Glowing Spacefish Join Crew Aboard the ISS

An experiment to study bone density in space uses transparent “fishonauts”

Spacefish! The medaka fish aboard the ISS are genetically modified so certain cells glow red or green. JAXA

Thirty-five new occupants arrived at the International Space Station in late October. Three were astronauts, the rest were fish.

“This is the first experiment in the world to take care of animals for such a long time in the space station — for two months,” says Akira Kudo of the Tokyo Institute of Technology. “Normally, animals are cared for for just two weeks. Only astronauts stay longer than that.”

Kudo is the principal investigator for a study called Medaka Osteoclast, or MOST, examining how the bones of the medaka fish — also known as Japanese killifish, which are popular both as pets and research animals — will respond to microgravity. (Medaka fish were the first vertebrates to mate in space; four of them successfully laid and hatched eggs in an experiment aboard Columbia in 1994.)

The fish are living in a specially designed space aquarium called the Aquatic Habitat, partitioned into two, 1.5-pint sections. Housed in the Japanese Kibo module, the habitat has temperature control, water circulation and bacterial filtration systems, and an oxygen supply from a modified artificial lung machine. It also has an automatic feeder — no fish flakes floating around. Like diligent home aquarists, astronauts have to test and clean the water twice a week for the first three weeks, then three times every 14 days after that.

The Aquatic Habitat aboard the ISS houses the spacefish.

Japanese astronaut Aki Hoshide started the experiment by sacrificing and preserving eight of the fish in a stabilizing solution as controls, and moving 16 more from the transport unit into the Habitat. Today, after two weeks of swimming in microgravity, Hoshide removed six more medakas and preserved them in a type of formaldehyde; they’ll return with the astronauts next week on the Soyuz. Other station crew members will care for the remaining 10 fish, preserve them after 60 days, and send them back to Earth on a SpaceX Dragon capsule. Kudo plans to dissect them in ultra-thin slices to examine their bone densities.

His main goal is to understand the formation of osteoclasts, cells that absorb bone, and how microgravity affects the interaction between these and osteoblasts, bone-forming cells. Scientists already know that bone density decreases in space, and Kudo suspects it has to do with increased osteoclast production.

Medaka fish are particularly useful for this study because they’re transparent, which allows easy viewing of their bones and organs. They are also easy to modify genetically: those aboard the station have fluorescent proteins that cause osteoclasts to glow green and osteoblasts to glow red. (How nice that they’ll be aboard for Christmas!)

The aquarium also is set up for observation. Astronauts and scientists on the ground are able to watch the fish swim in loops, rather than in straight lines, because there’s no sense of up or down to orient them. The medaka are  rapid breeders, so there’s a strong possibility for fish fry (fish babies, that is, not a dinner buffet) in space, up to three generations in the time they’ll be aboard — that would be a first for space fish. Further experiments will study organ formation, and the aquarium is also designed to house frogs.

At JAXA’s Tsukuba Space Center, Kudo can watch a live video feed to check whether the fish are swimming and eating normally. The medaka are already of great interest to the six space astronauts, who can look in on the fish as they go about their work. Kudo says: “We call them fishonauts.”

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