By all accounts, jellyfish are creatures that kill people, eat microbes, grow to tens of meters, filter phytoplankton, take over ecosystems, and live forever. Because of the immense diversity of gelatinous plankton, jelly-like creatures can individually have each of these properties. However this way of looking at them both overstates and underestimates their true diversity. Taxonomically, they are far more varied than a handful of exemplars that are used to represent jellyfish or especially the so-called "true" jellyfish. Ecologically, they are even more adaptable than one would expect by looking only at the conspicuous bloom forming families and species that draw most of the attention. In reality, the most abundant and diverse gelatinous groups in the ocean are not the ones that anyone ever sees.
To report sightings of jellyfish and other marine organisms, go to http://jellywatch.org/