The Fish That Spun, Flipped, and Laid Eggs Aboard China’s Space Station

They were fish out of water. Swimming in a fishbowl 380 kilometers above Earth, they flipped, spun, and struggled to find their bearings — until they adapted. For 43 days in 2024, four zebrafish orbited China’s Tiangong space station, surviving in space longer than any fish before them.

Last April, the zebrafish traveled aboard the Shenzhou-18 spacecraft, becoming the first residents of the “Tiangong fishbowl.” There they lived in a closed aquatic ecosystem, shattering the previous space survival record of 16 days — set by German scientists studying swordtail fish.

In 2025, China is sending more, with another six zebrafish set to return to orbit as scientists expand the experiment. They will study how vertebrates adapt to microgravity, a key step toward sustaining human life in space.

Wang Gaohong, a researcher at the Chinese Academy of Sciences and lead scientist of the experiment, said the zebrafish’s time in space was far more significant than it seemed. “The typical lifespan of zebrafish is two to three years. Their 43 days in space is equivalent to a human spending three to four years there,” Wang told state-run Xinhua News Agency.

A small tropical freshwater fish measuring 3 to 5 centimeters, the zebrafish — so named because of its blue-striped body, which gives it the appearance of a galloping zebra when swimming in schools — is a key model organism for biological research.

Rapid reproduction makes zebrafish ideal for genetic studies — a single pair can produce over 300 eggs per spawning. With over 70% genetic similarity to humans, zebrafish are widely used in developmental biology and disease research. Scientists call them “mice in water,” Wang explained.

Unlike mice, however, zebrafish are better suited for space experiments because they don’t produce solid waste or release gases that require complex filtration systems.

“Scientists will focus on studying the effects of microgravity on proteins in the muscles and bones of vertebrates,” Wang said, discussing the second “Tiangong fishbowl” experiment. These findings could extend the duration of human space missions and lay the groundwork for interplanetary habitation.

The study also contributes to sustainable life support in space. “Aquatic creatures like fish and shrimp, with their high feed conversion rates, could be an ideal source of protein,” Wang added.

Despite their genetic similarities to humans, zebrafish were never meant for space.

Twenty days into their orbit, the fish showed signs of disorientation. In the weightless environment, they flipped upside down, spun in circles, and swam erratically — struggling to find stability in microgravity. But over time, they adapted.

The “zebrafish astronauts” underwent rigorous selection, much like their human counterparts. “The four zebrafish that entered the space station were selected from 200 candidates. They need to be adults in good health, and they also need to have a stable temperament and be compatible with one another,” Wang told Xinhua.

Once in orbit, they entered a self-sustaining world — a sealed, 1.2-liter tank, about the size of two water bottles. “Aquatic plants produce oxygen through photosynthesis to supply fish respiration, and fish excrement provides nutrients to the aquatic plants,” Wang explained, adding that the fish and plants acted as both producers and consumers.

Scientists also developed a specialized feeder to sustain them in space: Using a syringe, astronauts delivered toothpaste-like “space meals” to the zebrafish. The fish not only survived beyond researchers’ expectations but also successfully laid eggs, marking a breakthrough in studying reproduction in microgravity.

On Nov. 4, the zebrafish eggs, along with water samples collected at three different stages, were brought back to Earth by the Shenzhou-18 crew and are now undergoing further scientific analysis.

Fish have a long history in space research. In 1973, the mummichog became the first fish to orbit Earth aboard the American Skylab station. Three years later, zebrafish followed, sent to space aboard the Soviet Salyut 5. Cosmonauts observed behavioral changes as the fish adjusted to microgravity. More recently, in 2015, Japan used 18 zebrafish in muscle research, offering insights into astronaut health management.

As for the record-breaking fish themselves, they made their final journey differently. To prevent decomposition aboard Tiangong, astronauts transferred them to a cargo spacecraft, which burned up on reentry — going out not with a splash, but in a flicker of flame.

Editor: Apurva.

(Header image: A zebrafish bred by the National Zebrafish Resource Center of the Institute of Hydrobiology, Nov. 3, 2024. From @湖北日报 on Weibo)