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As Arctic waters warm and sea ice melts due to climate change, many species of jellyfish and other zooplankton could expand towards the North Pole, threatening to disrupt ecosystems. The “jellification” of the Arctic may have already begun.

“There are impacts on the ecosystem that we can barely predict,” says at the Alfred Wegener Institute in Germany. “Some of these species – we know nothing about their ecology.”

Havermans and her colleagues combined several datasets on the distribution of the eight most recorded species of jellyfish and their gelatinous relatives across the greater Arctic. They looked at a representative set of species, ranging from the tiny hydrozoan Aglantha digitale, which are just a centimetre or two in length, to the venomous lion’s mane jellyfish (Cyanea capillata), which can grow tentacles that are more than 30 metres long.

They then modelled how the range of each species would shift in the second half of this century in response to warming waters, melting ice and other changes to the ocean under a medium-to-high emissions scenario. Their model accounted for vertical changes throughout the water column, which are particularly important to jellyfish, some of which only live at particular depths.

Most species would see their range expand as well as shift toward the North Pole, mostly due to the loss of sea ice. The lion’s mane jellyfish – which can compete directly with fish due to its size – had the largest predicted expansion, with its range almost tripling in area. All other species saw a substantial expansion except for the deep-water Sminthea arctica, which saw a small contraction.

There are already some indications that jellyfish have started expanding north – sometimes with undesirable effects. Havermans points to fjords in the Norwegian archipelago of Svalbard, where jellyfish have outcompeted cod, disrupting fisheries. “It can really take over and then there are almost no fish in there,” she says.

Other reports of surging jellyfish numbers in recent years have given rise to discussion of a wider “jellification” of the world’s oceans, although identifying clear trends is challenging due to a lack of data, says Havermans.

at the UK Centre for Environment, Fisheries and Aquaculture Science says impacts from such an expansion will also depend on how other organisms respond. The added competition could prove detrimental to some species, he says. For other predators like the spiny dogfish or scavengers deep in the sea, the new arrivals could provide a gelatinous source of food.