Thousands of Moon Jellies were launched as part of the Spacelab Life Sciences-1 mission to test the limits of biological adaptation. This nine-day orbital trip aimed to understand how development and orientation are affected by the lack of gravity. The experiment offered valuable insights into the vulnerability of gravity-sensing systems in a weightless environment. Their ultimate “survival” depended on their ability to function normally once they returned to a gravitational environment.
The Reason Jellyfish Were Selected for Spaceflight
In 1991, the Space Shuttle Columbia carried the Spacelab Life Sciences-1 (SLS-1) module on the STS-40 mission, the first dedicated solely to life science research. The mission included over 2,000 Moon Jellyfish (Aurelia aurita), primarily in their larval polyp stage. These organisms were chosen because their balance-sensing mechanisms are analogous to the inner ear structures found in humans.
Why Jellyfish Development Was Studied
Researchers studied the rapidly developing jellyfish to observe microgravity’s effects on gravity-sensing organs from their earliest stages. The short life cycle allowed observation of a complete transition from polyp to juvenile medusa (ephyra) during the nine-day mission. This experiment sought to gain insight into the causes of space motion sickness in astronauts.
The Biological Mechanism for Gravity Sensing
Jellyfish perceive orientation using specialized sensory structures called rhopalia, located around the rim of their bell. These organs regulate the animal’s swimming rhythm and sense of balance. Within each rhopalium is a statocyst, a fluid-filled sac containing a dense, solid mass known as a statolith.
How Statoliths Work
Statoliths are composed of calcium sulfate crystals that act as the animal’s internal plumb line. On Earth, gravity causes these crystals to settle, pressing against sensory hairs within the statocyst. This pressure signal informs the jellyfish which way is down, allowing it to maintain its vertical position. The statolith mechanism is functionally similar to the otolith system in the human inner ear.
Developmental Observations in Microgravity
During the spaceflight, larval polyps successfully underwent strobilation, the asexual reproduction process that produces the free-swimming juvenile ephyra. This demonstrated that microgravity did not inhibit the fundamental biological steps required to transform the polyp into a medusa. The newly formed ephyra developed a full complement of rhopalia, suggesting the overall morphological development of the gravity organ appeared normal.
Structural Changes in Statoliths
Closer inspection revealed structural differences in the gravity-sensing parts. Ephyrae developed in orbit exhibited a statistically higher number of statoliths per rhopalium compared to Earth-bound control groups. This suggested that the body’s regulatory mechanisms for mineralizing the calcium sulfate crystals were altered in the absence of gravity. Despite this anomaly, the space-developed ephyrae were observed to pulse and swim effectively in orbit, though their movement was circular since there was no “down” to orient toward.
Functional Impairment After Re-entry
The challenge for the space-developed jellyfish began upon their return to Earth’s gravity. While the organisms were structurally intact, they were functionally impaired when subjected to the one-gravity environment. Ephyra that had never experienced gravity during the development of their balance organs were severely disoriented after landing. These jellyfish exhibited a persistent swimming abnormality known as “looping syndrome,” struggling to orient themselves and often swimming in circles or spirals. This functional failure confirmed the profound impact of gravity on the proper calibration of a developing sensory system.