Jellyfish tell up from down through calcium sulfate crystals that ring the bottom edge of their mushroom-like bodies. The crystals are housed in little pockets lined with hair cells, and when the jellyfish moves, the crystals roll around, signaling to the brain which way is up by stimulating those hair cells. The pockets seemed to develop normally in space, but the astro-jellies later had trouble figuring out how to swim around in normal gravity. They had abnormal pulsing and movement when returned to Earth compared to non-astronaut jellyfish.