Olindias tenuis is a shallow-water hydromedusa from the Caribbean that during the day is cryptic, residing among seagrass and algae, and after sunset swims into the water column. The medusae forage by swimming towards the surface, and then slowly drifting downwards with the exumbrella uppermost and the long primary tentacles hanging below. Positive buoyancy of the umbrella reduces the sinking rate. Both the activity cycle and buoyancy appear to be regulated by light, since the intensity of light alters the normal activity cycle both day and night. Feeding occurs once the medusae have entered the water column. At Puerto Rico, Olindias fed mostly on calanoid copepods; but chaetognaths, polychaetes, fish larvae, and amphipods are also consumed. Possible ""lures"" on the tips of the long, primary tentacles may offer both visual and vibratory stimuli to prey. The nocturnal emergence of Olindias may be adaptive for feeding on nocturnal, demersal plankton and for avoiding diurnal, visual predators.