On a still and sultry summer day I paddled my canoe a mile or two up the Sudbury River from Concord into Sudbury. At a quiet bend in the river I parked the canoe in the weeds near a rock where a pair of eastern painted turtles were basking in the noon sun. Out in the middle of the river, a fish leaped into the air. A great blue heron, wading nearby, flew off. I lowered a pair of hydrophones into the water (about 3 feet deep here) and recorded these delicate clicking and chewing noises of creatures unseen and unknown.