Toxic sea slug threat not eradicated
Four years ago no one really gave the little grey side-gilled sea slug a second glance.
Then dogs got sick and some died after beach walks - mostly Narrow Neck and Cheltenham, but also as far north as Leigh.
July and August were the worst months, but this year - nothing.
The Cawthron Institue in Nelson pin pointed tetrodotoxin (TTX) poisoning, and sea slugs as the source.
The toxin is one of the more deadly compounds known, institute coastal marine ecologist David Taylor says.
It is found in other warm climate animals, including puffer fish and deadly rain forest frogs.
A funding scramble got research going into this new threat.
Commonly found in mussel beds, the native slugs are found all around the coast.
Testing in Whangarei, the Hauraki Gulf, Tauranga and Wellington has shown varying toxin levels - the highest in the Gulf, the lowest in Wellington, Massey University PhD student Yeserin Yilirim says.
Auckland Museum's slug specimens, including one from 1990 and two collected from 90 Mile Beach in 1923, were sent to Cawthron and tested positive for TTX, meaning the toxin wasn't a new thing, marine science curator Tom Trinski says.
Soft body organisms like slugs have few predator defences. A similar type of organism, nudibranchs, also contain a toxin but these are brightly coloured to ward off predators. The grey side-gilled sea slug is dowdy by comparison.
The slugs' eggs also have the toxin.
Snapper thrive in mussel beds and the toxin would likely have been a deterrent to them, other fast moving carnivorous fish, starfish, and whelks (sea snails), Dr Taylor says.
But the toxin hasn't turned up in South Island slugs, suggesting water temperature may be a factor.
Research by doctors Suzy Wood and Paul McNabb at Cawthron show Cook Strait seems to be a barrier.
This could change in the future as sea temperatures rise with climate change Ms Yilirim says.
TTX occurs in warmer temperatures, and rising sea temperatures look to be behind the toxin starting to turn up in some types of shell fish in the Mediterranean that were previously free of it.
This came to light after locals who customarily gathered the snails became ill.
The hunt is on to find out where North Island slugs are getting their poison from, and why Gulf slugs are more toxic.
It looks like they're feeding on something in mussel beds while very young, Dr Taylor says.
The toxin is known to be naturally present in some bacteria, but a Waikato University team under Professor Craig Carey studying bacteria as the likely source haven't found anything definite yet.
The slugs would have been much more common in the Gulf in the past, living in massive mussel reefs, particularly in the Firth of Thames. Commercial dredging wiped the reefs out 50 years ago.
Then in 1970 the Asian date mussel arrived, released from ship ballast. They are now more common in some harbours than native mussels and form beds lasting a couple of years before dying off.
That's what happened just off Narrow Neck Beach in 2009.
Diving surveys at the time of the poisoning found the odd sea slug around the harbour, but around Cheltenham and Narrow Neck there were thousands of them 200 metres to 300m offshore, Dr Walker says.
Since then the beds have died off, so sea slug numbers have dropped at those beaches to perhaps to one or two per 100 square metres, Dr Taylor says.
''In the future there could be another outbreak with a perfect storm of invasive Asian date mussels off the beach, toxic sea slugs, and waves washing them ashore,'' he says.
People just need to be aware of them and treat them with caution when on the beach, as they would stingrays and jelly fish, Dr Walker says.
Around 80 per cent of the country's commercial greenshell mussel production happens around the South Island, and Cawthron has has found large slug settlements on Golden Bay and Tasman Bay mussel lines. But they're not toxic down there, Dr Taylor says.
The remaining mussel production is around the Hauraki Gulf, but there is no funding available to work on these commercial mussel farms, he says.
Mussels are tested for any toxins as part of commercial operations and they don't carry it. Processing also sees slug and egg residue washed off the shells.
''The next research we would like to do is to see what role the mussel farms are playing. Probably a role of replacement habitat where you once had native mussel beds everywhere - which we've managed to destroy - and now replaced them with farms,'' he says.
Expansion of commercial Gulf mussel farming has been mooted, along with plans to increase mussel production around Northland.
This as the government looks to increase the aquaculture industry to $1 billion by 2025.
There are also ambitious plans to try and restore some of the Gulf mussel reefs.
So don't write off toxic sea slugs on the beach just yet.
NOT SO TASTY
Soft body organisms like slugs have few defences against predators.
A similar type of organism, nudibranchs, also contain a toxin but these are brightly coloured to ward off predators.
The grey side-gilled sea slug is dowdy by comparison.
"Snapper thrive in mussel beds. The toxin is likely to deter them, and other fast-moving carnivorous fish, starfish and sea snails from eating the slugs," Cawthron Institute coastal marine ecologist David Taylor says.
- © Fairfax NZ News
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