The zebrafish that accompanied the Chinese astronauts on last year's Shenzhou XVIII mission set a record for being the longest surviving fish in space, according to the professor researching them.

Sun Yonghua, director of the China Zebrafish Resource Center of the Institute of Hydrobiology at the Chinese Academy of Sciences in Wuhan, Hubei province, said the longevity represents a breakthrough in the cultivation of vertebrates and aquatic plants in space.

"It is the first time that zebrafish have survived for 43 days in such a closed aquatic ecosystem," said Sun, adding that the completely enclosed aquarium enabled unobstructed observation aboard China's Tiangong space station.

On April 25, two male and two female zebrafish along with hornwort aquatic plants from the institute were taken on the Shenzhou XVIII manned space mission to China's space station.

On Nov 4, the crew returned to Earth with their samples.

At the station, astronauts collected water samples and ensured the zebrafish were fed, and observed abnormal behaviors such as the fish swimming upside down, rotating and circling in the microgravity environment, Sun said.

"Previously, German scientists set a record of 16 days for what they called a closed equilibrated biological aquatic ecosystem, but they conducted experiments using swordtail fish," he added.

Scientists and researchers from institute have been using recovered water samples and other samples from closed aquatic ecosystems to analyze the impact of the space environment on the growth, development and behavior of vertebrates, according to Sun.

"Prior to this, the Chinese space station had already achieved the full life cycle cultivation of plants. And this launch of sending zebrafish into space is the first long-term cultivation of vertebrates in the Chinese space station, laying a scientific foundation for our future research on aquatic biology and life medicine in space, and providing scientific support for long-term human stays in orbit and deep space exploration," Sun said.

Wang Gaohong, a researcher at the institute, said that like humans, fish also face adaptability issues when surviving in a space environment.

"Researchers have worked together to create a comprehensive ecosystem, where aquatic plants produce oxygen through photosynthesis to supply fish respiration, and fish excrement provides nutrients to the aquatic plants," he said, adding that they are both producers and consumers, forming a seemingly small and simple but actually complex scientific experimental system.

Sun said the food that zebrafish eat in space was specially developed by the institute.

"The way zebrafish swim and hunt is very different from that on Earth, so the food is like a toothpaste. And it squeezes out a little bit inside for the zebrafish to eat, trying not to give them extra feed as it may worsen the water quality," he said.

Zebrafish are common ornamental small fish, with body lengths from 3 to 5 centimeters, and are named after the blue stripes covering their bodies. They are lively and active, swimming in groups.

Zebrafish reach sexual maturity in three months from birth, and can reproduce throughout the year. A pair of healthy zebrafish can lay over 300 eggs at a time.

"When we take zebrafish to space, we are likely to see them produce the next generation," Sun said.

The professor said zebrafish are a promising model animal, describing them as "mice in the water".

Scientists and researchers are looking into how to reproduce zebrafish in space. The genome similarity between zebrafish and humans is as high as 80 percent, making them an important model for research into human diseases, he said. More than 500 laboratories across the country are carrying out zebrafish and related research and studies, Sun said.

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