GRAVITY-DIRECTED ORIENTATION OF POSTGASTRULAR DEVELOPMENT AND EMERGENCE OF PRENAUPLII FROM CYSTS OF ARTEMIA (ANOSTRACA)

Prenauplii of brine shrimp hatched from cysts adhered to a stationary substratum emerged upward (from the top of the cyst) instead of random ly, The direction of light and point of cyst adhesion during postgastr ular embryogenesis had no effect on this direction of emergence, which was 76% upward, 23% sideways, and 1% downward (P = 0.168), suggesting a gravity effect. In cysts incubated and hatched without a constant o rientation to gravity prenauplii emerged in random directions. The emb ryos of stationary cysts, including decapsulated cysts, were motionles s during postgastrular development until final emergence (late E1 stag e). Therefore, gravity directs orientation of the embryo during develo pment in stationary cysts and dictates a head-up orientation of the pr enauplius prior to emergence.