Stimulus dependence of the development of the zebrafish (Danio rerio) vestibular system

It has been suggested that stimulus dependence is a general feature of all developing sensory systems. We tested this idea for the developing zebrafis h vestibular system using a bioreactor the National Aeronautic and Space Ag ency designed to simulate microgravity for cells in culture on earth. We re placed the culture medium with aquarium water and maintained zebrafish eggs /hatchlings in the bioreactor for either 72 or 96 h postfertilization. Thes e experimental animals displayed a swimming behavior that was indistinguish able from the control animals when illuminated from above. However, when il luminated from below, experimental animals swam not only dorsal surface up, but also lying on their side; they corkscrewed, swam vertical loops, and o ccasionally even swam upside down. When incubated in the bioreactor for 96 h, the saccular otolith was significantly smaller than normal, suggesting t hat otolith development was either delayed or slower than normal. When incu bated in the bioreactor for 72 h, some animals were missing one or more oto liths. In contrast, control animals all had two otoliths on each side. This supports the idea that otolith development was delayed. Immediately upon r emoval from the bioreactor at 96 h, experimental animals showed some signs of compensatory eye rotation, but with a much less clear relationship betwe en the orientation of the eye and the direction of gravity than the age-mat ched control animals. This difference was still obvious 1 day later. These results support the idea that development of the vestibular system in zebra fish is dependent on the presence of the normal stimulus the system is desi gned to detect, (C) 1999 John Wiley & Sons, Inc.