Ciguatoxins (CTXs) are a group of marine
biotoxins that are associated with illnesses such as the most frequently
reported seafood-borne ciguatera shellfish poisoning (CFP). CTXs are produced
by benthic Gambierdiscus dinoflagellates
and have highly stable, strong skeletal structures, allowing them to retain
their structures under a range of different environmental conditions. Due to
this, CTXs are able to bioaccumulate in the food web via trophic transfer.
There have been 29 CTX congeners found to
date, which have been grouped according to their geographic location, for
example: Pacific CTX (P-CTX), Indian CTX (I-CTX) and Caribbean CTX (C-CTX). Each
of these locations harbour different CTX congeners with different potency
properties.
In a study by Meyer et al (2016), the focus
was placed on P-CTX bioaccumulation in liver and mussel samples of 22
individuals of shark across 9 species from 4 locations along the east coast of
Australia. Sharks were chosen due to their high susceptibility of
bioaccumulation as a result of their relatively high trophic level (average
trophic level of 3.65), long life span and slow growth and metabolism. The
locations were selected based on Gambierdiscus
abundance, CFP incidents and tissue availability (i.e. one site with links high
incidence of Gambierdiscus and CFP, a
temperate site where Gambierdiscus was
found in high numbers with no link to CFP incidence, a coastal-reef site where
neither CFP nor Gambierdiscus had
been identified, and one well studied inshore coastal environment with
intermediate CTX presence).
CTX was extracted using approximately 50 g
of mussel tissue and 50 g of liver tissue were collected from each of the
studied sharks, and frozen at -20 OC. The samples were freeze-dried
and duplicate samples were extracted with methanol:hexane. After using SPE
methodology to extract the toxin, HPLC-MS/MS analysis involved the injection of
10 ml P-CTX-1, P-CTX-2 and P-CTX-3 toxin standards every 20 samples.
The results showed that the 22 mussel
samples and 11 liver samples used in the experiment had a lack of detectable
concentrations of the 3 most bioaccumulated CTXs (P-CTX-1, P-CTX-2 and
P-CTX-3). The authors of this study suggest reasons for the results they
obtained. Firstly, the methods used were specific for teleost species, and so
may not work as efficiently with elasmobranchs. Secondly, CTX may be
biotransformed in elasmobranchs due to their metabolic properties and so may be
undetected. Thirdly, 22 individuals may be too small a number to yield any
reliable results from. Finally, they suggest that their inconsistent data of Gambierdiscus blooms may have caused
them to misjudge CTX hotspots.
Unfortunately, this study has many
limitations, to the point where the authors mention a few of these at different
parts throughout the paper. In many of the shark species studied, instead of
testing whether the species is ciguatoxic or not, they have gone by local
knowledge and simply accepted the word of the locals without any scientific
evidence. Though this has come from a previous source, I feel that there is too
hanging on faith for this study to use that information. Also, 22 individuals
seems a very low number to be able to make any conclusive argument with, and
whilst the author mentions this, it is still a very limiting factor. The final
major limitation with this study was the CTX detection limit. Whilst no
detection was seen in any of the samples, this doesn’t necessarily mean that
there is no bioaccumulation of CTX in these sharks. Since CTX is highly potent,
levels of it below this detection limit may still be biologically significant. Whilst
it had many limitations, I think the idea behind this study showed promise, and
I believe that in the future the results obtained will be different than the
ones seen in this study.
Reviewed paper: Meyer, Lauren., Capper,
Angela., Carter, Steve., and Simpfendorfer, Colin. (2016). An investigation
into ciguatoxin bioaccumulation in sharks. Toxicon. 119: 234-243. http://www.sciencedirect.com/science/article/pii/S0041010116301611
Hi Amy,
ReplyDeleteThanks for your post. This paper is certainly flawed and a lot of extra study is needed before anything about shark CTX bioaccumulation can be gleamed. I feel that the authors have missed a key element in the bottom-up variables in bio accumulation - namely macroalgal cover. Gambierdiscus congeners are better described as 'epiphytic' rather than benthic and macroalgal cover would play a major role in CTX production and could be easily quantified. I was wondering what your opinion was on the authors dismissal of their own results - when they themselves note their methodology is so flawed that their results should not be trusted, what was the point of the study in the first place? Do you think if the results had 'fitted their classical expectations' they would've been so quick to criticise their study, or would the methodology have then been suitable? Great to know what you think.
Thanks,
Davis
Hi Davis,
ReplyDeleteYou've raised a fair point. This paper definitely missed out a lot of the wider factors surrounding this study, like the bottom-up variables as you said. With regards to your question, it's hard to know for sure what the authors would have done. However, my guess is that the biggest concern they had with this paper was the methodology rather than the results they obtained, so due to this I think that even with different results they would have been unhappy with their methodology. I think if their methodology had been different and been done to their satisfaction, they would have accepted the results they got. I think they would've been a lot happier with their methodology if they had gathered all the information needed for themselves, rather than 'taking other people's word for it' and relying on faith.
I hope this answers your question.
Thanks,
Amy