About a year ago sea slugs like the specimen shown in these videos hit the headlines after they were shown to be responsible for several dog poisonings on Auckland and Coromandel beaches.
Historical specimens from our collections established that the toxicity of Pleurobranchaea maculata is not a new event and in fact, that in Auckland, it has been toxic for at least 16 years.
Discovering toxic sea slugs
In July, August 2009 there was an unexplained spate of dog deaths on Auckland beaches, particularly Narrow Neck and Cheltenham, along with wash-ups of dead marine organisms, such as pilchards, porcupine fish and blue penguins in a wider area.
The deaths co-incided with a drop of brodifacoum poison on Rangitoto Island by the Department of Conservation – part of their pest control programme.
Very quickly a large number of agencies became involved and equally quickly a number of potential causes, including brodifacoum poisoning, were eliminated.
The Cawthron Institute became involved because of their skills in toxicology, particularly of algal bloom events, and there was a slight possibility we were dealing with toxic algae. However, extensive testing failed to pin-point any of the 26 common marine toxins.
Instead Cawthron were able to establish from the testing of a beach-cast grey side-gilled sea slug (Pleurobranchaea maculata) and the stomach contents of two of the dead dogs that a toxin new to New Zealand, tetrodotoxin (TTX), was responsible. The slugs were the carrier.
Tetrodotoxin (TTX) is the same toxin as that found in some tropical pufferfish and the Australian blue-ringed octopus, as well as a wide variety of other animal species. It is a neurotoxin, and causes paralysis of the muscles, while leaving the heart and brain relatively unaffected.
One of the muscles TTX affects is the diaphragm and victims usually die of asphyxiation. TTX is deadly even in small doses, and 1-2mg of TTX is enough to kill a 75kg human.
Our little sea slugs contain between 1-8mg of TTX – enough to kill up to 8 adults. They are therefore even more dangerous for children or dogs. For a child it could be fatal just to put their fingers in their mouth after touching a sea slug.
Why are the slugs toxic?
We don’t know why the slugs contain TTX, but it occurs in a wide variety of organisms, for example blue-ringed octopus, the Japanese Fugu pufferfish, toads, some sea stars, and a number of bacteria. In some species, such as the pufferfish there is some evidence the TTX is produced in association with symbiotic bacteria.
Further research by Cawthron Institute, Massey University and Waikato University will look at whether the sea slug is able to manufacture the TTX itself, whether it gets the TTX from its diet, or whether it is produced in association with symbiotic bacteria.
This research will use freshly collected specimens from around New Zealand, historical specimens from Auckland Museum and Museum of Victoria (Australia). It will also try to breed the slugs in captivity.
Have the slugs always been toxic?
Although we don’t yet know if all individuals of the grey side-gilled sea slug are poisonous, in the past year Cawthron Institute has tested freshly collected specimens from Auckland Harbour, Manukau Harbour, the Coromandel west coast and Nelson and all were toxic.
Grey side-gilled sea slug specimens in Auckland Museum’s collections from 1994, 2000 and 2007 tested positive for TTX and show that slugs collected from Auckland Harbour have been toxic since at least 1994. It is therefore highly likely they have always been toxic.
One of our specimens from 1989 (from Stewart Island) tested negative. However, this may have been a false negative – we don’t know yet how stable TTX is in preserved specimens and TTX may be discarded when we replace the alcohol in our specimen jars.
Why didn’t we know it was toxic before?
We weren’t aware about TTX in sea slugs until last year, when MAF Biosecurity reported the unusual cluster of dog deaths for Auckland beaches. This suggests that if there have been any previous dog deaths due to TTX poisoning they have gone under the radar because they were isolated cases. There was simply no reason to test our samples for the toxin before this.
Why are the slugs a problem around this time of year?
Grey side-gilled slugs lay long coils of eggs at the end of winter/start of spring, after which they usually die. The dead slugs often wash up on beaches, particularly with on-shore winds.
At the end of winter, you should keep children and dogs away from the high-tide line, because this is where slugs and slug eggs on seaweeds are washed up. Don’t let them touch or pick up seaweeds or slugs – there is no antidote to the fast-acting toxin these sea slugs innocently carry.
If you would like to know more about the sea slugs, feel free to ask me a question in the comments below or read more on our website.