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THE EFFECTS OF MICROGRAVITY ON THE SWIMMING BEHAVIOUR OF STARFISH LARVAE.   B.J. Crawford and D.L. Jackson. Dept. of Anatomy, U.B.C. Vancouver, B.C. Dept. of Biology, Dalhousie University, Halifax, N.S. Canada.

During the course of early development, embryos/larvae of the starfish Pisaster ochraceus develop distinct swimming patterns. During and shortly after hatching the embryos tumble about on the bottom of the culture dishes and make short excursions into the water column. By the early-to-mid gastrula stages healthy embryos swim in straight lines through the water column and rotate on an animal vegetal axis which passes through the long axis of the embryo. Once the mouth has formed, the larvae, which are heavier than water, spend a lot of time oriented vertically in the water column with the expanded anterior end upwards. When this behaviour is occurring, the larvae change the pattern of rotation. The axis of rotation still passes through the posterior part of the embryo but the anterior end describes a wide circle around the rotational axis, thereby allowing the animal to present a larger surface area to resist gravity (parachute behaviour). Thus, the later swimming behaviour may enable orientation to gravity despite the fact that the larvae have no obvious organ for detecting it. In the vertical orientation the preferred direction of rotation is clockwise (CW). When swimming horizontally the larval rotation is primarily CW but they will change from CW to antiCW and back.  The swimming behaviour of larvae raised in the Aquatic Research Facility aboard STS-77 was recorded and analysed using a PC-based image analysis system (OptimasЖ 6.0). Larvae raised in a 1G centrifuge appeared to orient themselves along the gravity gradient and to exhibit parachute behaviour. Larvae raised in µG swam in randomly oriented tracks, the majority of which were straight. Many of these tracks exhibited oscillations with a period of 2-5 seconds which corresponds to the period of rotation of the larvae exhibiting parachute behaviour in controls. The results confirm that these organisms have a method of detecting gravity which may be due to differences in density in different regions of the embryo. It also suggests that the parachute behaviour is innate and can develop without gravitational clues.

(Supported by Canadian Space Agency.)