Grey side-gilled sea slugs
In 2009, some grey side-gilled sea slugs (Pleurobranchaea maculata) were found to be toxic following the deaths of several dogs on Auckland beaches. It was not previously known that these slugs contained tetrodotoxin (TTX) – the toxic substance that killed the dogs. The slugs are now considered New Zealand’s most toxic creatures.
Grey side-gilled sea slug
The grey side-gilled sea slug Pleurobranchaea maculata. The discovery of tetrodotoxin in this slug in 2009 makes it New Zealand’s most toxic creature. In this photo, the grey side gill can be seen reflected in the tank glass.
Description
This slug can be up to 10 cm in length. It is pale grey and the mantle densely patterned with short, brown lines. It has rhinophores on the sides of the head. As its name suggests, the grey side-gilled sea slug has a feathery gill on the right side of its body. It has no shell, is soft bellied and slow moving. It is active when it is hungry – both day and night.
Grey side-gilled sea slug sketch
Simple sketch of the grey side-gilled sea slug showing the main parts.
Habitat
These sea slugs are native to New Zealand and are found on both the North and South Island coastlines. Based on research conducted so far, it appears the North Island slugs are toxic in varying degrees, while the South Island ones are not. Scientists have yet to discover why. The slugs are found in a wide range of marine coastal habitats including rocky coastline. They are found subtidally to about 250 m water depth. They are most commonly observed during winter and spring when they lay egg masses.
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.
Food
The grey side-gilled sea slugs are scavengers. Their diet includes sea anemones, marine worms and molluscs. They even eat each other!
Toxicity of the slug
Many species of slugs contain toxins to deter predators from eating them because they don’t have a hard shell for protection and are very vulnerable, but finding deadly TTX in this species was a surprise.
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.
Scientists from Cawthron Institute are working to determine the source of the toxin TTX. The source of TTX appears unrelated to the food chain – TTX has not been found in any of the sea creatures or algae that form the food chain the slugs are a part of. Scientists thought it may be produced by bacteria found in or on the slug, but the scientists have not yet been able to isolate any bacteria that produce the toxins. It is possible that the slugs are producing TTX themselves, and this is being investigated.
Whatever the source, these slugs are deadly. Dogs can die from ingesting them and so can humans. One slug can contain enough TTX to kill at least four adults. The lethal dose of TTX to humans is 1–2 mg (about half a teaspoon of slug).
Symptoms in humans from TTX poisoning include numbness and tingling around the mouth and nausea. Paralysis can occur. Medical attention should be sought immediately.
It is not unusual to see slugs washed up on the beach. In the past, this has not been a concern, but with the discovery of TTX in these slugs, they should be avoided. The sighting of grey side-gilled sea slugs should be reported to your local council.
The toxicity of these sea slugs is of national importance, and it may become an on-going issue. A number of organisations are involved in research and are monitoring these slugs, including the Cawthron Institute, the Auckland Council, the Auckland Regional Public Health Service, regional councils, Waikato University and the Hauraki Māori Trust Board.
Nature of science
The discovery of TTX in the grey side-gilled sea slugs in 2009 is an example of how science knowledge is not static but changes with time. It is also an example of how a number of scientists from different organisations collaborate to help communities monitor a potentially dangerous situation.
Related content
Explore why dogs died on Auckland beaches in 2009. Meet scientists from Cawthron Institute – Dr Paul McNabb, Dr Shaun Ogilvie, Dr Dave Taylor and Dr Susie Wood – as they look into the cause of these deaths. The discovered high levels of deadly tetrodotoxin (TTX) in grey side-gilled sea slugs.
Useful links
View these Auckland War Memorial Museum YouTube videos of the moving grey side-gilled sea slug:
Listen to a 2010 RNZ audio clip about research into the grey side-gilled sea slug.
