The siphonophores we're studying also have a gas-filled float, but it's much smaller, and they drift beneath the surface. Siphonophores trail long strands of venomous tentacles, which snare and paralyze smaller zooplankton. These get drawn into the digestive parts of the colony. During the day, most siphonophores are down deep, but at night they're found near the surface. Although they can't swim against currents, they can move up and down in the water.
Many zooplankton also undertake a daily vertical migration.
Each day, many species undertake daily migrations that are exhausting in human terms. Just before sunset, they start to swim up from the deep water where they spend the most of the day. One patch was about 33 feet down, maybe caused by small snail-like animals called pteropods. These animals have hard shells that can produce a very strong echo. The other patch was much deeper and probably indicated siphonophores.