Arndt, from Team Sunweb, led the breakaway group across the finish line in a 167-kilometer (104-mile) stage that ended with a sprint in Igualada after riders endured a steep climb of more than 7 kilometers (4.3 miles) and withstood slippery wet roads under heavy rain in the ensuing downhill near the end.
"I knew I had to chase every attack and keep things together because I knew I could win from a sprint," the 27-year-old Arndt said. "This is one of the best days of my career. I've always said I want to win a stage in every Grand Tour and this is a step forward on that goal. When I crossed the line I still couldn't believe I'd won a stage on La Vuelta."
Alex Aranburu of Spain was second, and Tosh Van der Sande of Belgium finished third. Miguel Angel Lopez relinquished the leader's red jersey for a third time in this year's Vuelta after being held in the peloton that crossed the line more than nine minutes behind Arndt.
The gap allowed Edet, from team Cofidis, to take the overall lead and gain 2 minutes, 21 seconds over second-placed Dylan Teuns, the Belgian who held the red jersey earlier in the week. "It might very well have been the opportunity of a lifetime to take the leader's jersey and I had to seize the opportunity," Edet said. "I had 'La Roja' in the back of my mind since the start of La Vuelta."
Lopez, a Colombian, was three minutes off the lead going into a crucial ninth stage on Sunday, when riders will face several climbs and a 4-kilometer (2 1/2-mile) gravel track near the end in Cortals d'Encamp.
"We stayed cautious in the finale because the road was wet and that allowed the attackers to gain time," Lopez said. "There is no problem: one day with 'La Roja,' one day without. Tomorrow will be a beautiful day. I hope to be among the best."
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