This past spring, hordes of bizarre sea creatures began swarming the coast of Oregon. They had bumpy, tubular bodies, gelatinous skin, and they emitted a strange glow. Sometimes called “sea pickles,” these creatures are more accurately known as pyrosomes, as Eleanor Ainge Roy reports for the Guardian. And much to the bewilderment of marine scientists and fishers, they are spreading quickly.

Millions of pyrosomes are now clogging up the West Coast, ripping apart fishing nets, getting caught on fishing hooks, and washing up onto the beach. They have invaded the waters of British Columbia, and have been spotted as far afield as Sitka, Alaska. During a cruise to study the critters, one team of researchers scooped up 60,000 pyrosomes in five minutes.

Though they look like single organisms, each pyrosome is in fact a colony of tiny multi-celled animals called zooids, Craig Welch explains for National Geographic. They reproduce asexually, feed on plankton, and are bioluminescent. Typically, pyrosomes are found in warm waters like the Ivory Coast or the Mediterranean Sea, where some species can grow to up 60 feet long and wide enough for a person to fit inside.

#ONCabyss Upper water column is full of Pyrosomes pic.twitter.com/2924OnMRko — Paul Macoun (@PaulMacoun) June 17, 2017

By and large, however, pyrosomes are mysterious creatures. Marine biologists rarely get a chance to observe them, since they tend to stay far below the surface of the ocean, away from the shore. So scientists aren’t entirely sure why pyrosomes have proliferated to such extremes along the Pacific coast.

Hilarie Sorensen, a graduate student at the University of Oregon, is part of a research team studying the baffling bloom. Writing in a National Oceanic and Atmospheric Administration (NOAA) blog, she suggests that the pyrosomes “are being delivered to coastal waters from farther offshore, and that warmer ocean conditions over the past three years are creating an ideal environment for them to thrive.” But other explanations—like atypical sea currents and a change in the animals’ diet—are also possible.

The ecological impact of the pyrosome bloom also remains unclear. According to Welch of National Geographic, some scientists worry that if the animals die en masse, they will leach oxygen out of the water and endanger other marine life. Pyrosomes also pose an economic threat to fisheries. In Sitka, fishermen have reportedly stopped trying to fish for salmon because the waterways are so clogged with the jelly-like creatures.

Sorenson and her colleagues have embarked on two cruises to catch and observe pyrosomes. During one expedition, cameras captured thousands of the creatures floating at a depth of 100 meters. But more research is needed to unravel the many mysteries of these peculiar sea pickles.