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Toxic sea slug

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Toxic sea slug

The grey side-gilled sea slug (Pleurobranchaea maculata) is commonly found around New Zealand and south eastern Australia, and has also been seen in Japan and Sri Lanka. It was recently discovered to be deadly to humans and other animals such as dogs.

Identifying the toxic sea slug

The grey side-gilled sea slug is pale grey with a dense dark brown mottling. It has no shell. The fully grown slug is about 10 cm long, and has a feathery gill partly hidden on the right side of the body.

Two stalk-like ‘rhinophores’ project from either side of the slug’s head, which also has a fringed veil. The slug moves rapidly with a smooth gliding motion.

The slug is an opportunistic and voracious carnivore, with a varied diet but a preference for sea anemones. In captivity it will greedily take chopped mussels and fish pieces and is not above cannibalising other members of its species.

Tetrodoxin (TTX) poison and sea slugs

The grey side-gilled sea slug, its eggs and larvae were found to be poisonous after a spate of dog deaths on Auckland beaches in August 2009.

Cawthron Institute identified the toxin in the sea slugs as tetrodotoxin (TTX), which is also found in other animal species such as tropical pufferfish and the Australian blue-ringed octopus.

Tetrodotoxin (TTX) is a neurotoxin. It causes paralysis of the muscles, including the diaphragm, which affects breathing. It does not affect the heart and brain.

When eaten, TTX is deadly even in small doses. 1-2mg of TTX, or about half a teaspoon of sea slug, is enough to kill a 75kg human. There is no antidote.

Where the toxic sea slug is found

In New Zealand the grey side-gilled sea slug is found in all marine environments - from silty harbours to rocky open coasts, and from the intertidal zone (where land and sea meet) to at least 250m water depth.

More sea slugs than usual seem to be washing up on Auckland beaches at the end of winter now. Dive surveys suggest that the large beds of Asian date mussels (Musculista senhousia) in these locations might be an influence – a source of food for the slugs.

Be careful on beaches at the end of winter when the  toxic grey side-gilled sea slug can often be found on the sand.

Be careful on beaches at the end of winter when the toxic grey side-gilled sea slug can often be found on the sand.

Auckland War Memorial Museum - Tāmaki Paenga Hira.

At the end of winter more sea slugs than usual can wash up on Auckland beaches. This is due to a combination of factors. The slugs lay their eggs at this time of year and they often die after spawning, after which any onshore winds can carry them ashore. Also, dive surveys in locations with large beds of Asian date mussels (Arcuatula senhousia) suggest that there are greater numbers in these areas because the mussels are a source of food for the slugs.

Further resources

Cite this article

Blom, Wilma. Toxic sea slug. Auckland War Memorial Museum - Tāmaki Paenga Hira. First published: 6 April 2016. Updated: 12 November 2019.

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