The bell jelly is not a true jelly but rather a hydromedusa. Usually smaller than true jellies and not nearly as colorful, a bell jelly has a translucent bell and a clapper-shaped mouth stalk.
Bell jellies also have 100 or more wispy tentacles, and red ocelli, or eyespots, which are sensitive to light. Bell jellies are probably vertical migrators—they remain in dark, deep waters during the day and come to the surface at night.
A bell jelly spends about half its time near the seafloor, where it feeds on small benthic (bottom-dwelling) creatures. Bell jellies often "hop" up from the seafloor; this movement may stir up sediments, uncovering potential food. They also swim to the surface and sink back down to the seafloor, a process that lets them collect plankton as they drift.
Bell jelly populations are dwindling, possibly due to dredging, pollution and collection for neurobiological research.
Bell jellies used to be abundant in bays and estuaries along the West Coast. But their nearshore seafloor homes have been disturbed by dredging, urbanization and pollution runoff. Jelly populations, especially hydromedusae, are declining in heavily impacted coastal areas. To understand the causes of population increases and decreases, we must know more about the life cycles of these animals.
Bell jellies respond to bright lights at night (such as camera flashes) and quickly swim away.
Since their red coloration disappears in deep water where light is low, bell jellies are virtually invisible to deep sea predators.