The combination of extraordinary heat and drought that hit the Western United States and Canada over the past two weeks has killed hundreds of millions of marine animals and continues to threaten untold species in freshwater, according to a preliminary estimate and interviews with scientists. “It just feels like one of those postapocalyptic movies,” said Christopher Harley, a marine biologist at the University of British Columbia who studies the effects of climate change on coastal marine ecosystems. To calculate the death toll, Dr. Harley first looked at how many blue mussels live on a particular shoreline, how much of the area is good habitat for mussels and what fraction of the mussels he observed died. He estimated losses for the mussels alone in the hundreds of millions. Factoring in the other creatures that live in the mussel beds and on the shore — barnacles, hermit crabs and other crustaceans, various worms, tiny sea cucumbers — puts the deaths at easily over a billion, he said.