Sea slugs and TTX
Investigations into the death of some dogs on Auckland beaches in 2009 led scientists from the Cawthron Institute to discover the presence of high levels of deadly tetrodotoxin in grey side-gilled sea slugs. This makes these grey side-gilled sea slugs our most deadly creature. It is also the first discovery of tetrodotoxin in and around New Zealand.
Tetrodotoxin
Paul McNabb and Dr Susie Wood of the Cawthron Institute in Nelson discuss the toxin tetrodotoxin. Paul explains the effects of tetrodotoxin on the human body and talks about just how poisonous it is. Susie explores where tetrodotoxin might be found and how its origins and reasons for existing are still a mystery.
Jargon alert: Natural toxin – some scientists refer to toxins produced by living organisms as natural toxins or biotoxins.
Tetrodotoxin
Tetrodotoxin (TTX) is an extremely potent neurotoxin that damages the nervous system and/or the brain. Symptoms can include numbness, headaches, loss of memory, vision and/or thinking ability and death. TTX is most notorious as the toxin that causes Japanese pufferfish poisoning. However, it is now known to occur in many different species.
How did TTX get into the sea slugs?
The big questions for the scientists are where did the TTX come from, and how did it get into the grey side-gilled sea slugs?
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.
These questions have not yet been answered. Dr Susie Wood from the Cawthron Institute is helping to investigate three possibilities concerning the source of TTX:
Is the toxin produced by bacteria and moved through the food chain (bioaccumulation)?
Is the toxin produced by bacteria harboured within the sea slugs?
Is the toxin produced by the sea slugs themselves (endogenous production)?
Finding the origin of TTX
Dr Susie Wood of the Cawthron Institute in Nelson describes research that aims to solve the mystery of the origin of tetrodotoxin. She presents three hypotheses: it comes from bacteria, it bioaccumulates in the food web or the slugs are producing it themselves. Susie gives an outline of some of the research being done and presents their conclusions so far.
Jargon alert: TTX – short form of the toxin tetrodotoxin.
Bioaccumulation
Dr Dave Taylor, a scientist from the Cawthron Institute involved with the sea slug mystery, is a diver. He and some others went diving in the Auckland Harbour to collect sea creatures and seaweed that were in the same area as the sea slugs. These samples were brought back to Cawthron and tested for TTX. None of these samples tested positive for TTX.
Asian date mussels
Dr Dave Taylor of the Cawthron Institute in Nelson explains what the Asian date mussel has to do with the grey side-gilled sea slug and the dogs dying on the Auckland beaches. Asian date mussels are an introduced species that thrive in the Auckland Harbour. It appears that Asian date mussels provide the perfect habitat for our toxic grey side-gilled sea slugs.
The scientists found that the sea slugs were attracted to a large population of Asian date mussels. These mussels are not native to New Zealand but were introduced in the 1970s – probably on the bottom of an oil rig. A prolific growth of the mussels in Auckland Harbour attracted the sea slugs that were feeding off them. TTX was not found in any Asian date mussels tested. Asian date mussels move around and are now no longer in the area close to the beaches where the dogs died.
It is interesting to note that, in other countries where TTX is found in organisms, these organisms are unrelated to each other in food chains. TTX is found in sea and land animals, for example, Japanese pufferfish, blue-ringed octopus, starfish, marine worms and snails, some frogs and Californian newts.
It seems that TTX in sea slugs may not be due to bioaccumulation.
Bacteria
In the case of the Japanese pufferfish, it is believed that the TTX is produced by bacteria and then passed on to the host, the fish. These bacteria can even be found in pufferfish eggs.
Testing slug bacteria for TTX
Dr Susie Wood of Cawthron, Nelson describes the process involved in testing sea slug bacteria for tetrodotoxin. She explains how bacteria are removed, isolated and grown in agar plates and then marine broth. The broth is then removed from the bacteria. Chemicals are used to break up the bacteria, which is then analysed for tetrodotoxin using liquid chromatography-mass spectrometry (LC-MS).
Jargon alert: Liquid chromatography-mass spectrometry (LC-MS) is a process using specialised equipment to detect toxins in substances. The process can accurately determine the molecular mass of different toxins, giving scientists the clue to work out what toxin it is.
TTX – abbreviation of the toxin tetrodotoxin.
Susie is testing bacteria from the slugs for the presence of TTX. She has tested up to 200 different species of bacteria found within and on the grey side-gilled sea slugs. So far, there are no positive results for TTX.
Endogenous production
It could be that TTX is produced within the slugs themselves, but this is not conclusive yet. Susie is breeding toxic grey side-gilled sea slugs at Cawthron and has found the presence of TTX in slug eggs and larvae.
Another mystery is that not all grey side-gilled sea slugs are toxic. All grey side-gilled sea slugs found around the North Island and tested at Cawthron were found to contain TTX. However, none of the grey side-gilled sea slugs from the South Island have been found to contain TTX.
Nature of science
Scientific knowledge is developed by a process of on-going inquiry. The discovery of TTX in the grey side-gilled sea slugs is new, and there are still mysteries to be solved concerning where TTX came from and how it got into the sea slugs.
