Is there anything left in our oceans that's safe to eat?

Ciguatera Fish Poisoning (CFP) is a foodborne disease associated with the consumption of coral reef fishes contaminated with ciguatoxins (CTX’s), a neurotoxin produced by the Gambierdiscus genus of dinoflagellates. Recently there has been an increase in atypical ‘ciguatera-like’ cases involving giant clams (Tridacna gigas), in south pacific regions, an area of the world which consumes high numbers of this species. This paper aimed to assess the bio-accumulative capacity of CTX within the tissues of the giant clam, and thus determine whether the species is a potential health risk to its consumers.

Giant clams were exposed to live or lysed cells of either TB92 (a highly toxic G.polynesiensis strain) containing 5.83 _ 0.85 pg P-CTX-3C equiv.cell-1, or HITO, a weaker toxic strain of G.tacxicus containing only (2.05 _ 1.16) _ 10_3 pg P-CTX-3C equiv.cell -1. Both were administered over a 48h period at a concentration of 150 cells mL-1 . Following this, CTX presence in giant clam tissue was assessed using mouse neuroblastoma cell-based assay, CBA-N2a (N2a cells are fast-growing mouse neuroblastoma cell lines) which was used prior to contamination experiments to determine potencies.  Cultures of TB92 and HIT0 were established and enriched in a natural seawater medium, and grown at 26 ± 1 ° C under daylight fluorescent tubes in a 12:12 h (light:dark) photoperiod with permanent aeration. Cultures were harvested at the exponential/early stationary growth, as this point is the most prolific for CTX. Contaminations were carried out in closed environments, in 20L tanks containing 37ppt sea water, with stabilised temperature and dissolved oxygen (28 8C and 8 mg L_1,) respectively . 3 tanks and 1 tanks with giant clams were exposed to TB92 and HIT0 cells respectively, with a 5th tank containing animals kept in the same environmental conditions (control).  Each tank was equipped with a pump set at a 200L h-1 flow in order to favour the suspension and/or dissemination of Gambierdiscus cells/dissolved toxins in the surrounding environment of the giant clam. The whole mass was extracted from the shell and thoroughly rinsed twice to remove all traces of incubation water which was potentially contaminated with dissolved CTX’s or Gambierdiscus cells. Toxins were extracted from the tissue and assayed using CBA-N2a’s.
Results revealed that strain TB29 had a high toxic potency, whilst HIT0 was found to be weakly toxic in CBA-N2a.  Extracts obtained from control animals and giant clams exposed to either lysed or live cells of HIT0 also showed no toxicity, which was confirmed by LC-MS/MS analyses. All 6 extracts from giant clams exposed to TB92 were found toxic in CBA-N2a assays. Results also showed that exposed animals to TB92 were able to retain 3.65±1.34 (lyzed) and 3.06±0.96% (live cells) of the administered toxins, whilst the giant clams exposed to HIT0 cells retained a toxin amount insufficient to induce toxin accumulation.
The paper successfully shows that giant clams are able to bio-accumulate algal CTX’s in their tissues, highlighting however that bioaccumulation only occurs under highly toxic strains of Gambierdiscus polynesiensis.  This suggested the utilisation of two uptake routes: either by direct ingestion or by filtration of seawater containing these CTX’s.  With the fact that similar toxin levels were observed in giant clams exposed to either intact or lysed cells suggests the use of a digestive uptake route.

However, although the animals were subjected to thorough rinsing before extraction, it is possible that contamination from incidental adsorption of dissolved CTX’s onto tissue surfaces of giant clams altered the toxicity readings. Unlike similar papers, the total toxin load supplied to giant clams in this experiment is representative of what is often found in natural blooms. However, the time-scale of this project is not representative of the true duration of toxic algal blooms, which often persist for several days and occur frequently over short periods, as in these conditions giant claims could be subjected to “chronic” accumulation of ciguatera toxins. Although the toxin uptake rate of claims within the experiment is considered low to what may be predicted from in-situ species, the TB92 toxicity was still well above the recommended safety limit for human consumption, with only 2u g of toxins in giant clam tissue being considered as sufficient to induce intoxications in humans. Therefore, giant clams in the natural environment are expected to have an even higher contamination risk if in areas of reoccurring toxic algal blooms. Additionally, discrepancies between in-situ levels can be explained by the known variations in CTX production of Gambierdiscus when grown in-vitro.

The paper concludes with a summary which highlights that on-going educational programs are needed in areas such as the south pacific, in order to minimise the risk of ciguatera intoxication from the consumption of giant clams. The authors appear to have plans to ‘address’ the discrepancies between in-vitro and in-situ toxicity levels in a future paper which will utilise more appropriate experiments.  However, despite these drawbacks, the paper does convey the important message that ciguatera toxins are not solely accumulated by reef fish and that the consumption of giant clams can lead to equally severe health risks.

 Reviewed paper : Roué, M., Darius, H.T., Picot, S., Ung, A., Viallon, J., Gaertner-Mazouni, N., Sibat, M., Amzil, Z. and Chinain, M., 2016. Evidence of the bioaccumulation of ciguatoxins in giant clams (Tridacna maxima) exposed to Gambierdiscus spp. cells. Harmful Algae, 57, pp.78-87.

Chinain, M., Darius, H.T., Gatti, C.M. and Roué, M., Update on ciguatera research in French Polynesia.