Effect of microgravity on the swimming behaviour of larvae of the starfishPisaster ochraceus

Abstract

Swimming larvae of the starfish Pisaster ochraceus rotate around their animal–vegetal axis every 2–5 s ( 12–30 rpm) and exhibit two patterns of swimming behaviour. They may swim with the animal pole forward in any direction or orient vertically (animal pole upward) and remain relatively stationary. Vertically oriented larvae adjust the pattern of rotation so that they present a larger surface area to gravity (holding behaviour). To determine how gravity is involved in the development and pattern of larval swimming behaviour, Pisaster larvae were raised in the aquatic research facility aboard the NASA space shuttle Endeavour (Mission STS 77). Control larvae raised in 1 g appeared to orient themselves along the gravity vector and to exhibit holding behaviour. Tracks of the larval swimming-pattern studies with a motion-analysis system demonstrated that larvae raised in microgravity swam in randomly oriented straight lines or broad arcs. Some of the tracks exhibited oscillations with a period of 2–5 s, while others did not. The results suggest that the holding behaviour, which normally serves as a response to gravity, develops despite the absence of gravitational clues. Possible mechanisms that the larvae may use to orient to gravity are discussed.