2009 MAY BE REMEMBERED as the year of the jellyfish. In newspapers and magazines around the globe, headlines heralded news of "outbreaks," "attacks" and "plagues" of these gelatinous sea creatures, which seemed to be stinging swimmers, fouling fishing nets and crippling power plants everywhere from Sri Lanka to Scotland, Australia to Japan and Mexico to Massachusetts.

Most scientists who study jellyfish decried the sensationalism, noting that the animals biology, combined with unpredictable ocean currents, make periodic jellyfish blooms a natural phenomenon. Yet even some of the experts have begun to wonder if something unusual is going on. "In certain coastal waters, there seem to be more jellyfish than there once were," says Jennifer Purcell, a marine scientist at Western Washington University who has studied jellyfish and their kin for nearly three decades. A report from the National Science Foundation (NSF), released in December 2008, goes further. "In recent years, massive blooms of stinging jellyfish and jellyfish-like creatures have overrun some of the world's most important fisheries and tourist destinations," states the document. "Worldwide reports of massive jellyfish blooms are triggering speculation that jellyfish swarms are increasing because of human activities."

For now, most scientists say that view remains "speculation" at best. "Because fisheries researchers traditionally considered jellyfish as by-catch, and not really important to ecosystems, the animals are poorly studied," says Purcell. "We have very few long-term records of jellyfish populations."

million years, "true" jellyfish are invertebrates in the phylum Cnidaria. Scientists recently have begun to realize that the animals, along with a dizzying diversity of their gelatinous cousins, play critical roles in ocean ecosystems. As highly efficient predators, jellyfish eat vast quantities of fish eggs and larvae as well as zooplankton fed upon by adult fish. As prey, gelatinous creatures sustain a variety of species, including seabirds, salmon, sea turtles and other jellyfish. "If you don't understand the ecology of jellyfish, you're missing an important component of marine ecosystems," says Jack Costello, a marine biologist at Providence College in Rhode Island.