FOR YEARS, NOT QUITE UNDERSTANDING what was being offered, I declined a friend’s invitation to visit his family’s summer cottage on an island off the coast of Maine. Finally, I went. The island was reached only by boat, but the population was large enough to support, barely, a one-room school. (High school was on the mainland.) The cottage was modest, hardly 50 feet from high tide. As I recall, the family had bought 150 acres by the shore for very little money at the end of the Depression. I arrived in afternoon fog, so the surroundings didn’t make much impression. My bed was beneath the roof in the open upstairs, where there was a window at either end. I woke up the next morning and looked out at a clear day with golden sun lighting near and distant islands. On the horizon was Mount Desert. The scene was so unexpectedly beautiful that I cried, the only time nature ever affected me that way. For one meal, we gathered mussels at low tide from the clusters among the rocks. They were steamed and as fresh and sweet as could be. I returned each summer for several years, but eventually my summers were filled with other things. Not long after I stopped going, my friend told me that the mussels were entirely gone. What happened? he asked. I’d written about mussels, but I didn’t have the answer. I know now that mussels have disappeared in many places in Maine because of invasive green crabs, warming water, and acidification from increasing carbon dioxide in the atmosphere (part of it is absorbed by the ocean, where it forms carbonic acid). In some areas of Maine, overharvesting of wild mussels is also a problem. And yet in general in the world, the warming climate has allowed mussels to expand into places that were previously too cold.