In the region of the North Atlantic Ocean known as the Sargasso Sea, circling ocean currents accumulate mats of Sargassum seaweed that shelter a surprising variety of fishes, snails, crabs, and other small animals. A recent paper shows that in 2011 and 2012 this animal community was much less diverse than it was in the early 1970s, when the last detailed studies were completed in this region....