Freshwater jellyfish hunt turns up trumps

12 February 2010

A prolonged spell of warm weather could see Hamilton Lake filling up with tiny jelly fish. That's one of the fundings of a Summer Scholarship study by Waikato University biology student Kevin Eastwood.

He has been working out how far these odd little creatures, originally from China, have spread since they were first discovered in Lake Taupo in the 1950s. He's found they're in many more lakes than previously thought - including Hamilton Lake - although most of the time they're almost invisible.

Kevin’s supervisor, Dr Ian Duggan, explains. “The freshwater jellyfish has two stages in its life cycle. For most of their lives they exist as a tiny polyp, about half a millimetre long, a bit like a sea anemone except they don’t really have much in the way of tentacles. The jellyfish, or medusa, is the sexual stage, although in New Zealand they’re all males. No females at all. They bud off from the polyps and grow up to about 20mm in diameter. They can bloom in enormous numbers for a couple of months, then they die off, and may not reappear for years.”

Kevin and Ian have collected rocks from lakes as far north as the North Shore of Auckland, and have recently returned from a collecting trip to the lakes of the southern North Island.

Not many people realise there are freshwater jellyfish in New Zealand, says Ian. “As we were sampling we’d often get people talking to us, and they’d give you some funny looks when you mention what you’re doing.”

Dr Ian Duggan and Kevin Eastwood

FISHY BUSINESS: Waikato University's Dr Ian Duggan, left, and Kevin Eastwood look for the jellyfish in a Waikato lake.
Photo: Waikato Times.

Kevin has done the painstaking work of examining the rocks under a microscope for signs of the polyps and has become very adept at spotting them.

“They look like bowling pins, and on their top ends they’ve got what appear to be studs,” he says. “That’s the nematocysts, the stinging cells they use to capture their prey. They live in colonies generally, two or three to a colony, and they feed on zooplankton; we’ve been feeding them brine shrimp.”

Kevin, who is about to start the final year of his BSc, says he has in the last few days found polyps in samples from several southern North Island lakes. The jellyfish stage has not previously been seen in the North Island south of Taupo, and Lake Tutira in Hawkes Bay, though there are a couple of South Island records. Ian says it’s possible that in some lakes where the polyps are present conditions may never be right for the jellyfish to appear.

“I just got the idea that if we looked for the polyp stages, we’d get a far better idea of where they actually existed, and I figured they would exist in places where the jellyfish stage had never been found. There was an opportunity to get funding from the Summer Scholarship scheme for promising undergraduate students who may want to go on to research later, I applied for some money, got it, and Kevin was the lucky recipient. I had more people who applied to look for the freshwater jellyfish than for any other project, I think because they’re one of those sexy animals that people want to work on.”

New sites for the species confirmed by this study include Turtle Lake at the Hamilton Gardens, Lakes Hakanoa and Puketerini in Huntly, Lake Gilmour in Waihi, and Western Springs in Auckland. They also confirmed the presence of polyps in lakes where jellyfish had been recorded previously.

 “In fact we managed to see the jellyfish swimming around in Quarry Lake (beside Lake Pupuke in Auckland) the day we were sampling,” Kevin says.

 “We can say that they are in a lot of lakes that they haven’t been recorded in before; they are more widely distributed in the North Island than we thought.”

The jellyfish is now found widely around the world, and most populations outside of Asia are single-sex, reproducing asexually, suggesting the original colonisations have been of only one or a very few individuals. They are harmless to humans, and there is little evidence they’re having adverse effects on freshwater communities in New Zealand, although the short-lived summer jellyfish blooms can involve enormous numbers of animals. “At Lake Kaituna out at Horsham Downs, they’ve been recorded at about 13 per 100 litres,” Ian says. “That doesn’t sound like much, but when you’re in a boat, looking down, it’s really noticeable.”

Sustained water temperatures above 25 degrees appear to be necessary to promote a bloom, although the precise triggers remain a mystery. “It’s hard to predict,” says Ian. “You’d think that if it was just down to the weather then all the lakes in an area would have a bloom at the same time, but in places like the Rotorua lakes you seem to get a bloom in one lake one year, and in a completely different lake another year. It doesn’t seem to be consistent.”

The research has raised more questions, says Ian. “You do research like this and you find lots more things that we have no idea about. Not much is known about the polyps; most of the research done around the world is based on whether the jellyfish themselves pop up. They seem to spread around from lake to lake quite easily, we’ve even found them in recently formed lakes, but we’re not sure how they do it. That’s one mystery that needs to be solved some time in the future.”

Kevin would be interested to hear from anyone who has seen freshwater jellyfish in a New Zealand lake, email [email protected]