DNA floating in seawater is now enough to let scientists monitor the health of America's dolphin populations‎

DNA is everywhere in the world's oceans—not only packaged inside cells from skin, scales, mucus, feces, and blood, but also floating freely. Sequencing such "environmental DNA" (eDNA) from open water has long been used as a cost-effective way of gauging the number and identity of species in a region, especially when they are rare and elusive or living at great depths.