Barrel jellyfish are found in Carmarthen Bay and Tremadog Bay
Scientists are tagging jellyfish to study their role in marine life off the Welsh and Irish coasts.
Although consisting of 98% water and some being the size of dustbin lids, biologists say they are "tough, robust animals", making tagging possible.
Around 10 Lion's Mane and 20 Barrel Jellyfish will be studied with particular interest in jellyfish as a food supply for leatherback turtles.
Experts from Swansea, Belfast and Cork will also examine how long they live.
Meanwhile, beachcombers who find any of the jellyfish washed up dead could claim a reward.
The research project is paying £25 for handing in the bright orange-topped tags, which are expected to begin washing up along the coast of Wales or the east coast of Ireland sometime over the next year.
The tags work as waterproof mini-computers.
Marine biologist Dr Jon Houghton said the idea "seems daft" but was necessary because experts know as much about jellyfish as they do about unicorns.
The study will take place at four sites
Researchers from three universities, Swansea, Cork and Queen's University Belfast, will target four sites - Rosslare and Dublin on the Irish coast and Carmarthen and Tremadog Bays in Wales.
Dr Houghton from Queen's University in Belfast told the Radio Four programme World on the Move that even though jellyfish are 98% water, researchers can attach the dive tags or time depth recorders to the jellyfish using a simple cable tie.
He said a boat draws along the jellyfish, some about the size of a dustbin lid and weighing more than 14lb (6.3kg), and researchers either net them or jump in the water to fix the tie by hand.
He said: "If you think what a jellyfish looks like, it's like a giant mushroom, so just underneath the cap of the mushroom there's quite a thick stem, and you can just about tie a cable tie around that with a tag attached to it. They are quite tough, robust animals."
"The tag is attached to a wee float that, when the jellyfish dies, becomes dis-attached, floats to the surface and washes ashore.
The time depth recorders are attached to a small float so they can drift ashore
Dr Houghton said the study aimed to find out more about how jellyfish use their environment, how long they live and how they use the tides in the four locations.
He said: "What we've found is that leatherback turtles always end up in these four bays, so by tagging the jellyfish with these dive computers, what we're looking at is if they (jellyfish) go up and down in the water column with the tides to help maintain themselves.
"There's a lot of mythology surrounding jellyfish - we often think they have come from this sort of amazing jellyfish Atlantis somewhere out in the ocean and drift in every summer.
"It seems daft because they are so familiar, but you might as well be tagging a unicorn for the amount of information we know about these fellas."