Dept. OtolaryngologyHead & Neck Surgery, Univ. Texas Hlth. Sci. Ctr., San Antonio
Gravity and linear acceleration are sensed in fish by a saccule, utricle (as in mammals) and a lagena, each with a solid otolith. As in mammals, the pull of gravity or linear acceleration on the otolith either depolarize or hyperpolarize the hair cells, changing the firing rate in vestibular nerve fibers which send a coded representation of the stimulus to the central nervous system. Previous experiments in which eggs or larvae of a marine mollusk (the sea hare Aplysia) or fish larvae were raised on a centrifuge, demonstrated that the size of the otolith or statoconia (in Aplysia) were reduced, in a graded manner, as the g-field was increased, suggesting that some control mechanism was acting to "normalize" the weight of the mass. In the experiments described here, pre-mated adult female swordtail fish (Xiphophorus helleri) were flown in the CEBAS aquarium system on STS-89 and 90 (Neurolab). Developing larvae were removed from the adult ovaries after the shuttle landed.
The size of otoliths was compared between ground- and flight-reared larvae of the same size. For later-stage swordtail larvae, with spine lengths from 3 to 6 mm from STS-90 (16 days), the growth of the otolith with increasing spine length was significantly greater in the flight-reared fish for all three otoliths, from the saccule (saggita), utricle (lapillus) and lagena (astericus). However, juvenile fish, 1 cm long at launch, showed no significant difference in otolith size between flight- and ground-reared animals. In very early stage larvae from STS-89 (9 days), with spine length of 1.5 to 3.5 mm, the utricular and saccular otoliths were actually larger in the ground-reared larvae. Thus, it appears that late-stage fish larvae reared in space do produce larger-than-normal otoliths, apparently in an attempt to compensate for the reduced weight of the test mass in space. However, the results from very early-stage larvae and juvenile fish suggest that there is a fairly short critical period during which altered gravity can affect the size of the test mass. Recent work on the development of the otolith organs in the zebrafish suggest that the inability of the early stage embryos to react to reduced gravity is due to the formation of the otolith before the sensory hair cells are able to respond and thus assess the weight of the otolith. (Supported by NASA: NAG2-952 and NSF: IBN-9529136)