Depending on whom you talk to, jellyfish are either fascinating, a nuisance, a toxic menace, or some combination of the above.

Jellyfish plop into the media spotlight when their presence causes beach closures, or when an unlucky swimmer meets a jelly's toxic tentacle. They stimulate debate among scientists: some say that rising numbers of jellyfish are a sign of climate change and pollution, since the animals thrive in warmer, more acidic waters. Others say we don't know enough about their natural cycles to blame population booms on human activities.

Still others say there's no such thing as a general jellyfish trend, because each species behaves differently.

"Any statements such as 'warming oceans will mean more jellyfish' are suspect in my book," says Allen Collins, a zoologist with NOAA's National Marine Fisheries Service. "That would sort of be like saying that greater urbanization is going to lead to greater populations of birds. It may be true for pigeons, starlings or English sparrows, but it certainly isn't true for wood peckers or owls, or for birds in general."

Scientists are also still trying to identify and catalogue all the jellyfish species out there. There are four classes of jellyfish: true jellies, box jellies, stalked jellies, and lookalike jellies. True jellies (scientists call them Scyphozoa) have tentacles dangling from all around their mushroom-shaped bells. Box jellyfish (known as Cubozoa), have one or more tentacles extruding from the corners of their cube-shaped bodies. Many of these jellies have venomous stings that can land an unsuspecting swimmer in the hospital. Stalked jellyfish (called Staurozoa) are trumpet-shaped and live attached to surfaces by a stalk instead of swimming free.

Then there are jelly lookalikes. The Portuguese man-o-war (Physalia physalis), for example, resembles a jellyfish, but it’s actually a colony of creatures known as siphonophores. Like box jellies, some of these look-alike species (called Hydrozoa) have venomous stings.

Taxonomy, or the science of classifying living things, is fundamental to jellyfish research, says Bastian Bentlage. A graduate student in invertebrate zoology at the University of Kansas, Bentlage spends a lot of time classifying jellyfish and identifying species. He says we can learn a lot about a species by figuring out where it belongs in the jellyfish family tree—no easy task, even for the experts.

Jellyfish are challenging to study. They are found around the globe, including the Arctic and the seas around Antarctica, and it would take an army of scientists to identify, tag, and plot the migration patterns of all these flimsy, soft, transparent creatures.

But modern technology is helping. Citizen science projects like the Monterey Bay Aquarium Research Institute’s JellyWatch, and EcoJel in Ireland call on beach-goers to report jellyfish sightings. The data and images submitted are used to map where and when various jellyfish species can be found.

Steven Haddock, co-founder of JellyWatch, hasn’t had any problems with identifying species that citizens have spotted. By combining multiple reports from the same region with what is already known about the area, he can deduce which species has been spotted. Getting reliable information during seasons when jellyfish are not as visible is more challenging.

"You don't know if a lack of information is a lack of observers or the fact that there aren't jellyfish in that area," says Haddock. "This is a similar problem to some that we face when dealing only with newspaper reports as an indicator of abundance: you only hear about it when they are abundant enough to get noticed."

The work of scientists and conscientious beach-goers is a step forward in understanding jellyfish. Rather than pronouncing them dangerous invaders, the latest research is in favor of getting to know jellyfish and the roles they play in the global ecosystem.