Vertical migration and diel activity patterns of hydromedusae: studies in a large tank

Vertical distributions and swimming behaviors of 7 species of hydromedusae ( Aequorea victoria, Bougainvillia principis, Gonionemus vertens, Mitrocoma cellularia, Phialidium gregarium, Polyorchis penicillatus , and Stomotoca atra ) were observed over 24-h periods in a 2 m high, 1500 1 transparent tank. In this tank, most species performed well-marked diel vertical migrations that were mediated by swimming. Manipulation of the light regime showed the did swimming behaviors to be light-dependent rather than intrinsic, even in species that do not possess recognized photoreceptors. Correlations between vertical migration and spawning times for several species of medusae suggest that in cases when vertical migration reduces the distances between individuals (e.g., in mass movements to the surface) just prior to spawning, enhanced fertilization success may result from such movements.