Sönke Johnsen scuba dives in the middle of the ocean, far from land, miles above the seafloor. There are no shipwrecks to explore, no reefs to admire, just a disorienting oasis of blue.

What he looks for is hard to see. Indeed, he and his companions often stare intently at one another, searching for distortions passing among them, slightly more visible against the dark color of a wet suit. And then they carefully catch them and place them in glass jars.

“You’d be surrounded with all these animals,” said Dr. Johnsen, a professor of biology at Duke. “But you could barely see them, because they were transparent.”

The oceans, which make up more than 90 percent of the earth’s livable space, are full of almost invisible animals. That is because life there is different from everywhere else.