Toxicity levels in sea slugs

Dr Susie Wood of the Cawthron Institute in Nelson talks about how toxic grey side-gilled sea slugs are. She explains that not all these sea slugs are toxic. It seems that North Island slugs are toxic to varying degrees while those found in the South island so far are not toxic at all.

Jargon alert: TTX – short form of the toxin tetrodotoxin.

Transcript

DR SUSIE WOOD
All of the grey side-gilled sea slugs around New Zealand are not toxic. So we know that there’s several populations. We’ve found one in Nelson out in Tasman Bay and also one off Kaikōura which are not toxic, and we also know that the levels in the populations around the North Island vary.

We don’t really have any idea why there’s such a huge difference in the levels of toxins between populations and why they’re non-toxic, so it really comes down to answering that question of what the origin of the toxin is, and once we can establish the origin, hopefully we’ll be able to work backwards and work out why there’s non-toxic populations.

We’ve recently done some surveys around the Hauraki Gulf, and we’ve sampled from the original beaches where the dog poisonings occurred and also from some other sites close by. In particular, one site – a rock off Rangitoto Island – the levels do vary between the different populations, but they are still really, really high. In particular, this site off Rangitoto, they’re as toxic as they were when the dogs died, perhaps even higher than they were.

One thing we know is that the levels do change throughout the year. So most recent research has shown that the levels of TTX are highest in the egg-laying season – that’s between June to August – and we’ve also found that the levels of TTX are very high in the egg masses at that time of year.

We suspect that the adults are passing on the toxin into their eggs initially and then also onto the larvae in the juvenile slugs as a protective mechanism. So you can imagine these slugs, they’re soft-bodied organisms and their egg sacs are just sort of floating attached to rocks or other shellfish, and so they don’t have any other way of protecting themselves, so a lethal toxin is a pretty good protection mechanism.

The sea slugs have potentially always been toxic, so we have tested some museum samples. The Auckland Museum holds a collection of these slugs and they had samples from back in the early 90s, and we were able to take a little sample from the slug or from the liquid around the slugs and test that. And we could actually detect trace levels of TTX, so we know that probably for at least the last 20 years, these slugs have been toxic. So how much further back than that we don’t know, but it’s one of these things, once we start looking for these toxic compounds, we, you know, usually find them.

Acknowledgements:
Jarrod Walker
Cawthron Institute
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