The grey side-gilled sea slug (Pleurobranchaea maculata) is commonly found around New Zealand and south eastern Australia, and has also been reported from Japan and Sri Lanka. It has recently found 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.
At the end of winter, you should keep your family and pets away particularly from the high-tide line on beaches where these slugs, or seaweeds carrying their eggs, wash up.