San Francisco Chronicle: Pacific Ocean shellfish – the mussels and snails, the clams and cockles – are heading for a mass invasion into the North Atlantic that could alter the entire ecology of both oceans as sea ice vanishes from the warming high Arctic, two California scientists predict. It has happened at least once before, the scientists say – about 3.5 million years ago, when a relatively brief period of natural global warming kept the Bering Strait open and hundreds of species of marine life migrated …