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Snails, Fish and Gravity Sensors: On a Mission to Find Answers

May/June 1998

This April, dozens of snails and fish went where only a few men and women have gone before: outer space. Traveling on the Space Shuttle Columbia, the snails and fish were part of a research project funded by NSF to study the development of gravity sensors in space by animals in the early stages of life.

The snails and fish flew aboard Neurolab, a shuttle research mission dedicated to the study of the life sciences. Neurolab focused on the most complex and least understood part of the human body--the nervous system, which faces major challenges in space.

Gravity-sensing systems have the same basic structure in all vertebrates, whether fish or humans. The gravity-detecting organ is lined with sensory cells that send signals to the brain when they are triggered; the "triggers" are small, rock-like particles of calcium carbonate, referred to as statoliths in snails and otoliths in fish (and in humans). In humans, this system is a component of the inner ear. A simpler system exists in snails, one that is far easier to analyze and develops faster, say scientists.

"Data from the aquatic experiments aboard Neurolab are helping us understand the basic mechanisms that maintain postural and spatial orientation," explains Christopher Platt, program manager in NSF's Division of Integrative Biology and Neuroscience, which funded the experiments. "Gravity is always present on Earth, so it's been hard to explore its role in development and in controlling movement. Neurolab allowed unique tests to take place, ones that will shed light on how gravitational sensors work by showing the changes in sensory structure or nerve function that appear in its absence. These studies may tell us how long exposure to lack of gravity may lead to abnormalities in the otolith organs, which is relevant to long-term spaceflight and to certain kinds of posture and balance problems in people on Earth."

Other benefits of the aquatic studies aboard Neurolab include development of an electrode that offers potential use as a connection to the nervous system in people with deafness caused by hair cell damage. The electrode might also someday be used as an interface to signal motor prostheses how and when to move.

Tracking the progress of the snails and fish flying aboard Columbia were scientists on "The Aquatic Team," as they were known to shuttle crewmembers. Researcher Michael Wiederhold of the University of Texas Health Science Center at San Antonio monitored freshwater snails and swordtail fish in the beginning stages of their development into adults.

Wiederhold is trying to answer such questions as what physiological changes occur in the components of the gravity sensors of animals in space, whether signals sent from the inner ear to the brain are altered, and, if alterations do occur, whether behavior of the animal changes. "Looking at changes that occur in organisms in space is very practical," says Wiederhold. "Long-term space missions and space exploration point to a time when it will be necessary to begin generating life--including human beings--away from the Earth. Studies such as those on Neurolab are, therefore, vital."

Upon return from their spaceflight in Neurolab, the freshwater snails and swordtail fish were compared to a control group on Earth to determine whether the size of their statoliths and otoliths increased while they were in "microgravity." On Earth, the pull of gravity eventually signals developing statoliths and otoliths to stop growing. "In space, however," says Wiederhold, "without this signal, they should develop to a larger size than they do on Earth. And if indeed they increase in size, how will that affect these animals? That's what we hope to discover through these experiments."

Scientist Steven Highstein of the Washington University School of Medicine in St. Louis, Missouri, is also studying aspects of the inner ear, but his research involves the inner ears of astronauts flying aboard Columbia, as well as those of oyster toadfish aboard Neurolab. Says Highstein, "The otolith organs of these fish experienced the removal of gravity at the same time as the otolith organs of the humans on board." Since this system is very similar in fish and humans, by recording data from the nerves of the otoliths in the oyster toadfish, researchers will soon be better able to understand changes in the same signals sent by the astronauts' inner ears as both humans and fish adapt to microgravity.

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