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 (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?
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)?
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.
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.
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.
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.
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.