Invasive water flea arrived 60 years ago

Scientists now think a species of invasive freshwater crustaceans got to New Zealand about 60 years ago, possibly by clinging to recreational fishing gear or clothing.

The water flea, Daphnia pulex, breeds prolifically and crowds out native water fleas. It also breeds with native water fleas to create hybrid species.

Its arrival in New Zealand was noted over recent decades but some very thorough science by Zhiqiang Ye and colleagues from Arizona State University appears to have nailed down the arrival time, likely source and how it spreads within this country.

Using several different genetic techniques, the researchers concluded that the ‘‘New Zealand population originates from a single asexual clone’’.

Daphnia are capable of sexual reproduction, but if there are no males to mate with, the females can clone offspring that can also clone themselves.

‘‘Our results suggest that all of the South Island Daphnia pulex originated from a single genotype, therefore, the invasion is unlikely from multiple clones,’’ Zhiqiang Ye wrote in an email.

It probably was not a single individual water flea that arrived here, but multiple individuals who were identical clones and transported together. Genetics showed they originated in North America.

Zhiqiang Ye estimated the arrival date was about 58 years ago, in the early-1960s.

These water fleas produce ‘‘ephippia containing embryos’’, notably tough eggs that can survive harsh conditions. They float on water surfaces for long periods and adhere to fishing equipment, skin, fur, feathers and clothing.

It is possible the earliest arrivals came on these surfaces and then detached in New Zealand lakes. This is probably how the water fleas moved around New Zealand as well. These species are tiny and best seen with a microscope.

Zhiqiang Ye and colleagues also wondered if the fleas came over in a cargo of live salmon. Chinook salmon were brought to New Zealand from California starting in 1901.

But the molecular data suggested an early-1960s date, a time of increasing recreational angling and international tourism.

Water samples were collected from 14 New Zealand lakes, including Lake Kapoai near Dargaville. The other 13 were from the South Island and included lakes Coleridge, Ha¯ wea, Pu¯ kaki, Tekapo, Wakatipu and Wa¯ naka.

Dr Carolyn Burns, of Otago University, and Dr Michael Lynch collected the South Island samples in 2018. Both are experts in these fields, especially daphnia.

These species are tiny and best seen with a microscope.

Burns is one of New Zealand’s most-honoured scientists and Lynch runs the world-renowned Biodesign Center for Mechanisms of Evolution in Arizona.

Zhiqiang Ye led the work on the New Zealand samples and identified the South Island samples as a sub-species called Daphnia pulicaria Forbes. The North Island samples were hybrids of that species and New Zealand natives.

It is not known how the native water fleas got to New Zealand, but they have been genetically linked to a population in Japan. These water fleas are thought to have invaded Japan between 680 and 3400 years ago.

Again it is not known how that happened, but there has been speculation that ‘‘very rare or extreme events, such as a volcanic eruption, caused migratory birds to alter their flyways and destinations’’, Zhiqiang Ye said.

The tough eggs might have adhered to their feathers and later detached somewhere novel.