Zebrafish aboard China’s Tiangong space station were reported swimming upside down, rotating and circling during a 43-day experiment. “Confused” is an informal way to describe these unusual orientation behaviors in microgravity—not a diagnosis, and not proof the fish were distressed. The same experiment also saw the fish spawn, as researchers studied how animals behave and how a small aquatic ecosystem cycles resources in space.
Why were the fish swimming upside down?
In microgravity, the familiar gravity cue that helps animals orient themselves as “up” or “down” is absent. China’s early account described inverted swimming, rotation and circling as abnormal directional behaviors. Wang Gaohong, a researcher with the Chinese Academy of Sciences, explained: “Like humans, fish cannot discern up from down in microgravity, leading to disoriented movement.” Chinese Academy of Sciences, 15 January 2025.
That helps explain the headline’s “confused” wording, but the reports do not establish that the fish experienced distress or a diagnosed condition. They document observed movements and offer an explanation tied to orientation in microgravity.
What was aboard the space station?
Shenzhou-18 launched on 25 April 2024 with four zebrafish and four grams of hornwort, an aquatic plant also known as Ceratophyllum. The fish and plant were transferred to a small life-ecology experiment facility in the Wentian module. China’s Manned Space Agency says the in-orbit experiment ran for 43 days and that the fish spawned during it. China Manned Space Agency, 2025
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The facility controlled conditions including temperature and light. Its design linked plant and fish activity: the plant photosynthesized and supplied oxygen, while the fish used oxygen and their waste contributed nutrients for the plant. Researchers used this arrangement to investigate animal behavior and material cycling in a confined aquatic system.
What did the experiment show—and what did it not show?
It recorded fish behavior and spawning in orbit
The reported findings include inverted swimming, rotation and circling, as well as spawning during the 43-day run. These observations offer researchers material for studying vertebrate development and behavior under microgravity. The Chinese Academy of Sciences described those research aims.
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It tested a small ecosystem, not a permanent habitat
The plant-fish cycle was a controlled, small-scale research system. Its operation does not establish that a large habitat could run indefinitely without resupply, nor does it demonstrate long-duration life support for people or determine health outcomes for astronauts. Those are broader goals for space biology and life-support research, not conclusions proven by this short experiment. Chinese Academy of Sciences; China Manned Space Agency.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What happened to the samples, and what comes next?
A November 2024 account said water samples from the experiment returned to Earth aboard Shenzhou-18 on 4 November for study. China Manned Space Agency, November 2024.
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In January 2025, the Chinese Academy of Sciences reported plans for a follow-on experiment using six zebrafish and six grams of Ceratophyllum to study bone- and muscle-related proteins. That report described a plan; it does not establish that the follow-on later flew or what it found. Chinese Academy of Sciences, 15 January 2025.
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