Nuclear power plants increasingly face a new enemy: the humble jellyfish.

These aquatic animals—and algae and other plants—get caught in and block the cooling water intake pipes of nuclear power plants, preventing nuclear reactors from getting the huge amount of water they need every day to cool their reactor cores and associated equipment.

Usually, screens prevent aquatic life and similar debris from being drawn into the power plants’ cooling system. But when sufficiently large volumes of jellyfish or other aquatic life are pulled in, they block the screens, reducing the volume of water coming in and forcing the reactor to shut down.

Jellyfish and algae have assaulted nuclear power plants in the United States, Canada, Scotland, Sweden, Japan, and France. In Scotland alone, two reactors at the country’s Torness power station had to shut down in a single week when the seawater they used as a coolant was inundated with jellyfish. (Because of their tremendous need for cool water, nuclear power plants are often located next to oceans and other naturally occurring large bodies of water.)

The problem is not entirely a new one in the energy industry; the first known jellyfish “attack” on a (coal-fired) power plant happened in 1937, in Australia. But while biological fouling has long been an issue, the number of such events has been on the increase in the past five years or so, and could increase further because of environmental change. The sheer number and size of the animals seems to be increasing as well; in some incidents, there have been more jellyfish than water, jellyfish biologist and senior marine scientist Monty Graham of southern Alabama’s Dauphin Island Sea Lab has reported. Sometimes, the jellyfish concentrations can be quite dramatic, with as many as 50 to 100 of the animals per cubic meter of water. News photos show jellyfish taken from power plant intakes filling containers the size of the bed of a pickup truck. Occasionally, schools of jellyfish are so large and thick that they can be seen from the air, as shown in this video footage from LiveScience.

Scientists are unsure as to the reasons for these periods of sudden, rapid increase in population, or “blooms.” It is known that some species of jellyfish, such as the moon jellyfish (Aurelia aurita), can thrive in over-fished and degraded waters. Jellyfish populations have been seen to increase when ocean waters warm, but the recent blooms cannot be easily linked to climate change alone, because there is insufficient historic data available for jellyfish populations on long time scales.

Nonetheless, researchers suspect that there may be a climate-related mechanism at work, in the form of warmer ocean temperatures combined with environmental changes such as fertilizer run-off, overfishing, and ocean acidification. It could be, for example, that the increasing acidity of the oceans—caused by the rising level of carbonic acid in seawater due to the oceans’ uptake of atmospheric carbon dioxide—interferes with the process of calcification, in which sea creatures use calcium in the water to make their shells. If that’s the case, then acidification would reduce the number of shelled creatures (and non-shelled animals that are heavily dependent upon calcium, such as corals), but leave jellyfish untouched. In this new-found absence of competition, the jellyfish might go forth and multiply.

But if the cause—or causes—of jellyfish proliferation remain unknown, the effects are clearer. There have been dozens of cases in which jellyfish caused partial or complete shutdowns of coastal power plants in the past few decades, as well as shutdowns of desalination plants.

Each shutdown can be costly. When Torness nuclear power station in Scotland had to shut down in 2011 due to an influx of jellyfish, its parent energy company lost about £1 million (approximately $1.5 million) each day that the plant was not generating energy. The situation has gotten so bad that the UK government created a £383,000 (more than $592,000) grant to research preventive measures.

A freak event that keeps recurring. Shutdowns caused by jellyfish have occurred all over the globe. In 2011, the Shimane nuclear power plant in Japan had to shut down due to an influx of jellyfish. The same problem occurred twice at Sweden’s Okarshamns nuclear power plant—home to the world’s largest boiling water reactor—which was forced to shut down because of a bloom of moon jellyfish in 2005, and stop generating again for three days in 2013. Jellyfish have been a problem for decades at California’s Diablo Canyon nuclear power plant. As far back as 1984, jellyfish caused Florida's St. Lucie plant to shut down; it happened again in 2011—this time with a two-day shut down.

Such supposedly freak events may become even more common in the future, because of degraded environmental conditions that favor jellyfish. The Asian press has reported near-annual swarms of the massive species known as Nomura’s jellyfish (Nemopilema nomurai)—a six-foot diameter, 440-pound species that used to arrive on Japan’s coasts only about once every 40 years. Japan’s Shinichi Ue, a professor of marine science at Hiroshima University, warned in November 2014 that the world will be “in big trouble” if its leaders “fail to get serious about countermeasures against jellyfish.”

For now, the responses to jellyfish fouling of intake channels of nuclear reactors include flushing blocked screens with water or having divers hand-clean the screens.

Algae, too. And jellyfish are not the only problem. Many forms of aquatic life can cause problems with the cooling water intake system at nuclear power plants. Recently, Cladophora—a taxonomic grouping that includes many similar species of green algae—have been of particular concern, causing problems at nuclear reactors along the Great Lakes multiple times.

The Cladophora situation follows a familiar pattern. Similar to jellyfish, Cladophora benefits from environmental degradation, including increased fertilizer runoff, and it apparently thrives in the warmer waters found near the discharge points for water used to cool nuclear power plants.

Recently, the arrival of the zebra mussel, or Dreissena polymorpha—an aggressive, invasive species accidentally introduced to the United States in the late 1980s—seems to have allowed the Cladophora population to boom. This one-to-two-inch long, filter-feeding mussel forms large colonies that clarify the water, allowing light to penetrate and help Cladophora grow; at the same time, the mussel provides a substrate to which the algae attach.

In 2003, algae accumulated at the intakes of the Pickering nuclear generating station in Ontario, Canada, causing operators to preventively shut down its Unit 7 for two days. A more problematic event occurred in 2005, when three of the four operating units at Pickering shut down because of a large algae incursion. That same year, Ontario’s Darlington generating station reduced its electrical output as a result of algae and silt blockage in its water intake system; to protect equipment, personnel shut down Unit 1.

Ontario Power tried to correct the problem by installing a diversion net by the water intake and improving its operating procedures, but with mixed success. Ontario Power estimated that Cladophora fouling of cooling water intakes at its Pickering and Darlington nuclear power plants on Lake Ontario cost the company more than $30 million in lost power generation over a 10-year period.

Similar algae-intrusion events occurred at the FitzPatrick nuclear power plant in New York State four times in 2007, forcing expensive upgrades that included stronger parts, more powerful motors, guards, and better availability of cleaning equipment.

Jellyfish economics. Biological fouling in nuclear power plants has long required monitoring, evaluation, and action. But International Atomic Energy Agency reports warn that monitoring and processes that address biological fouling will need to change, because nuisance species seem to benefit from the warmer waters caused by climate change.

From the Pacific to the Atlantic and in the freshwater seas of the Great Lakes, nuclear power plants have been attacked—and shut down—by jellyfish or algae, costing millions of dollars. The increasing number of shutdowns caused by jellyfish blooms and algae may, in their own small way, help to deflate a key argument of those who promote nuclear energy: that it is cheaper to operate than other energy sources. In this way, nuclear power may run afoul of spineless enemies.