averages from five individuals. The linear model indicated a significant (p < 0.001) decrease in TTX concentrations after 150 days.

Fig. 5

Tetrodotoxin (TTX) concentrations, in the organs of Paphies australis maintained in captivity and fed a TTX-free diet for 150 days, measured using liquid chromatography-tandem mass spectrometry. Results at each time point are a total of 10 pooled P. australis individuals (n = 1).