Long‐distance horizontal migrations of zooplankton (Scyphomedusae: Mastigias)1

Perennial swarms of sexually immature medusae of the germs Mastigias occur at high densities in three marine lakes in Palau, Western Caroline Islands. Numbers per lake are 1.76 ± 0.10 × 105, 1.61 ± 0.17 × 106, and 4.69 ± 0.51 × 106. Local density within a swarm may exceed 1,000·m‒2. Behaviorally generated swarms form in the early morning and late afternoon and are temporarily dispersed by directional horizontal migration of the entire swarm twice a day to opposite ends of the lake. Distance swum each day is up to 1 km, depending on the size of the lake. Compass direction of migration is different in each lake and appears to depend on both light and basin morphometry.