In 1973, NASA sent two fish into space to test their swimming ability in zero gravity; they initially swam in loops but later used light as cue to move normally after 21 days

When NASA launched the Skylab 3 mission in July 1973, astronauts were not the only passengers being studied. The spacecraft also carried two small mummichog minnows and 50 fertilised eggs as part of an experiment into how living creatures maintain their sense of direction in weightlessness.

Scientists chose the fish because their vestibular system, which helps an animal maintain balance and orientation, has similarities to that of higher animals. The experiment aimed to see what happened when the fish lost the constant pull of gravity that normally helps them determine which way is up and which way is down.

The fish travelled in a plastic aquarium that recreated some features of their natural environment. NASA gave the container a dark background and a lighted surface so the fish could still use visual cues while in orbit.

The fish started swimming in strange loops

The first few days in space showed just how much the fish relied on gravity.

After about three days in orbit, the astronauts opened the aquarium and observed the fish swimming in unusual circular patterns. They moved sideways in loops while keeping their backs toward the light. The loops varied in size, and the fish moved in both left and right directions.

NASA researchers linked the behaviour to the fish's difficulty in interpreting signals from their vestibular system in the absence of gravity.

On Earth, tiny structures called otoliths help fish sense movement and orientation. Gravity normally acts on these structures, providing information that helps the animal maintain its position. In weightlessness, that familiar signal changes dramatically.

The researchers therefore saw the looping behaviour as a sign that the fish had temporarily lost their normal sense of orientation.

After 21 days, the fish adapted to weightlessness

The unusual swimming did not continue indefinitely.

NASA's later analysis found that the looping gradually became less frequent during the mission. By the 21st day, the two fish appeared to have adapted to weightlessness and had returned to a much more normal swimming pattern.

But the fish had not simply regained their Earth-based sense of direction. Instead, they appeared to find another way to work out their orientation.

The fish consistently kept their backs toward the light, suggesting that they had begun relying on visual information as a substitute for the gravitational cue they normally used. NASA's experiment records describe this visual orientation as an important part of how the fish adapted to their new environment.

The fish could still return to looping behaviour if astronauts shook the aquarium, showing that their adaptation was not exactly the same as having a normal sense of balance.

The fish eggs revealed another difference

The experiment also included 50 fertile eggs, allowing scientists to observe fish that developed in space rather than adapting after growing up under Earth's gravity.

The eggs began hatching after about 19 days, with most hatching during the fifth and sixth weeks of the mission. That happened roughly two weeks later than the control eggs kept on Earth.

The newly hatched fish immediately oriented themselves with their backs toward the light. Unlike the older fish, however, they did not show the same initial disorientation unless astronauts shook the aquarium.

NASA later noted that juvenile fish initially showed obvious disorientation in space but gradually adapted to weightlessness and relied increasingly on visual cues for orientation.

What the experiment showed

The Skylab fish experiment gave scientists a glimpse into how animals can adapt when one of their most familiar environmental signals suddenly disappears.

The fish initially struggled because their vestibular system could no longer respond to gravity in the usual way. But over time, they learned to rely more heavily on what they could see, particularly the direction of the light, to maintain their orientation.

The experiment also became useful for later research. NASA's records note that the observations from Skylab helped lead to further experiments, including studies during the Apollo-Soyuz mission, to investigate vestibular disturbance and fish development in space.

Skylab 3 launched on July 28, 1973, and its crew spent about 59 days aboard the space station. Alongside the two fish and their eggs, the mission also carried spiders, mice and fruit flies for other biological experiments.