Sea salps — gelantinous, marine organisms that look like jellyfish — have clogged water systems used to cool nuclear reactors in South Korea, forcing two units offline.

It’s the second time in less than three weeks Korea Hydro & Nuclear Power Co. shut the Hanul No. 1 and No. 2 units, after salps clogged water intake valves. The reactors, which each have a capacity of 950-megawatts, resumed operation last week before shutting again Tuesday.