Tougher rules may be needed to stop potentially deadly tropical algae hitching a ride into New Zealand on corals imported for aquariums.

Research by Master of Science student Kirill Woodhouse identified 62 species of tropical algae on 100 corals obtained from retailers and aquarium hobbyists.

“A couple of algal cells hidden on a live coral could be enough to introduce a new species if water from an aquarium is poured down the drain,” says Kirill, who recently completed his degree at Te Herenga Waka—Victoria University of Wellington.

Kirill tested 10 of the 62 algae species to see if they could tolerate seawater temperatures in Auckland and Wellington. The 10 samples tested were growing well in holding tanks, an indication they may be able to adapt to rapid environmental changes, he says.

One sample (Chaetomorpha vieillardii) survived for four weeks at 14℃, Auckland’s minimum winter sea-surface temperature. This species is commonly kept by aquarium hobbyists.

“Temperature tolerance limits are the largest determinant of algae survival. As this strain can tolerate low temperatures, grows into very large unattached balls and can reproduce from single cell fragments, it is a biosecurity threat.”

Further work is needed to assess the likelihood of the algae establishing in the event it was accidentally released into the marine environment, Kirill says.

None of the 10 samples survived more than two weeks at Wellington’s cooler sea-surface temperatures.

“In some ways this was encouraging. But it wouldn’t take much of a rise in temperatures to allow other tropical algae to thrive, especially around northern New Zealand.”

The risk of successful algae incursions is only likely to increase with more marine heatwaves predicted as the climate warms, Kirill says.

“They can quickly spread and out-compete native algae, disturbing algal communities that are used as habitat by fish and invertebrates. Introduced algae can cover substrate, killing other algae and even killing non-mobile invertebrates.”

Kirill believes stricter quarantine and monitoring are essential to stopping these invaders in the future.

Import rules require corals to stay up to six weeks in quarantine.

“I don’t think the period is long enough to be able to visualise the algae, catch it at the border and destroy it before it gets into someone’s tank. The trouble is, algae can last as a spore and stay dormant until the environment’s right to start growing.

“You might be able to see it once it starts germinating, but the spores are tiny and there’s all sorts of other stuff on the rock to which the coral is attached—detritus, bacterial films, microinvertebrates.”

Longer quarantine times and changes to how corals are brought into the country could help reduce the risk.

“Corals might have to be detached from rocks and put on to clean concrete plugs. So you chop off the coral at the base where there is live tissue, and then glue it to a clean ceramic plug before releasing it to the quarantine facility.”

The public also needs better information on the proper disposal of unwanted fish and aquarium wastewater to reduce the risk of an unintentional release of algae, Kirill says.

“Many aquarium enthusiasts are unaware of the potential impacts of releasing specimens or wastewater into the wild,” he says.

Kirill and his Master’s supervisor Professor Joe Zuccarello have co-authored a paper on the research for the New Zealand Journal of Marine and Freshwater Research.