Toxic Sea Slugs

Fact sheet and media information.

What do we know about this sea slug?

Sea slugs are common marine animals, found all around New Zealand. There are many species in New Zealand, but one species in particular, Pleurobranchaea maculata, has been linked to the deaths of dogs on Auckland beaches in 2009.

Some sea slugs are toxic, but this sea slug has not previously been known to contain this particular toxin. Sea slugs naturally produce toxins to deter predators, such as fish, from eating them because they are vulnerable, being soft-bodied and slow-moving.

Where are the toxic sea slugs found?

It is also not known whether all sea slugs of that species are toxic, or whether the toxicity is restricted to sea slugs found in the North Shore and Coromandel area. It is possible that these toxic sea slugs are more widespread.

Identifying the source of the toxin

The Cawthron Institute identified the toxin found in the dogs’ vomit as being tetrodotoxin (TTX), which is also found in puffer fish. NIWA marine biologist, Dr Hoe Chang was involved in the early stages of the investigation. It was first thought that the deaths may have been caused by a toxic algal bloom, but Dr Chang used water samples and satellite imagery to show that this event was not due to a toxic algal bloom.

Where can tetrodotoxin (TTX) be found?

TTX can be found in naturally occurring marine bacteria.

Little is known about the source of the toxin in these sea slugs, however, it may lie in the shallow sub-tidal algal communities, adjacent to these beaches. The sea slugs may be ingesting the bacteria with their food.

There are three possible explanations for the presence of TTX in the sea slugs. “We need to explore whether the sea slugs produce TTX, whether it comes from a symbiotic relationship between bacteria and the sea slug, or whether it’s something the sea slug ingested,” says Dr Chang. At this stage we do not have answers to these questions.

Can TTX harm humans?

What is known is that dogs that eat organisms containing TTX can die, and so can humans.  

The lethal dose of TTX to humans is 1–2 mg. A human would need to eat 2.6 grams of sea slug in order to get a dose of 1mg of TTX. That’s not much, probably half a teaspoon of sea slug.

What are the symptoms of TTX poisoning?

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 sea slugs washed up on the beach, and it is not normally a cause for concern. However, in the current situation, any sea slugs on any beach should be avoided. If you do see a sea slug washed up on the shore, you should report it to your local council.

Other organisms contain TTX

TTX has been found mainly in puffer fish, and they don’t produce the TTX themselves. In the case of the puffer fish the toxin is produced by bacteria, and then passed on to the host – the fish. Puffer fish and the TTX-producing bacteria could have a symbiotic relationship, and these bacteria can even be found in puffer fish eggs.

TTX has also been detected in crabs, newts, ribbon worms, and blue-ringed octopuses.

Interestingly, the highly poisonous puffer fish is eaten in Japan, even though its poison can be fatal. When prepared for human consumption, the gut, the nervous system, and other parts of the puffer fish are cut out, and then other parts of the fish can be eaten. In Japan they are eaten like sushi, paper-thin slices of fish dipped into soya sauce, which they call fugu. A chef needs at least six years’ apprenticeship, and to have a licence, before being allowed to prepare puffer fish.

What is being done right now?

The toxicity of these sea slugs is of national importance, and it may become an ongoing issue. The Auckland Regional Council (ARC) are leading the response to this issue. A multi-organisation Technical Advisory Group has been established, and it includes the Auckland Regional Public Health Service, MAF Biosecurity New Zealand, regional councils, the Department of Conservation, Cawthron Institute, NZFSA, NIWA, and others. Consideration is being given to the need for a comprehensive, long-term management and surveillance strategy.

Members of the Technical Advisory Group are updated by newsletters from the ARC.

1. Scientists are investigating which parts of the sea slugs are toxic; possibly the acid glands or the gut. Cawthron Institute Technical Manager, Paul McNabb, is trying to determine whether the toxin is passed on genetically or isolated in the organism.

2. Scientists are investigating the geographical areas where these toxins are found, and at what time of the year. It is not known whether a New Zealand marine bacteria species is involved in the production of TTX. This has not been investigated before in New Zealand. It is not clear whether changes in environmental conditions around Auckland and Coromandel coasts are responsible for this outbreak.

3. MAF Biosecurity New Zealand screen credible reports of affected animals on beaches, through the Exotic Pest and Disease hotline (0800 80 99 66).

Where can you get additional information?

To obtain public health advice,go to the Auckland Regional Public Health Service website or contact:

Species fact file
Common name:   Grey side-gilled sea slug
Scientific name: Pleurobranchaea maculata
Habitat: A wide range of marine coastal habitats including rocks. It can be found from low tide to 250 m.  It occurs all along the coast and across the continental shelf. 
Description:   The body is translucent grey. The mantle is actually covered with minute puckers and folds. Along the edge of the oral veil are small, branched papillae.  It is active during the day or night, i.e. whenever it is hungry. It has rhinophores on the sides of the head and the mantle is much smaller than the foot and there is no shell.
Size ranges: Up to 100 mm in extended length. The species shows a geographical gradient in size, with the largest specimens found in southern New Zealand.
Abundance: Common in summer and autumn

For further information about the sea slug see: NZ Coastal Marine Invertebrates 1, edited by Steve de C. Cook, and published in 2010 by Canterbury University Press.

Pleurobranchaea maculate - grey side-gilled sea slug. [Cawthron Institute]
Research subject: Biosecurity